Jun 14

Fully Programmable Modular Bench Power Supply – Part 4

After a fair amount of playing about, measuring and trying things I finally arrived at a working design. I am happy I have what I think is a good starting point of rest of the project. I have something that is working and feels much more stable under a varying capitative and resistive loads, I have a high degree of confidence that I am now on the right track. Having resolved the issues with stability I have now added a current shunt, high-side current sense amp and current error amp for current regulation, I have also added a couple of amps in comparator mode to sense the regulation state to show if the regulator is in constant volts or constant current mode or out of regulation all together.

PSU Schematic Version 0.4a

To sense current I have used a high-side current sense amp type MAX4080F, this is a really nice single chip solution designed for the exact purpose, it has a wide supply range and the “F” version has a x5 gain factor. Using a high-side current sense amp means that I do not need to sense in the ground return path which in turns helps keep things simple. The MAX4080F reads the voltage dropped across the current shunt which is made up of 10 x 1R resistors in parallel. The voltage read is multiplied by 5 and delivered to the output pin. In this configuration a load of 0-5A gives me a voltage output of 0-2.5v which is fed back to the current error amp limiting current to what has been set on the CC_REF input.

Here is a quick video taken to show the constant current limiter kick in as a filament lamp warms up and current demand drops. Turn the PSU output on and the cold lamp draws current beyond the limit set (indicated by the red LED), but as the lamp warms up and draws less current the PSU transitions from CC mode to CV mode smoothly.

In Part 3 I mentioned the idea of having a very stable low-power full voltage range regulator, this is what I created around Q5 and Q6, with R8 acting as its load. The IRF540 Q4 is then simply acting as a voltage follower which scales up the power handling, its source simply follows the voltage on the collector of Q5. This configuration works really well and is much more stable (more on that in a second), but its not perfect. I am not entirely convinced about the use of a FET as the final pass device any more. Its easy to drive because of the low drive current needed, but being a linear regulator circuit the output FET never does the one thing that its really good at which is being turn hard on, so despite having a positive grid bias supply that will allow this, its never used. The FET device is perfect for the pre-regulator switch (Q2) so I don’t need to change that but it would seem like there is an opportunity to simplify this circuitry further in the final output and driver section, so this is not yet a finished design, but its certainly a workable one. The decision may well come down to the bill of materials in the production, if we are using the FET for the pre-regulator, then we may as well use the same device as the pass transistor – we shall see….

For now I am not expecting to use the pre-regulator, it is in effect wired out by being switched on by default, If you were to build this circuit as as and do not wan’t pre-regulation (as would be the case in the 0-6v configuration) you would simply exclude Q2, Q3 and their associated bias resistors and put a link on the board between the drain and source pins where Q2 would have been. The pre-regulator will be introduced later and is specifically required to lower the overall heat dissapation of the regulator circuit under certain load conditions. The pre-regulator switching control will be done by the micro processor in the main but I do need to add some direct analogue feedback mechanism to ensure the regulators dynamic response is not impacted by any software or hardware latency the micro controller would introduce in this setup. Basically the idea is to sense any significant change (ac coupled) on the control point at the junction of D8 and D9 and if detected turn the pre-regulator hard on for a set period of time, lets say 250ms or so which will ensure full power is immediatly available to the drain of Q4 which will regulate as required. During this forced on time, Q4 will dissipate all of the heat generated by the power dropped across the regulator. Because of this forced on time, the micro controller will have time to sense the large amount of power being dissipated by Q4 by sensing the voltage differential between PRV_MON, I_MON and V_MON and will start driving PRE_REG to take over, in effect this is a lazy pre-regulation control that will only kick in once the load has stabilised, the micro controller will slowly wind down the power delivered into the resevour caps by switching the 100Hz drive from the bridge rectifier on the rising and falling slope of the half cycle, keeping a reasonable differential across Q4 for the given load. The pre-regulator control switching will be synchronised to the line frequency which will be sensed via the AC_SENSE signal.

