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| ADC - Shunt resistor spikes |
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| nixie84:
Hello everyone I try to measure current on DC motor using shunt resistor - 5V Atmega ADC. I use 0.1 shunt resistor. On multimeter i have: 0,03V -> 0,3A - no load 0,1V -> 1A - on load 0,5V -> 5A - stall current Everything looks OK. But on a scope i see spikes with a higher voltage than i expected and multuimeter shows. First circuit: Circuit 1 Spikes are above 10V but only for couples of microseconds. Second circuit: Circuit 2 Spikes repeated less frequently. About 7,5V. The diodes should limit voltage to 0.6. Third circuit:Circuit 3 Like in second circuit. Why is this happening? Is this dangerous to 5V Atmega? If so how to prevent it? Thanks! |
| Kleinstein:
The mechanical contacts at the carbon brushes can cause current spikes, e.g. from some kind of make before brake action. The wire wound resistor used will have some parasitic inductance than can make current spikes look larger than they actually are. Some 7.5 or 10 V spikes could cause trouble to the AVR. Die Diodes are really effective only with some series resistance. Ideally there would also be some resistance in front of the ADC input to limit current here if the voltage goes out of range. Diodes parallel to a shunt are a known way to protect shunts, but it need strong diodes and may lead to some leakage currents that can be relevant for the smaller range. |
| nixie84:
Fourth circuit: Circuit 4 Something like that? DIY 0,1 Ohm bifilar resistor ;-) Spikes appear less frequently but are still at a high level. |
| ArthurWozniak:
Hello, Use some filters, as Snubber filters. They are good for smoothing those voltages spikes ;D Best regards, |
| nixie84:
I assembled the electronic circuit from the diagram. It works probably ok. The 2nd channel of the oscilloscope shows output. 1st channel show spikes on motor. What are the other ways to solve this problem? |
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