Updated 16th May 2012
In a previous post: Versatile Dual Power Supply Printed Circuit Board I described a dual power supply design that I hoped would fulfil 90% of my needs. To this end, it was very successful, and several have been built and installed in various pieces of equipment. These have been mostly of the ‘fixed’ type of supply, and for types using the LM317/LM337 adjustment of the two presets has not usually been a problem, simply set and forget. When, however, a dual supply is require that is continuously variable – say, for example a bench supply, the individual adjustment of the presets is a pain. Of course the problem can be partially solved by the use of a dual-ganged pot, and judicious use of trimmer resistors, but it is unlikely that such a system will offer precise voltage tracking between + and -. A better solution would be the use of a single pot to set both voltages simultaneously, and for precision setting this could be of the 10-turn variety.
There are several dual-tracking regulators around, but these are generally expensive, and may necessitate special ordering. I present here a reasonably simple solution that substitutes an op-amp and a few resistors for the -ve control pot, that is suitable for supplies delivering between 3.3v to 15v +/- at around 1.5amp. In a separate post I will discuss simple bootstrapping of the op-amp so that the voltage range can be extended upward, and also a power- booster for high current use.
Now referring to the two schematics below, it will be seen that they are essentially mirror-images of each other. If we remove resistor R2 in the second schematic, and connect the mirror-image of the voltage found at the junction of R1 and R2 in the first schematic, full tracking between the +ve and -ve outputs will be achieved.
Now refer to the third schematic, drawn by me, in which I’ve combined the two circuits below, and added in the mirror-image voltage generator.
In the above, IC1, R2 and R7 comprise an amplifier with a gain of -1. Put simply, any positive voltage appearing at the ‘ADJ’ terminal of the (upper) LM317, will be inverted (made negative), and the result applied to the ‘ADJ’ terminal of the (lower) LM337. The net result is control over both outputs achieved with the single adjustment pot R1. (5Kohms)
As always, the Devil is in the details, and the following should be noted.
Firstly the +ve and -ve supply for the op-amp IC1 are derived from the +ve and -ve outputs themselves – this may seem strange, but has been done for the following reason. Using the monolithic op-amp TL071, the supply voltages are limited to absolute maximums of +/- 18 volts. This would mean that if the op-amp supplies were derived from the raw DC inputs, (the ‘IN’ terminal of each regulator) some way of ensuring this never exceeded 18 volts would be required. Far simpler is to set the absolute maximum output voltage to around +/- 15 volts and use that instead. This is achieved by placing a 6.8K ohm (R10) resistor across the adjustment pot (R1), so that the largest value of the combined (paralleled) resistors is around 2.8K. Since there is always a voltage drop of around 1.2 volts across R3, this allows the op-amp to control a full range from around 3 volts to 15. (ohms per volt is approx 200)
Although not essential, without the offset adjustment trimmer R8, the voltages will track to within a tenth of a volt or so. Inclusion and set-up of R8 and R9 will allow tracking accuracy to within a few hundredths of a volt.
High stability 1% tolerance metal film resistors should be used for R2, R3, R4, R7, and R10. Other resistors are 5%.
Setting up is simple. Set R8 to it’s mid-position, then set the voltage adjustment pot so that +15.00 volts is obtained at the output of IC2. With a voltmeter on the output of IC3, adjust R8 so that the obtained voltage is the exact mirror of that obtained at the output of IC2, i.e. -15.00 volts. Tracking should be maintained within a few hundredths of a volts throughout the range 3.3 – 15 volts.
During testing, the following was observed.
I discovered a very low-level parasitic oscillation at the output of IC1 on the 1st version of the PCB. Adding a 47pF capacitor (C12) across R7 cured this, and the Schematic and Board reflect this small addition.
With an ambient air temperature TAMB of around 19oC, the op-amp IC1 warms up slightly. With the circuit supplying +/- 15 volts I affixed a temperature probe and left things to stabilize for around 10 minutes, when the plastic case temperature of the op-amp measured a little over 28oC. As is usual with observations, the method of measurement has a quantum-like effect, and I attribute some of this temperature rise to the fact I had to tape the probe onto the op-amp case, affording it some insulation from free air.
I checked drop-out voltages by supplying the unit from a VARIAC. The results were as follows, and were what I expected:
Images of the PCB foil and component layouts are given below. The Downloads section has links to the Eagle project files.
A front panel
I used FrontDesigner to put together a front panel for the PSU. Sod’s Law states that if something can go wrong – it will, and because of the design, the voltage control is roughly anti-logarithmic. FrontDesigner only caters for linear and logarithmic scales, so I plumped for a linear scale (total rotation 300o) and annotated this with the resulting voltages for each major division.
A high-definition export of the panel design is available in the downloads section.
Update 16th May 2012
A while ago, I received a helpful comment from Harry Brunt regarding this post, which referred to an article describing a design about which Harry remarked I could ‘omit the need for a trimming resistor and possibly improve the tracking if you include the negative voltage regulator within the negative feedback loop of the op amp.’
Harry himself has experimented with this design, though sadly the link has gone the way of many others, and is no longer valid. Harry reported that the design worked, but that tracking was not as accurate as he would have liked, and has augmented the schematic, so that tracking within a few millivolts can now be achieved.
The design is an interesting alternative to mine, providing as it does, tracked dual supplies down to nearly zero volts, and I have built the circuit and can verify as to Harry’s reports on the excellent tracking. I have shown Harry’s schematic below. I substituted an LF351 for the LM108 op-amp Harry suggested, as the latter isn’t stocked at my local electronics store. I matched the two 10K 1% resistors to within 10 ohms of each other, selected from a batch of 10. Other resistor values are not critical, and as Harry suggested, offset trim resistor on the op-amp doesn’t appear necessary, and wasn’t tried.
A zip file containing PDF and jpg of the above schematic is here: http://joebrown.org.uk/images/DualPSU/fwpsu.zip
End of update
The Eagle schematic for the Dual Tracking PSU is here: http://joebrown.org.uk/images/DualPSU/SplitSupply_Tracking.sch
and the printed circuit board layout here: http://.joebrown.org.uk/images/DualPSU/SplitSupply_Tracking.brd
The FrontDesigner project file is here: http://joebrown.org.uk/images/DualPSU/DualTrackingVariablePSU.FPL
and a high definition exported image file is here: http://joebrown.org.uk/images/DualPSU/DualTrackingVariablePSU_FPL.JPG