Need some simple help with a scope filter

Thread Starter

NashvilleFiero

Joined Jun 28, 2008
3
Hey Guys!
This is my first post.

I know this circuit is fairly simple, but I need some help. I need a circuit to put in line with my test leads so that I can filter out the DC signal from a car alternator. I can look at the waveform on my Snap-On scope.

Can anyone help?
Thanks!
Brit
 

SgtWookie

Joined Jul 17, 2007
22,210
All you really need is a small capacitor, perhaps 1uF. If you use an electrolytic, have the + side towards the alternator wiring. A poly cap would be much better. If your scope has extremely high input impedance, you may need a resistor to ground (say, 47k) on the output of the cap.

Capacitors block DC, but allow the effects of AC to pass right through.

See the attached for a simulation of a portion of an alternator, the bridge rectifier, outputs along the way, and the C1/R1 combination I've mentioned. Notice that the simulated O-scope trace from point (E), the green trace is virtually the same as the original output (D), the yellow trace, but is settling down to become centered around 0v; which you need to help diagnose problems with the rectifier bridge or the diode trio/exciter diode.
 

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Thread Starter

NashvilleFiero

Joined Jun 28, 2008
3
All you really need is a small capacitor, perhaps 1uF. If you use an electrolytic, have the + side towards the alternator wiring. A poly cap would be much better. If your scope has extremely high input impedance, you may need a resistor to ground (say, 47k) on the output of the cap.

Capacitors block DC, but allow the effects of AC to pass right through.

See the attached for a simulation of a portion of an alternator, the bridge rectifier, outputs along the way, and the C1/R1 combination I've mentioned. Notice that the simulated O-scope trace from point (E), the green trace is virtually the same as the original output (D), the yellow trace, but is settling down to become centered around 0v; which you need to help diagnose problems with the rectifier bridge or the diode trio/exciter diode.
Great info. Thanks!
 
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