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4hv.org :: Forums :: General Science and Electronics
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Measuring very low resistances

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EDY19
Fri Feb 17 2006, 09:57PM
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
 		With the inductance also figured in, your SCR won't even see that 18kA that the resistance figure gave you.
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Eric
Fri Feb 17 2006, 10:37PM
Eric Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116
 		EDY19 has a point. You've got to consider the whole circuit, all the resistances (the cap's effective resistance too, which is probably greater than the coil's resistance) and the coil's inductance. Figure all that out and then simulate it.

Link2
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Bjørn
Fri Feb 17 2006, 10:41PM
Bjørn Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
 		If anyone tries this then remember to think about safety. The low voltages involved often makes people think it is completely safe. Depending on the coil there might be enough energy released when you disconnect it to hurt both you and your instruments.
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Ben
Fri Feb 17 2006, 10:48PM
Ben Vigilatny
Registered Member #17 Joined: Thu Feb 02 2006, 02:47PM
Location: NL
Posts: 158
 		Another way to measure small resistances would be to use a wheatstone bridge .
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FastMHz
Fri Feb 17 2006, 11:22PM
FastMHz Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287
 		Thanks for all the responses and suggestions. I knew that the amperage from this test would be higher than real due to other resistances, which is what I wanted. I wanted to make sure I wasn't in danger of destroying my SCRs.
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Part Scavenger
Sat Feb 18 2006, 01:21AM
Part Scavenger Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
 		I've got a schematic for a milliohm adaptor if anyone wants to see it. It's somewhat complicated IIRC, but I've not taken a good look at it for years; it might not be as I remember. I can post it if anyone is interested?
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FastMHz
Sat Feb 18 2006, 04:06AM
FastMHz Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287
 		If it' not too much trouble I'd like to see it...I've had other instances where small resistance measurement would be useful.
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Bored Chemist
Sat Feb 18 2006, 10:40AM
Bored Chemist Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
 		I'd also be interested- I just got a platinum resistance thermometer with a resistance of about 3 ohms.
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kell
Sat Feb 18 2006, 04:59PM
kell Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
 		Do like I did, get a lm317 and make a 1 amp constant current supply.

Hook your voltmeter leads to the coil, put the amp through it, and voila.
battery
power
|
|     ______
|  in|      |out
'----|  317 |------,
     |______|      |
      adj|         /
         |         \
         |         /
         |         \
         |         /
         |         |
         |         |
         +---------'
         |
         |
       load
         |
         |
       ground


The load will have a constant current throught it because the 317
keeps a fixed voltage across the resistor (a little over a volt).
You will have to fiddle with the resistor.
I used nichrome wire so I could tap it at just the right spot.
This is very stable once you get it dialed in.
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Part Scavenger
Sun Feb 19 2006, 02:59AM
Part Scavenger Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
 		Here it is. Resolution supposedly .00001Ohms depending on multimeter. Sorry for the quality, but it is readable except for R4 which is 499ohms. Dial-up stinks. I'm kinda amazed I got it this clean and into 58Kb. You might have to download and zoom in, but it's all there.

Enjoy!


Included in this article is a megaohm adaptor if anyone's interested.
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