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Back to the regulator stability topic, I have removed all the capitative loading around the driver and output circuit and instead reduced the bandwidth of the control loop by including frequency-dependnat negative feedback on both the voltage and current error amps. The aim was to significantly lower the gain of the loop above the point where the feedback becomes positive and the servo action becomes unstable. The simple first order filtering built around R33/C11 and R34/C12 provides a very stable control loop in this configuration. DC conditions remain in tact but as the frequency rises in the circuit, then the negative feedback starts to kick in via C11 and C12 lowering the gain in the feedback loop. The filtering could be improved and there is definitly some improvements I need to make at very low output voltages (0-50mv). The basic regulator circuit I have ended up with is pretty classic and has been done many times before so there is nothing new, but for me what I have achieved is creating a circuit from the ground up and getting my own understanding up to a level that allows me to properly comprehend the behaviour and problems that can occur with such a circuit and some basic ideas on how one would tackle the problems when they do arise.

The regulator in the configuration shown means there is a x10 voltage gain, so for 0-3v in I get 0-30v out, and for current limiting, for 0-500mV in I get a 0-1A current limit. Doing some basic testing I was able to drive a load well above the rated power of 30W (almost 50 watts) without a problem and with good load regulation. The regulator tracks very accurately according to my Fluke 289 and Agilent 34401A into both load and no load conditions. I was able to short the output out while it was driving into a half load, full load and no load with no damage or noise problems – the current limiting works nicely. I have yet to do any testing around dynamic response, I don’t have a programmable DC load so I need to work up a simple test rig to do this which I will leave for another article.

I have based the entire design on a single rail power supply for the op amps, using single supply op amps type LM358, these are common, low cost devices. I have tried using LT1013 precision op amps in this circuit but there is not any improvement over the LM358’s, primarily because the circuit remains accurate because of the control loop that ensures this, so precision op amps are not needed.

As a final check, I was measuring the output noise on a scope which was looking pretty disappointing, I was getting 25-60mv of white noise and some very minor high frequency ringing, I think the latter could well have been scope leads etc. Slugging the output with a big electrolytic cap helped reduce the noise but it was still present. Having tried a few things I suddenly realised that my test setup was flawed in relation to noise testing. I was using the 0-6V range on my Agilent E3631A to derive my CV_REF control voltage which is bad. Firstly the Agilent PSU generates some noise, about 2-3mv worth of it and secondly, my regulator circuit is a x10 amplifier so i was amplifying whatever noise the E3631A was generating by a factor of ten. I tried the same test but driving the CV_REV signal from a 1.5v AA battery cell and low and behold the noise figures plummeted to below 2mv on the output into a load at 16v – lovely. I have not carried out tests of any substance yet but the early results are very encouraging.

So I now have a working regulator, what next? In Part 5 I am going to set the regulator aside for a while and turn my attention to the digital control circuit. I am going to use a micro controller with a DAC and ADC to try to create stable and accurate control reference voltages to drive the regulators CV and CC reference inputs as well as relatively accurate and high resolution metering to measure the output volts, current delivered and pre-regulator voltages. I want to achieve good accuracy for both reference voltage generation as well as metering but plan to try various configurations, DAC/ADC’s and software approaches to see see what can be achieved at a sensible component price point.

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Jun 08

Fully Programmable Modular Bench Power Supply – Part 3

Previously in Part 2 I created a simple regulator circuit using a pass device and an op-amp, I was basically putting the well understood theory of this kind of circuit into a practical circuit and verifying my own basic understanding. Apart from no-load DC conditions which were fine and dandy, any sort of even moderate resistive or capacitive load drove the circuit into wild instability, clearly the breadboard its self, thin connecting wires and long component leads as well as a lack of any star earthling were all going to contribute to this, so it was time to refine the circuit a bit and build it on a more sturdy vero board with some attention to layout, earthing and current handling. Here is the circuit I built: –

PSU Schematic Version 0.1

The initial prototype is a throw-away, that means I build it on vero (strip) board, use low/no cost components, cheap sockets for the IC’s I want to re-use and as many components as possible from the junk bin – when I am done with my testing I strip anything worth keeping for the next project and throw it in the bin – the point is, don’t expect it to be pretty…..

PSU Throw Away 0.2
PSU Throw Away 0.2

The significant change was adding gain to the circuit which is critical if I am to get the desired output range of 0-30 volts. The gain is obtained in Q1 which is in the classic emitter follower (if it was a NPN BPT) configuration. Having this configuration for the driver means I only need a swing of between 1 and 2v to get the full output swing of 0-30v. Needing a small voltage swing means I can run the op-amp at 14v. The driver and output is acting as a voltage amplifier which is what FET’s are good at doing. The diode D3 is ensuring that the op amp is running in class A in the required range driving into R8 which is its load. By doing this we eliminate any potential crossover distortion the op amp might exhibit because of its internal center point.

