RMS current.. 2 + 2 = 3?

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,560
Someone please catch my mistake. I'm trying to measure RMS ripple current at a capacitor that is filtering noise on a DC supply line. The AC component looks like a sawtooth wave with somewhat random period and amplitude. There is a series fuse to the capacitor that measures 0.2 ohm. My true RMS fluke 87V multimeter measures ~50mV RMS AC across the fuse. The math says I should have about 250mA ripple current. But this is not true because an automotive style 2A fuse blows in less than one minute, so I know the ripple is something more than 2A. Is the error in my method or in my measurements? I'm trying to resist lugging my scope out to where the circuit is, but maybe the multimeter isn't giving me accurate RMS AC voltage?
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,560
Yes the fuse is in series with the capacitor and nothing else, mainly for short protection. The frequency varies somewhat randomly. This is output from a DC generator that uses a shut type regulator, so the spikes and dips happen at whatever times the regulator feels the need to shunt the various coils. Here's a scope shot at circuit startup, before the fuse was added, so it's not really useful beyond showing the shape of the noise.

I haven't measured DC across the fuse as it's dedicated to the capacitor so there's no real measurable DC component, but I'll try that next time I'm out there just to see what the meter says.
 

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