Computing stuff tied to the physical world

Dual power supply

In Hardware on Jun 10, 2012 at 00:01

To generate a sine wave of ±10V for the Component Tester project, I’m going to need a suitable power supply.

The first option would be to take a dual-windings 12 VAC transformer, add a bridge rectifier, two beefy electrolytic capacitors, and violá: ±12V, right?

Not so fast… this is called an unregulated supply. It has a couple of drawbacks: 1) the voltages will actually be considerably larger than ±12V, 2) the voltages will change depending on the current drawn, and 3) the voltages can have a lot of residual ripple voltage. Let’s go through each of these:

  1. Voltage levels – a 12 VAC transformer generates a 50 Hz alternating current (60 Hz in the US) with an RMS voltage of about 12 VAC. For a sine wave, this corresponds to a peak voltage which is 1.414 times as high, i.e. 17 Volts peak to peak. With a bridge rectifier, you end up topping each of the two caps to 17V DC.
  2. Regulation – or rather: lack thereof. Since the input is a sine wave which only peaks at 17V, the caps will be charged up to this value only a couple of dozen times per second. In between, current drawn will simply discharge them, causing the voltage to drop. Large current = much lower voltage.
  3. Ripple voltage – this variation on the power supply is called ripple. It’ll be either the same frequency of AC mains, or double that value – depending on the rectification circuit used. So that’s a 50..120 Hz signal on top of what was supposed to be a fixed supply voltage (that’s why bad audio amplifiers can “hum”).

There’s a very simple solution to all these issues: add 2 linear regulators to generate a far more stable supply voltage (one for the positive and one for the negative supply). The most widely used regulator chips are the 78xx series (+) and the 79xx series (-). You give them a few more Volts than what they are designed to deliver, add a few caps for electrical stability, and that’s it. In this case, we need one 7812 and one 7912 to get ±12V.

But I’m not so fond of power line transformers in my circuits, because you have to hook them up to AC mains on one side – that’s 230 VAC, needing lots of care to prevent accidents. Besides, we only need a few dozen milliamps for this Component Tester anyway.

So instead, I decided to use a DC-to-DC converter – a nifty little device which takes DC in and transforms it to another level of DC. The nice thing is that there are “dual” variants which can automate both positive and negative voltages at the same time.

I picked the Traco TMA0515D, which generates up to 30 mA @ ±15V, using just 5V as input. Its efficiency is specified as about 80%, so the 900 mW it supplies will need about 1.125W of input power. At 5V, that translates to 225 mA, well within range of a USB port – how convenient!

Here is the circuit I’ve built up:

JC s Grid page 20

As you can see, it uses very few components. And the output is galvanically isolated from the input supply – nice!

Such DC-DC converters are surprisingly small, at least for low-power units like this one (black block on the left):

DSC 3304

With a bit of forethought, almost everything can be connected together with its own wires:

DSC 3305

It worked as expected (caveat: the 78xx and 79xx pinouts are different!), but there were two small surprises:

  • the unloaded DC-DC converter output was about ±25V, these units are clearly not internally regulated!
  • the outputs from this assembled unit are indeed + and – 12V, but with some residual switching noise:

SCR13

That DC-DC converter appears to be based on a 100 KHz switching regulator (5 µs between on and off transitions), and these spikes are making it all the way to the output pins, straight through those linear regulators!

It probably won’t matter for a component tester operating at 50..1000 Hz, but this too should be fairly easy to fix – by inserting a couple of ferrite beads for example: small inductors which filter out such high frequency “spikes”.

With analog circuitry, stable and smooth power supplies tend to be a lot more important!