The op-amp U1.1 is doing the error correction. The driver FET (Q1) is inverting, which means the negative feedback from the output of the regulator is actually fed into the positive input of the op amp, this is a different configuration to the first design where the driver was a current amplifier and non-inverting.

One interesting addition is the Vgb+ supply voltage. You will see that I have included pre-regulator pass device (Q2), the idea here being when there is a large amount of power being dissipated across Q3 the micro controller can detect this and drive Q3 which in conjunction with C5 and C6 will act as a bucket style pre-regulator. Because Q3 is acting purely as a switch we need it to be both very fast and very low on resistance and one of the things a power FET does very well is have very low on resistance. However, in order to get this low on resistance, you need to drive the FET’s gate pin at approximately +5v above the source pin. That means if you have 30v on the drain, and you want to see 30 volts on the source, then you need to drive the gate with +35 volts. Vgb+ is obtained by using a simple AC voltage doubler and a series regulator that’s referenced from +5, giving 5v above V+ regardless of input voltage.

So filled with high expectations I power up my new masterpiece for the first time and err, well not great. Unstable is an understatement, I had high frequency instability, low frequency instability and wildly different variations of the same with different loads. For the most part, a large electrolytic cap across the output brought it mostly under control, the high frequency stuff was largely killed off by placing C12 on the driver (Q1). Various loads across the range would yield different results and it was unpredictable and certainly not reliable in a way that I would be happy to power my next project from it!

So whats going on? why are these types of circuits so unstable? Well, I am sure there are very detailed scientific explanations but I don’t have anything like the knowledge to explain those; so in layman’s terms this is the best I can do. Under perfect DC conditions, the ideal circuit (which is how I tend to visualise electronic circuits) there is negative feedback and no phase shifts so you have a perfect DC Servo, so it locks the output to the input reference and as load is placed on the output the error amp drivers more or less to maintain the output at the input level – easy right? Unfortunately, all electronic components, wires, board layouts power sources and environments generate noise and have parasitic inductance and capacitance all of which can introduce phase shifts at different frequencies – this means that across a frequency spectrum our perfect theoretical circuit with its perfect negative feedback actually turns into positive feedback and your stable DC servo becomes an oscillator, your circuit can be stable at one frequency and totally unstable at another frequency and this can be occurring at the same time. Worst case you have numerous different stable and unstable conditions concurrently.

As I was measuring this behaviour I was observing a lot of fluctuation, things were varying in a random and unclean way, the analogue equivalent of a random number generator in software. To me the circuit felt very loose and reminded me a lot of those old fashioned black and white CRT TV’s I used to play with when I was a kid, The whole thing was powered by a big dropper resistor and change in the percentage of white/black proportion would significantly change the voltage levels in the TV’s electronics and this could be seen visually as instability as the picture would appear to breath and move around on the screen as things changes. Compare that to later solid state CRT TV’s where regulated power supplies were used, everything was much more stable visually.

I had managed to quieten the PSU down and drive into loads with good regulation and I achieved this by slugging the think with capacitance which has the effect of lowering the bandwidth of the circuit, which means the circuit has no, or substantially reduced gain at higher frequencies and any oscillations at those frequencies obviously go away. The problem with reducing bandwidth though is the impact on the dynamic response of the PSU under varying load conditions (more on that in a future article).

Types of capacitor also play a big part, for example I was only able to stabilise the high frequency issue using a 3n3 to 10n polyester cop, putting in the same value in multilayer ceramic did not work – that was yet another indicator that the circuit is simply too sensitive to instability – its loose.

My test conditions were done mainly under a 5-10 watt load at about 15v. I have attached some photos and some scope traces so you can see the sort of effects I was seeing.

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In summary, despite having it quiet and stable with various test conditions I was left with the impression that the circuit felt wrong. At this point I am suspecting many things, the FET’s are very fast so they were a concern, the feedback loop is running at high power levels so effects of parasitic capacitance and inductance are very pronounced. My overall take away was I need to start again and just for a while I thought, maybe I should just use an off-the-shelf IC solution – but I felt like I was giving up to easy. I don’t have the skills and experience to design this scientifically so I have to approach this with a bit of trial and error and a lot of instinctive sauce. That desire to not give up got me thinking – and an idea came to me – what if I created a very low-power regulator which was highly stable at the desired voltage range and then scaled that up with a simple current amp – could I get better results?

