Dual PSU - Introduction:

The first thing to decide is the basic requirements of the unit:

Initial Specifications:

• General: Dual outputs, for op-amp based experiments. Permanently configured to give positive and negative supplies, rather than a conventional dual unit that has a series-tracking function. This enabled me to employ a circuit design that I'd devised to give very accurate tracking.
  
  
• Outputs: ±0 to 18 volts, ±0 to 2 amps. Automatic crossover between Constant Current (CC) and Constant Voltage (CV) with status LED's
  
  
• Voltage and current adjustment: By separate course and fine controls for both voltage and current. This offers quicker setting and better resolution (in my experience) than 10-turn pots.
  
  
• Metering: Dual 7-segment LED displays. Using devices with a 1999 count enables 10mV and 1mA resolution. Each display can display voltage or current from either the positive or negative supply.
  
  
• Tracking modes: Either the positive or negative supply can be the 'master'. This might be useful for quick testing - for example it the positive supply is set for 9 volts and the negative supply is at 12 volts, then changing the 'tracking master' mode will quickly change between ±9V and ±12V
  
  
• Output switching: DC outputs switchable - when output is isolated, the current meters should display the selected current limit value, enabling the limits to be set safely before the circuit-under-test is powered up.
  
  
• Physical: I wanted to fit it into a 3U height, just under half-width. That fits nicely with some heatsinks that I've got, and my other kit.
  
  
• Other: Heatsink temperature should be monitored, and the outputs should be isolated in the event of overheating. This is because I don't intend to use it anywhere near full load continuously - I can safely use smaller heatsinks than what would otherwise be required for worst-case conditions.

During the 1992 summer break, I built the first prototype. It almost worked, but had several problems that weren't addressed until quite recently. As I only built the positive regulator, I didn't know for sure that my cunning tracking scheme would work accurately. Also, in my naivety I didn't plan earthing properly, instead trying to make 0V from thick heavy wire.... That's been consigned to history, and prototype no. 2 is shown below:

Pictures:

Here it is, working and looking pretty much like the original sketches. All it needs is some labelling. Inside there is still some tweaking to be done, but it's getting close to being on the shelf and in use.

It's built in a standard box from RS - all steel apart from the aluminium front and back panels. Unusually for RS, it wasn't too expensive - about £15. Both the top and bottom are removable for access. I had to add the side screws to make the case stable with both covers off. It's reasonably well ventilated, which is just as well!

Before getting involved with the circuit details, it would be worth having a nose about inside the unit. Also, it would be nice to know what the front panel controls do. With the help of Paint Shop Pro, here's a labelled version:

Unfortunately, the finished version won't look this good!

Most features should be obvious - I tried to make control layout logical. There's course and fine controls for both voltage and current, and green/yellow LED's to indicate CV and CC modes. Pressing the Meter Mode switches changes the meter indication - the red LED's in the display windows move to indicate the mode selection.

The tracking mode select is on the left. Normally both red LED's are off, and each supply is independently controlled. Pressing the button lights the first LED and the positive supply becomes the master. Pressing it again moves the LED down, and the negative supply is the master. However, since building the unit, I decided to change this to a 3-position toggle switch so that the tracking mode is remembered when the unit is re-powered. Tracking is very accurate, and when the unit is properly set up, it's quite rewarding to see both voltmeters indicate exactly the same reading at every setting of output voltage...

I realise that having the 0V terminal in green is slightly confusing, so I recently changed it to black, and changed the two negative black ones to blue. I could have provided 0V sense connections, but any losses in the 0V cabling would result in tracking errors. As the currents involved are relatively small, the +/- sense connections are probably unnecessary anyway...

The LED displays are a bit poor - they came from Maplin. The left-hand "18" segments are particularly bad. Also, the strange Maplin display filters make it worse - if the LED's aren't firmly touching them, then the filter blurs the image. I'm investigating a better quality solution that hopefully won't involve rewiring the panels...

I'm planning to replace the small rectangular LED's (and confusing labelling) with HP light bar modules, and produce a transparency that has the V/I labels on. I've discovered that I can print black in a high enough density on inkjet transparencies. The resulting printout is sticky, ie self-adhesive.

The power switch is illuminated - I've set it up so that it flashes to indicate a fault condition. Currently the only trigger is the heatsink temperature, but I'm planning a simple over-voltage detector. In fault-mode, the DC outputs are isolated - they're switched by relays.

What's Inside?

Here's what you see under the top cover:

Things are kind-of upside-down in there - the transformers are bolted to the underside of the chassis-plate. The large board is the Logic board, and deals with all the mode switching and relay driving. It also contains the analogue switches for the meters. The 20-way ribbon-cable connects to the Control board underneath - all relay drives and meter signals go down there. The 34-way ribbon cable connects to the Meter panel - 12 wires go straight from there to the front panel for switches and LED's - see later. The only other connection to the logic board is for the heatsink thermistor, just visible at the back. The inter-board wiring worked out quite well - it's always worth trying to use ribbons rather than having a birds-nest...

