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Circuit tester

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den052

New Member
I am looking for a circuit tester that simulates an amperage load on the wire or circuit under test. For instance, vehicles that I work on sometimes get small resistance in their connections such as a headlamp circuit. I would like to run current through the wire and check for voltage drops or the inability to carry the rated current. Something like a box that I could vary the settings from 5 - say 30 amps through the questionable wire depending upon the guage of the wire.

For instance, I had a situation where a power wire from the battery was feeding the vehicle computer, and the vehicle would shut off. I checked for 12v at the computer feed and it was ok. I also checked resistance of the wire and it was within specs. BUT when I applied about 15 amps through the wire back to the battery with a battery charger, I was getting a 2.0v drop from the charger to the battery post. I subsequently found a bad connection between the battery and the computer B+ terminal because of the ability to run amperage through the wire and heat up the bad connection.

thanks, Dennis
 

Sceadwian

Banned
This is for automotive use, why not just use a bulb? I'm sure you wouldn't have to spend more than 15 minutes in an auto-parts store looking for different bulbs that will draw varying currents. The upside of this is it's incredibly simple and the bulb lightning up will give you human understandable feedback about what's going on in the circuit. Voltage dips will show up as a dim bulb, and you can even see transients. The bulbs are effectilty resistors so you can check the voltage drop across the bulb and the voltage across the rest of the circuit and determine where faults might exist.
 

Hero999

Banned
An LM317 constant current regulator with a bypass transistor is what you require.
 

den052

New Member
Well, 5 amps could be drawn using a headlight, however 10-30 amps would require a awful big bunch of headlights???
In my fore-mentioned problem, I would have liked to draw about 20 amps from the wire, since it was a 10 guage wire. Using ohms formula, 20 amps would be required to dissipate 200 watts. To make a useful box with a selector switch would require some massive power resistors to accomplish that???
I was thinking that some company makes a selectable "load box"?
 

den052

New Member
Thanks em2006, I understand the schematic, what is the resistor or carbon rods made from? (appliance or etc????)
 

den052

New Member
Thanks to Sam2 also. I thought about a carbon pile since I am quite familar with them having used them to troubleshoot batteries or alternators. My problem is they are usually big and bulky (capable of dumping 300+ amps). I was looking for something smaller from 5-60 amps?????
 

Sceadwian

Banned
A carbon pile capable of lower amperages is going to be smaller as a result.
 

em2006

Member
den052 said:
Thanks em2006, I understand the schematic, what is the resistor or carbon rods made from? (appliance or etc????)
The resistor from the picture (load.jpg) is made by me.
I made various resistors using a resistance wire from 2 to 5 mm diameter,
to test welding inverters wich I repare (need load from 5 to 250 Amp).
I use resistance heating wire, Nickel-Chromium Alloy, 80% Nickel/ 20% Chromium.

If you can't find desired wire diameter, you can use more paralelled small diameter wires.

You also can use a small coller to avoid overheating if undersized wires are used.

About material, you can find some informatiobs here:
http://www.omega.com/ppt/pptsc.asp?ref=NI80&Nav=temh03
http://www.electro-tech-online.com/custompdfs/2006/11/NI80.pdf
http://www.leybold-didactic.com/phk/themen.asp?B=PhKE&PT=4.5&L=2
http://www.jlcelectromet.com/alloys.htm
 
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