In Part 4 I will share with you the great progress I made and a fully working regulator design.

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Jun 04

Fully Programmable Modular Bench Power Supply – Part 2

Now the overall high level system design and parameters are set (see Part 1), its time to get down to some practical design. As I specifically do not want to use one of those “out-of-the-box” all in one regulator chips the first thing we need is a working linear voltage regulator that we can build upon. Using the prototyping breadboard I created the following circuit. The objective was to set up and verify the DC conditions for a basic regulator. Unlike a classic regulator circuit where there is typically a fixed reference and a variable resistor (POT) in the feedback loop, this regulator calls for something slightly different because our micro controller and appropriate DAC will generate an accurate reference voltage between 0 and a couple of volts, the exact value of which will be set by the user; the regulator circuit must track this reference voltage and set its DC output to a multiple of that reference.

The following schematic shows my initial 30 minute attempt at building such a regulator.

PSU Schematic Version 0.1

I should state at this point that the circuit is basic and is missing lots of things that one would expect to find. The purpose of this initial design was not to create a perfect regulator but was to setup a circuit to verify the basic DC conditions and theoretical practicality of the circuit.

The circuit is built around a FET pass device (type IRF540) which is the power workhorse. Unregulated power into the drain with the load placed on the source. The op-amp is configured as a simple error amplifier. The op-amp will move its output up or down (depending on the input state) in order to get its two inputs as close together as possible. Because the op amp drives the power device and the negative input of the op-amp is derived from the output that the power device we have a closed loop servo circuit and this is in essence what a typical regulator is made up of.

Because of the closed loop nature and the tight feedback loop, one of the biggest problems with these types of control circuits is stability. Each component and any introduced capacitive or inductive loading will create phase shifts (all components exhibit parasitic capacitance and inductance). Phase shifts will put the circuit into positive feedback at certain frequencies so there is a good possibility that such a circuit will become an oscillator at certain frequencies. As a consequence, a great deal of attention needs to be paid to this problem in the design which needs to ensure that it works reliably and remains stable within the scope and specs of the requirements. The more voltage and current range required the more difficult it is to tune the circuit to be stable of the range. Component choice, DC conditions, speed/bandwidth, gain, noise and PCB/Track layout all matter a lot here. It’s for this reason that many power supply designs avoid discrete solutions in favour of a one-chip solution. Discrete regulators are hard to make, and even harder to make reliable and stable and to be honest, most people cannot be bothered when there is a $2 off the shelf IC that will do the job most of the time. However, a professional grade programmable bench power supply needs something more than what these one chip solutions offer.

Considering the breadboard approach to building the above circuit (see photo) under moderate loads the regulator circuit was pretty stable, actually I was very surprised just how stable it was.

Birds Nest Construction

I thought I was on my way but very quickly realised I was actually on the wrong track. The reason why this design is stable is because it does not have much gain, in fact it has a gain of just 2. So put 2 volts into the V_REF input and get 4 volts out. The driver transistor Q1 is in effect a current amplifier in this configuration – the op-amp has to swing its output pretty much the range of the desired output. For low voltage requirements where you con comfortably run your entire circuit from a single supply this is a very good approach because of the inherent stability of the circuit. However, this design requires a regulated output of 0-30v which would require the unregulated input voltage after the rectifier and reservoir caps to be about 45 Vdc. Not many op amps can run at this level, and those that can are only just within range so we would be right on the edge of the spec for the device which does not make for a robust design and I personally don’t like components being pushed to their limits – it feels wrong, a bit like driving your car continuously in the red like, you can do it but not many people want their car to scream like that all the time.

In order to facilitate ease of use and the modular design I have set out to achieve, I want to create a regulator module that runs from a single AC power source, a single winding from the mains transformer. Many bench PSU’s (for example, like the Agilent E3631A I recently repaired) have multiple internal power supplies where the control circuitry is powered separately from the main power source. This makes it easier to design for higher regulated voltages and helps with things like noise immunity too but because of the design goals I have set it’s not an option for this design.

