| Gallery, Projects and General > How do I?? |
| Measuring DC current >10a? |
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| Bluechip:
Or .... http://cpc.farnell.com/multicomp/sd-60-0-30a/meter-moving-coil-30a/dp/PM11439 I have bought meters from here and they have the appropriate shunt fitted. Well, the 15A one does anyway. See no reason why this should be different. This is the one I have: http://cpc.farnell.com/multicomp/sd-60-0-15a/meter-moving-coil-15a/dp/PM11438 Dave |
| PekkaNF:
I like clamp meters when I'm happy with approximate value and when I don't want to remove the wire (Cars, mains). But using them takes a little practice. Shunts are simple in theory, but need careful consideration as well. Shunts are easier to use if you can use relatively high ohmic value. If you use smaller ohmic value, where and how you measure voltage comes into consideration. There are losses on connections/wires etc. Kelvin-type resistors are easier to use if any accuracy is needed: http://www.farnell.com/datasheets/575641.pdf Most of the time we don't need to go this far, but it helps to know where inaccuracy comes in. Pekka |
| lordedmond:
Pekka At the iron works we used to measure DC 220 at 2000 amps plus on many machines and rotary converters we always used a very low resistance shunt these were calibrated with a hacksaw by cutting into the side to reduce the CSA No problem at all As to the OP You need to do as described but you would need to calibrate the shunt with a known load could be a couple of car headlamp lamps and a 12v battery |
| DavidA:
A while back I had the same problem and got around it by using a length of 1 mm copper wire (wound into a coil) and then an op-amp driving a 50 Micro Amp panel meter. The very high gain of the op amp used the small voltage drop on the coil to indicate 20 Amp on the meter. The wire , being copper, generated very little heat. I calibrated it by placing a DVM (on 10 AMP range) in series and turning up the current until I had 5 Amp on the DVM. Then moved the input across the coil until I got 25% of fsd on the 50 Micro Amp meter. That equates to 20 Amp fsd . I then removed the DVM. This was a rather bulky affair, but it worked until I could afford to buy a 50 Amp meter. Dave. |
| drmico60:
The problem with pure metals, such as copper, is that they have a high temperature coefficient which can effect accuracy. It is better to use a resistance wire such as constantan (so called because it has only a small temperature coefficient). For a given AMP range the shunt resistance is the same regardless of the shunt material so the heat generated is the same. Mike |
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