Need for testing short something draw 5,10 or 15 amp

[ben7]

Re: Need for testing short something draw 5,10 or 15 amp

I suggest that you get some high power resistors, and use them to limit the maximum current. Much fewer problems than with non-linear loads like LEDs and light bulbs. Also LEDs are kind of inefficient for this purpose. They are intended to be efficient, which is not what you want for your current limiter. You need something that will take more power, unless you just want a hell of a lot of LEDs sitting around...

People (I think some forum members on here have done so too) have made whole arrays of power resistors to load the output of power supplies, for performance testing purposes.

http://www.ebay.com/itm/Lot-of-10-Da...item462b68a01f
All 10 of those in parallel will give you 9.6 amps of current draw, if you have 12V across them.
FYI, they will get hot when you put that kind of power through them! (About 115W)

There are other things to do with the actual circuit you are testing, so this test may not indicate what part is faulty. For example, say a chip that is powered off of 5V has gone short. Also, say that the circuit board has a 12V to 5V regulator on it. That regulator would be overloaded by the short circuit on it's output, and it would get quite hot. But the shorted chip may not get as hot, or maybe not even warm!!! This leads you to a false perception of the regulator being the culprit, when the chip it is powering is the REAL problem.

[drussell]

Re: Need for testing short something draw 5,10 or 15 amp

I believe what the OP is trying to build is a 'Dim Bulb' tester, but a 12v one instead of the usual 120v that you would use to test-power up an old radio or TV or something.

He needs some sort of incandescent light bulb, not just a resistor...
You want to take advantage of the fact that the filament has a very low resistance when cold but will heat up if there is excessive current draw or a short in the powered load.

When I need a current limiter 'Dim Bulb' for 12V, I reach for a spare old-style automotive headlight... You can use the low beam, the high beam, or both in parallel depending on the wattage you want to limit yourself to.

[budm]

Re: Need for testing short something draw 5,10 or 15 amp

He is trying locate the shorts on the board, the board already has shorted component that he is trying to locate by looking for which component will heat up so it does not matter if he uses the lamp or resistor, just have to limit enough current just enough to heat the shorted component. Bulb is already mention on my post 11.

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

He needs some sort of incandescent light bulb, not just a resistor...
You want to take advantage of the fact that the filament has a very low resistance when cold but will heat up if there is excessive current draw or a short in the powered load.

He's not trying to check if there is a short, he knows there is a short, he's trying to smoke the shorted component out, or at least, heat it up to identify it.

An incandescent light bulb has two problems, as already discussed:

1) That really low resistance at switch-on will likely trip the PSU short circuit protection (because it's not a current limited lab PSU), and
2) The low resistance at turn-on means a big inrush current which may burn up the shorted part and damage the PCB.

[budm]

Re: Need for testing short something draw 5,10 or 15 amp

+1 Agent.

[drussell]

Re: Need for testing short something draw 5,10 or 15 amp

My bad... I thought I had read the whole thread but I missed two pages.. He does indeed need some sort of resistor.
Carry on...

[ben7]

Re: Need for testing short something draw 5,10 or 15 amp

My bad... I thought I had read the whole thread but I missed two pages.. He does indeed need some sort of resistor.
Carry on...

Hey, don't worry, I accept your apology