The design needs to take into account some system design constraints. No split supply for the op-amps, so we need to use single-supply capable devices. The op-amps and control circuitry need to run at a much lower voltage than the pass device and regulator output and the only way that is possible is by introducing voltage gain outside of any op amp. Voltage gain using an emitter follower (or equivalent approach) provides the level translation needed and lots of gain but that’s where the stability problems start.

In Part 3 I will describe the next evolution of the regulator circuit where I introduce gain and a considerable amount of instability and start to formulate a workable solution.

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May 25

Fully Programmable Modular Bench Power Supply – Part 1

After fixing a high quality power supply (see here if you are interested) it spurred me on to have a go at designing and making my own – I think anyone who does electronics as a hobby will at some point or another build a power supply to use on their bench – I have not done that myself before so I thought I would give it a go. My aim is to use my basic understanding of analogue electronics and create a fully programmable bench PSU that will perform at least as well as the Agilent PSU, I think this aim is reasonable on the basis that todays components are considerably better than those that were available 20 years ago. Apart from the performance characteristics, there are a system engineering characteristics that I also want to consider because I would like to make it possible for anyone else to build this PSU as a DIY project with the ultimate aim of creating a high quality PSU that is modular and can be built in various configurations and be built at a hobby user or small lab price point. Here is a photo showing the very first working prototype regulating at 5.010 volts.

The first working prototype regulator

Why a PSU project, there are hundreds of them already? Firstly, the two areas of technology I really enjoy are electronics/embedded and software development and this project requires a fair amount of both to be brought together. Apart from that, no other reason than because I think I can do a decent job – we shall see 🙂

I thought it would be a good idea to try and set out what I have in mind. My aim is to create a modular PSU system designed to be used in lab or test automation environments. The first thing I want to create is a module similar in concept to those audio amplifier modules you can buy for building a HIFI amplifier, the module will physically look something like this.

The PSU module will take a single AC input from your line transformer of choice on its input and will provide a fully programmable lab quality Constant Current/Constant Voltage regulated DC output. The module its self will not have any kind of controls or display, but instead will have a fully isolated serial I/O which will be connected to a controller with the idea being you can create a multi-channel PSU with isolated outputs while also providing a single earth referenced controller that can be safely connected to a computer or other test equipment in test automation environments. Once I have created these modules, my intention will be to make a a few variants of controller, an RS232 interface and a PC software controller, a simple stand-alone control board with an LCD display and a couple of rotary encoders to control a single module and a more comprehensive control board that can control up to four modules with a nicer display (TFT/VFD?) and other interfaces such as RS232, USB and Ethernet to create a full function standalone multi output bench PSU.

Focusing back on the PSU module, I would like it to support a number of configurations with just a few component changes, primarily this is to allow different voltage/current ranges and resolutions to be selected to suit different requirements.

The headline specs for the regulator module are as follows: –

  • Up to 50w of power
  • Output range options 0-6vdc 0-5A, 0-10vdc 0-5A, 0-15vdc 0-2.5A, 0-25vdc 0-1A, 0-30vdc 0-1A
  • Constant Voltage and Constant Current capable
  • Remote sense capability
  • Over voltage, over current, reverse power and short circuit protection at all power levels
  • Optional on-board pre-regulator to lower the modules heat dissipation for higher voltage ranges
  • On-board temperature monitoring
  • Fully isolated serial interface for programming, control and monitoring
  • Control resolution down to 1mV and 1mA in low voltage range

There are also some system engineering constraints I want to apply, these are: –

  • Low component count
  • Easy to source low cost components
  • Easy to build DIY
  • Physically robust construction
  • One single PCB design for all voltage range configurations

In terms of my design approach, and I must state at this point that I am no analog electronics expert, I really just have a passing understanding. None the less, I want to avoid using the easy option in the form of the classic single package regulator IC’s that most DIY PSU builders use. LM317T, LT3080 and the like. These are great components, don’t get me wrong, but what you don’t get with these is a professional grade PSU without putting a significant amount of other electronics around them, by which time you have pretty much lost any advantage you have gained over using discrete components. Apart from that, one of the main drivers for this project is to learn more about building this kind of project and to share that learning with others.

In Part 2 I will describe my first attempt at building a discrete linear voltage regulator, with a schematic of the first working circuit along with a description of what I found along the way.

