Powering Test Circuits

by Bob Carolan

Recently I've been building a lot of test circuits for checking other circuits that I tend to use in a mechanical testing project. Most of the test circuits have come from EPE and Babani books. They are all battery driven. So, after building my first couple of test circuits I decided that I would build them, in the same box and use the same battery using a setup for the power supply from the single battery akin to the mains wiring in the home ie. basically each test circuit is wired in parrallel to the battery and each test circuit has a switch in it. The details of the circuits are not neccessary here, only the power supply connections need to be considered.

The test results are given by a piezo-buzzer (the circuits were continuity testers and an op-amp tester). The circuit outputs were again all wired in parrallel to the buzzer without switches. A schematic diagram of the power supply and the output to the buzzer is shown in Figure 1.

C1 and C2 are continuity testers (refs 1 and 2), C3 is stripboard version of the handything (ref 3). All requiring 9V. S+ is 9V S- is 0V supplied from an LM317 regulated circuit using a simple model transformer. The circuit is as given in ref 4. S1-3 are any kind of single pole switch. I actually used unlatchable push switches since the circuit overall was designed to use a single battery and not a main psu. The outputs of each circuit go directly to the piezo-buzzer.

It was possible to do this:

I then decided that if I could use only one battery, I could equally well use a stabilized power supply instead. Since I had quite a lot of 79'' series and 78'' series regulators as well as some LM317 variable regulators.

So I decided there and then to build a separate mains PSU using these. In the case of the variable regulators I got some 5k pots from J&N factors (4 for 1) to adjust the supplied voltage in the standard way for these regulators, and I also obtained the necessary transformers from them, usually I get a mains primary rating close to the required supply voltage and for all these projects an equivalent current rating of about 500mA, a bit high, but you won't get cheap transformers from J&N at lower ratings. One then needs only the necessary rectifying diodes (1N007's) and smoothing capacitors (electrolytic and in some cases ceramic) which I get from my 'local' shop in London since I can get them quickly and won't save much buying them elsewhere.

Since this initial project I have built regulator testers, FET, MOSFET and more op-amp testers (single supply, dual supply and dual op-amp testers) and these require different supply voltages. Again they were all low current circuits, unaffected by noise.

Due to the different required voltages I could not use the method above. Instead I built all the circuits into a single large box leading the power supply connections out to 4mm plugs in the box lid. The relevant power supply module can then be connected by jump leads into the plugs. For the testing component sockets I use 8 pin dils, set in the lid of the box and wired to the test circuit. DIL sockets are cheap and almost anything can be fitted into them; transistors, FETs, regulators, and of course the op-amps themselves Figures 2 & 3 show the schematic for this set up and how the set up is fitted into the box. Needless to say I get all my boxes from J&N as well.

M1 has an external transformer (L,E,N connections). For use with the regulator testers the transformer connections go straight to the regulator circuit excepting the E connection. The regulator tester circuits R1 and R2 have their own rectification and smoothing sections one half of the 8 pin dil used for regulator testing is used for 78 series (3pins) and the other half for the 79 series (3 pins). For the single supply op amp testers (op1 op2) the rectified and smoothed output of M1 is used. This is an internal LM317 circuit (ref 4). As we shall see below the output of this psu is lead out to two 4mm plugs so that this supply may be used either with this box or for circuits eleswhere. Lastly op3 is dual supply op-amp requiring +,0 and - supply rails for which an external dual supply based around a centre tapped transformer, rectification, smoothing and 78,79 regulation is used. Also note that the single supply dual op amp has two +ve outputs; one for the first stage op amp and another for the second. As for figure 1 all ouputs can be connected up to a piezo buzzer. The regulator testers are just standard 78,79 psu circuits (ref5) whose voltage output can be checked. The op-amp testers are based on ref 6.

For connecting up the circuits of figure 2 wire pairs fitted with 4mm plugs are used to connect the needed supply to required tester circuit. The s+ s- sockets of the LM317 supply may be used for external circuits when required.

What I have described above is a simple, electrical more than electronic idea but it can save a lot of money and construction time. Either fewer batteries or PSU's are needed. I prefer to use the PSU method since it does not need periodic renewal, and shopping around can enable you to build them for prices not much different from buying the batteries. Also you can have no doubt that the power supply is running low causing problems with the circuit. Maybe others have done this kind of work before.