How to stop your inventions from blowing up…

by Stephen Hobley on November 26, 2010

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The first test you perform when switching something on is the “smoke test” – literally you are seeing if anything produces (magic) smoke when you apply power. This is the most costly moment in any project as a simple solder bridge can cause those $10 MOSFETs to fry and set you back a week in ordering replacement parts.

One useful thing to do is limit the current (as you should know it’s the amps that’ll get you, not the volts) when you first switch on. If you don’t have access to a current limiting bench supply the easiest way to do this is to put a light bulb in series with the device under test.

I do this a lot, and so the last time I went to the hardware store I collected together about $11 in parts to make a device that would help – a socket with a switch (a switch always beats yanking out the power cable when something starts to go wrong), a lamp holder, a case and some flex with a plug.

All you do is wire up the socket with the bulb in series with the live wire. Now the circuit will be current limited by the power rating of the bulb.

Not super pretty, but cheap and very functional...

ie. A 25W bulb will only let 25/120 = 0.208 amps pass through the circuit – anything more and the bulb will burn out. A 40W bulb will allow 0.333 amps, and so on.

So start with a low value and work up. You will know when the circuit is drawing all the current it needs because the lamp will no longer glow. When a surge occurs the lamp will glow brighter.

Note: Light bulbs are not linear devices – their resistance increases as they get “hot” – so when this circuit is first switched on their will be a tiny inrush of maximum current – so it’s not an infallible system, but it’s sure better than having no limiting at all.


{ 6 comments… read them below or add one }

Adam Casey December 3, 2010 at 10:22 pm

If the lightbulb is wired in series whatever is plugged in will not recieve 110 volts. This may not matter for most items. But a very sensitive instrument may be effected by lesser voltage.

Stephen Hobley December 3, 2010 at 10:47 pm

Astutely observed – and quite correct.

I use a variable transformer that lets me go beyond 110V and adjust the load voltage till I get 110V across whatever is being tested.

But for most things this does not matter.

David December 4, 2010 at 8:25 am

This technique is not new and has been used for years by radio amateurs and restorers when testing old equipment and for rejuvenating power supply capacitors which have not been used for some time. Failure to allow the capacitors to reform can result in a large bang and a hunt for replacements.

Nate February 22, 2011 at 12:11 am

Lightbulbs also make ideal current-limiting resistors for charging and discharging capacitors or banks thereof. I recommend this to car-audio freaks all the time: Using an SPDT switch with center-off, you can easily mount a bulb onto the same backboard as the cap and amp, and easily connect the cap to battery positive to charge it before inserting the fuse, or to ground to discharge it after removing the fuse. In either case, the light will dim as the capacitor comes to equilibrium.

electro luminescent March 3, 2011 at 12:04 pm

Wow. That is genius. I had no idea you could do that. Thank you for the info. I am pretty used to seeing magic smoke with my inventions!

Profpep March 30, 2011 at 8:07 am

A classic trick – I was taught it back in the 70’s when I had a college part time job at a TV repair shop. It could save you a fortune in expensive power transistors when fixing line out put stages and power supplies. Still use it nowadays, when fixing switch mode units. To help tame nasty in-rush currents, a couple of different NTC thermistors can be used; you can often salvage some from old PC power supplies, and you can parallel then up for higher currents.

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