Note the double-insulation on the mains switch - safety must always come first!

This is a close-up with both ribbon-cables removed. It shows the grey 12-way connector that connects to the front panel switches. This connects to a green loom that you'll see later.

The white Molex (just visible on the right of the 34 way latching socket) is the power input from the logic PSU. Power for the Logic panel is taken via the ribbon cable.

The 20-turn pots are for meter calibration - there's enough slack on the ribbon cable to let you get to them. Standard 4052 CMOS switches deal with the signals.

This board was a bit tricky because of the limited height. To get the most height for components, there's a sheet of Paxolin for insulation underneath it, meaning that the clearance is less than 3mm.

This board would've been much smaller if I'd used mechanical switching for the meter modes but they're so old-fashioned... I was also having problems fitting them on the front panel. Next time I'll be putting less in the box!

Underneath:

Here's the view with the lower cover removed:

You can see the Control board, on it's crude hinge mechanism. It works quite well in practice, which is just as well because there is still quite a bit of development to be done to this board. The 12VA transformer you can see powers the logic, LED meters and the relays.

Things become clearer when you hinge up the Control board: You can see the Logic PSU board (left) with it's connection to the Meter board, which is the other side of the aluminium sheet. That sheet is held to the front panel by all the controls, avoiding visible screwheads on the front panel. On the right is the main transformer (120VA), above it is the rectifier assembly. That's held in by 2 screws accessible from above - the lump lifts out easily for repair once some Molex connectors have been undone.

You can see the other end of the 20 way ribbon cable, along with the 50 way front panel connection. This was a bit of a risk, electrically speaking. Seems OK, fortunately - I made sure that lots of parallel connections were used for heavy currents. Normally you'd want the regulator output connections to be as short as possible for stability reasons.

This picture shows the same shot from a different angle, with the big ribbon cable removed...

The yellow rectangular lumps are relays - the large ones in the middle of the Control board are output isolation. They also deal with the current meter source - when the outputs are off, they show the chosen limit value. The smaller ones on the right select tracking modes. I considered CMOS analogue switches for them, but relays are much easier, and I've got plenty of them...

On the left end of the Control board are the connectors for the power transistors and unregulated DC voltages from the Rectifier pack. These are loomed together inside PVC sleeving to offer protection from the heat in the area...

Bench power supplies aren't normally this serviceable!

Front Panel Removal:

The front panel is a complicated assembly. Because of that, I made sure that it's easy to remove from the chassis. The aluminium sheet supporting the Meter panel and Logic PSU is attached to the main chassis by two M3 spacers - one of which also secures the Meter panel. But, once you've undone them, disconnected the 50 and 34 way ribbon cables, and undone the 2 side screws, here's what happens:

The whole lot becomes free, attached only by the mains wiring. If required, you can undo the 2 small screws in the back of the switch, and unbolt the earth tag - then it's completely free. In practice, you only need to do this to get to the Meter panel, as the Logic PSU is easily removed in-situ.

Here's another view, showing the back of the front panel. You can see the loom required to connect the 50 way header to the front panel pots, 4mm posts, and the CC/CV LED's (via left grey Molex connector). I won't doing this again in a hurry!

The 5 volt rail supplies the LED panel meters, and generates about 3 watts of heat. It doesn't warm up too much because the front panel is an effective heatsink. I wanted to avoid too much heat in this area because the ICL7107 DPM IC's get quite warm. Also, while the separate voltage reference IC is quite good, it makes sense to not heat it up unnecessarily.

The mains earth 4mm post is connected directly to the mains loom - this is securely connected to the main chassis. The 12 way Molex loom splits into two, and connects to the output switches/meter mode switches, and the small board holding the tracking mode switch and LED's. From there, the twisted white wires go to the power switch lamp.

Back Panel...

The back panel removes just as easily, should power transistor replacement be necessary. The only connections are the transistors (one connector), the thermistor (one connector) and the mains wiring (hopefully enough slack!)

Here's a close-up of the Rectifier pack, showing the main smoothing capacitors and rectifiers (that need heatsinking adding!) Apart from the screw-terminals for the AC input, all connections to this board are via the PVC-enclosed loom, bottom-left.

The aluminium-clad resistors are the current-sense devices. They don't generate much heat, but by clamping them to a heatsink, you can be sure that their tempco (already a respectable 50ppm/°C) doesn't affect the accuracy of the current measurements too much.

The smaller additional board was clearly an afterthought! Note the use of LED's - these are useful for 2 main reasons - quick visual indication of supply presence, and also as bleed devices. The green CV lights stay on for about a minute after you turn the power off!

Part 2 looks briefly at the circuit details...