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May 17

HP/Agilent E3631A Power Supply Teardown & Repair

I recently bought a faulty Agilent E3631A bench power supply on e-bay which I thought would be a nice addition to my *slightly excessive* electronics hobby workbench.  These power supplies are really nice; they are engineered and built like military equipment, good high quality materials and mechanically very robust.  An great indication of how good these things are is the second hand values, these things cost $900-$1000 to buy in reasonable condition so they are not cheap.  I bought this particular one faulty and thought I would have a go at repairing it.

My first impression of the electronics in side was not great, it seemed very seriously over engineered for what it was trying to achieve.  It seemed like the designers had a field day adding all sorts of crazy circuits because they could.  The ADC is made up of discrete IC’s, there is a custom logic chip in there as well as a CPU, ROM and RAM, there are numerous power supplies for bias and control circuits all floating around each other and most things seemed much more complicated than they need to be.  The one real surprise though was the opto isolation in the analogue domain. The CV and CC reference signals from the DAC for the +6v supply are isolated through high linearity opto couplers type HCNR200, this is something that would be crazy to do today when the cost of micro-controllers are so low and have all the goodies like DAC’ ADC’s and PWM’s making isolation in the digital domain a far more sensible design choice.

6vIsolation

In fairness though, I was making my initial judgements based on what’s possible with today’s components, things were very different 20 years ago so given its age it’s a pretty sophisticated piece of kit really.  Once working it does appear to work very well so my initial thoughts are not really founded on anything other than my own instinct to want things to be easier to understand and better as a result.

On with the repair….

First things first, after a quick check of the obvious big components like the series regulator transistors etc, I very quickly needed a schematic diagram. Agilent were less that helpful here, the manuals they put out now days specifically have the detailed schematics removed from the documents despite there being a reference to them in the index. When I contacted Agilent and asked for a schematic I was told in no uncertain terms (after a 4 day response time) that they no longer make the schematics available, but they do offer a £450 exchange repair service – come on HP/Agilent, by all means offer the service but don’t stop those of us who want to hack around from doing so.  The solution was to buy an original printed service manual which did include the schematics; e-bay and $10 got me what I needed.  As luck would have it, while waiting for the manuals to arrive in the post, I also managed to find a manual on the net which still had the schematics present – not from any official Agilent source I might add…

I set out to work on fixing it and found I had to strip it down completely, removing the two boards, front panel, transformer and wiring from the chassis and spread it out on the bench. If you find yourself needing to repair one of these, be prepared to commit serious bench space to the exercise.  I have taken a bunch of photo’s if the teardown so you can see what all the bits look like.

Home » HP/Agilent E3631A Power Supply Teardown & Repair » HP/Agilent E3631A Power Supply Teardown
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There were various faults with the PSU, numerous op amps and some CMOS logic IC’s were faulty as well as two open circuit 33k resistors. At a guess I would say there was some kind of big static or high voltage discharge into or across the outputs that caused the original fault. I had to isolate the various areas of the circuit and work on them individually, making assumptions about what should be present in terms of voltage levels and feed in lots of external signals to get to the bottom of each fault.  I struggled with the configuration of some of the analogue circuitry – fortunately for me I have a good friend who understands much more about analogue electronics than I do so some exchange of e-mails and sections of circuits with measurements kept me on track and expanded my own knowledge too – cheers Span.

Here are all the components I ultimately had to change…
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While fixing it I also managed to introduce some faults of my own. Specifically I managed to blow two of the HCNR200 opto couplers, easily done just with a slip of a multi meter probe shoring out pins 2 & 3 puts 15v with no current limit straight into the internal LED rendering it open circuit instantly.  I managed to blow four of them like this before I figured out what I kept doing – doh!

After working through these problems I finally got it working except — when placing a dummy load on the +6 output, the voltage I was measuring went up!  A bit more inspiration from my friend Span and a scope on the output and voila – it was bursting into oscillation under load, probably due to the 4 ohm wire wound load resistor. It turned out this was down to the fact that the electrolytic capacitors soldered onto the back of the binding posts on the front panel are actually there for stability reasons – obvious once you know. I had removed the output wiring from the front panel to make it easy to work on. Strapping 1000uF across the output solved the problem.

You can download the service manual which include the schematics

Having worked on this I have been inspired to have a go at designing my own programmable PSU from the ground up to see if I can match the specs but use more modern components and design approach – I will post info on progress if I get around to it.

[UPDATED:] I am getting around to it… http://gerrysweeney.com/fully-programmable-modular-bench-power-supply/

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