# Need safe power supplies for beginner class with indicator light [duplicate]

In an intro electronics class in high schools, we use 6v battery packs as a way of delivering safe power for experimenting. However, the logistics of constantly getting fresh batteries, and our distaste for using so many batteries is leading to a search for an alternative.

We would like an inexpensive power supply that delivers small amounts of current (Ideally both 5V @ .2A, 12V @ .2A for example). Building such a supply as a project would be possible for a class of the more advanced students. So the question is to find a design. However, it would have to be protected from short circuits, and ideally would indicate to the student (with an LED?) when they have made a mistake and short circuited.

I found one design for a DC circuit, though this one requires DC voltage in:

Obviously I could take the above design, add a transformer and full-wave rectifier, but that's both big and clunky as well as possibly not optimal.

In an earlier question, someone suggested a 5V wall wart. student safe power supplyI can find inexpensive 5V wall warts, but would ideally like 12V as well, and have no idea if a given wall wart is short-circuit protected and even if they are, I want an indicator SHOWING when there is a short-circuit. I would like to take one and build in short circuit protection with an LED or alarm showing when a short happens, as well as a power indicator showing when it is on.

Can anyone suggest a design that we can build or buy? Since we are talking dozens of units per class, the cost has to be kept low.

If you wanted to roll your own you could do it with a transformer, a full-wave bridge rectifier, a reservoir capacitor, a 7812 and a 7805, like this:

The regulators are self-protecting against overloads, so LEDs showing overcurrent would only be for the benefit of the students, but could certainly be added if desired.

Assuming a 12 volt RMS output for T1, C1 would charge up to about 17 volts, DC, which would put 5 volts across the 7812.

Then, a 200 mA load on the 12 volt line and a 200 mA load on the 5 volt line would mean the current through the 7812 would be 400 mA and it'd be dissipating: $\text{0.4A}\times\text{5V} = \text{ 2 watts,}$ which means you'd need a heat sink on it to keep it from going into thermal overload. With 12 volts on the input of the 7805 and a 200 mA load on it, it'd be dissipating 1.4 watts, so it would need a heat sink as well.

Update:

Per your your comment requesting one transformer and four regulators per table, here's a way to go about it:

I've only shown two regulator stages, for convenience, but any number can be implemented by sizing the transformer properly.

• An excellent design. Transformer T1 isolates the line from the circuit, protecting students while experimenting. Dec 27, 2015 at 16:54
• Could I have one box on a desk with the transformer, and have multiple 7805's in small boxes hanging off it? That would keep the weight and cost way down.
– Dov
Dec 27, 2015 at 19:13
• @Dov: How many regulator boxes per desk? Dec 27, 2015 at 19:46
• @Dov: Indeed you could, and that's a good idea. Probably the neatest way to do it would be to daisy-chain the AC through the boxes in order to keep from having a lot of cables (one for each box) coming from the transformer. I'll edit my answer sometime tomorrow to show that. – EM Fields 19 mins ago Dec 27, 2015 at 19:58
• @Optionparty: Thanks for the kind words, but an upvote would be nicer. Dec 27, 2015 at 20:28

I wouldn't recommend building a power supply for others to use, that starts with the mains, unless you are going to have I designed and qualified by correctly certified people. There are too many risks and liability issues. For one PSU for your own use, fine. I have one exactly as described above, use it often.

For example: what if a live wire comes loose and touches the case? You might discover after an accident that you violated ten rules about soldering, earthing, crimping of mains terminals, fuses required, professional certification to sign off on the design, FCC/CE/NEC compliance, etc.

Rather buy something that gets you down to below 20 V, and build from there.

You could buy a qualified 12 V AC wall wart and build the regulator/current limiter.

Or buy a 15 V, 20 A supply, and have a 15 V DC bus from which to hang all the little regulators.

Designing something safe and reliable, to be reproduced, will need a lot of work, on more than the circuit diagram.

• point taken, so see my comment on the answer.
– Dov
Dec 30, 2015 at 3:31

Any of the Android style PSUs should suffice. Cut the connector off and fit your own with an LED and resistor to indicate "Power OK". (Not quite the 'short circuit' indication you asked for, but much simpler to implement.)

Note that some Android style chargers send out fixed voltages on the data lines which control charging functions on the phones, etc. These should be left unconnected.

The big advantage with purchase of ready-made units is the reduced liability for you in the event of a malfunction caused by isolation failure and shock of one of your students.

For the 12 V units you could boost from 5 V up to 12 V or get a second wall wart unit. If you're doing mixed analog / digital circuits this may cause problems if the 12 V is connected without power on the 5 V digital circuits so the buck converter may be a safer option in that they should both come on together.

• I assume you mean a wall wart powering a USB cable?
– Dov
Dec 27, 2015 at 11:19
• Yes, that would suit for the 5 V units and should have the current limiting built in. The 12 V units are more difficult as it can be very hard to know how good the circuitry is inside and, for example, what's going to happen with a long short-circuit. Dec 27, 2015 at 11:22
• Instead of just a "Power OK" LED, make a small box with fuse or current limiter or even an adjustable linear regulator (LM317), powered from the Android PSU or a 12V wall wart. The wall wart should be out of sight and out of the firing line, but accessible if the teacher needs to replace it despite everything.
– user16324
Dec 27, 2015 at 12:19
• I wouldn't mind a circuit breaker (although the enterprising juvenile delinquent could hold one down so probably not). But a fuse is definitely out. The whole point of this is to not have to deal with lots of disposables. If a fuse blows every time the student short circuits, that's worse than batteries!
– Dov
Dec 27, 2015 at 13:18
• I agree. Auto-reset would be much less trouble in class. (To put your mind at ease, circuit breakers will trip even if the switches are forced. The switch should only 'arm' the switch. The trip level won't be very well defined, however.) Dec 27, 2015 at 14:22