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What is / how can I get a student-safe power supply for electronics projects?

I'm looking for a student-safe power supply for solder less breadboard. I'm developing a course to teach electrical engineering to grade school students as an after-school program. I'm wrapping up the first course and I've learned a lot, including that I need to find a safer power supply. We're mostly using 9V batteries and connecting them to the breadboard via alligator clips. After lots of searching, I'm realizing that I need to turn towards the expert community for guidance. I hope that with the rise of the Makers this question will become increasingingly relevant with time as more kids get into electronics.

Inextricably linked to this question is what is the minimum power source to use. I've liked using 9V batteries since we can connect to them using alligator clips; but I worry that they are just a little too powerful. I can imagine them somehow starting a fire with them.

My #1 priority is to keep the students safe. Kids just have no concept that they can cause damage to themselves and property. When I ask "where is that smoke coming from?" I'll get a gleeful response from the student that is seeing how many batteries he could hook together and short out. I really need to limit the damage they can do. I mainly don't want anyone electrocuted or anything to catch fire. Kids will routinely attach LEDs backwards, leave them like that and then act surprised when they explode.

After safety, I want to protect the equipment. Kids will hook up a LED backwards all the time.

After protection, I want to give the kids feedback as to what is going on. A light that indicates power is on would be awesome. A light or siren or circuit break that indicated a short would be awesome too.

Battery powered supplies are slightly preferred, just to keep upfront costs low and to avoid having to run outlets to each table.

Boundaries: We are only using simple LEDs, BJT transistors, resistors and maybe a timer IC.

Similiar Questions: How to protect an electronic construction kit (a toy) from damages?

References: Course details are here: http://www.ascendly.com/classes/iee

We currently sometimes use this power supply from the Squishy Circuits Store and/or a 9V battery with alligator clips.

Epilogue

I found it tougher than expected to find a suitable wall wart, so I'm listing what I finally found for future reference. I'm about to test the following combination:

Epilogue 2

A year-plus in, I've decided to drop the in-line power switch since it wasn't obvious when it was 'on' vs. 'off'. I'm instead relying upon the students to unplug the rat-tail from the supply, which is quick and easy. Also, since I'm primarily using this for a squishy-dough project, I'm experimenting with with using two button batteries instead of a wall power.

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    \$\begingroup\$ +1 for running a program where kinds can hook things backwards and blow them up. Would you consider adjustable benchtop power supplies? They have adjustable voltage, adjustable current limit. They display voltage and current. They cost more than wall warts, though. \$\endgroup\$ – Nick Alexeev Jan 30 '13 at 19:22
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    \$\begingroup\$ @insta: Sortof. You could say the same thing with half the words. \$\endgroup\$ – Olin Lathrop Jan 30 '13 at 19:22
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    \$\begingroup\$ @LeonHeller This one is not a malignant shopping question. \$\endgroup\$ – Nick Alexeev Jan 30 '13 at 19:26
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    \$\begingroup\$ @Leon: I don't see this as a shopping question either. \$\endgroup\$ – Olin Lathrop Jan 30 '13 at 19:34
  • \$\begingroup\$ Per wall warts: I'm not opposed to them, per se, but the related extension cords just add to a tripping hazard and some extra setup. So, not a big deal either way. \$\endgroup\$ – JJ Rohrer Jan 30 '13 at 19:43
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I know you said you don't want to run outlets everywhere, but I think a pile of wall warts is exactly what you want. Jameco has a large selection. 9 V sounds pretty good and that is one of the standard voltages, but you might want to consider 5 V (see below). Get something in the 500 mA to 1 A range.

These supplies are inherently isolated from the line, are usually short-circuit protected (check to make sure, you definitely want that), and draw so little AC power that you can string a bunch of outlet strips together without harm.

In the end, I think this will cost less and provide a better experience for the kids. I remember when I was a kid tinkering with this stuff how frustrating it was to have batteries run down, especially when you're not aware of when you are asking a lot from them. You also feel a lot less guilty abusing a power supply than running down consumable batteries.

You can start a fire with almost anything. If you do just the right thing, even a 9 V 500 mA supply can catch something on fire, but no more so than a 9 V battery and without the chance of the battery itself doing something bad and causing chemical burns.

If you are worried about LEDs getting damaged by them getting hooked up backwards, maybe you should get 5 V or 6 V supplies. Most LEDs can handle 5 V in reverse, and some 6 V. With 5 V supplies, you can eventually run logic cicuits directly without having to use a linear regulator. Lots of stuff will run well from 5 V. If you get 5 V supplies, get at least 1 A capability. That will be useful for running small motors. 5 W total power really isn't all that much.

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  • \$\begingroup\$ Thanks for taking the time to write a well-thought-out answer. \$\endgroup\$ – JJ Rohrer Jan 30 '13 at 20:52
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    \$\begingroup\$ One thing to add: DC electricity is very safe in small amounts. A wall wart would be safer because you could have a power strip and an extension cord so you could turn the power off remotely. Also, teach them basic safety. If they go out on their own and try it, they can get hurt. Label which is the positive and negative. If you're really worried about fire, have a mini fire extinguisher. I doubt with such low voltage that you could start a fire not trying to. Have policies that if they break something, they have to pay for it. \$\endgroup\$ – Anonymous Penguin Jan 30 '13 at 21:37
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    \$\begingroup\$ Continued: Only give them enough they need, they shouldn't have enough batteries to start a fire, unless other groups let them barrow or they steal. I have safely used 18 volts and with 5V wall warts they would need 4 to even match that. If you don't trust them to do this on their own safely, you shouldn't teach them how in the first place. Good luck and teach them as much as you can about safety. \$\endgroup\$ – Anonymous Penguin Jan 30 '13 at 21:40
  • \$\begingroup\$ "Have policies that if they break something, they have to pay for it." Their parent(s) have to pay for it. I would have a parental permission sheet to take the course and to spell out what exactly is the behavior the student will be required to demonstrate along with the above 'break it/pay for it' sentence. Not an unreasonable requirement for a grade school student in an after school program. Also Thank You for taking the time to do this. \$\endgroup\$ – rdivilbiss Feb 1 '13 at 18:41

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