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Question about AC ripple

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Alez

New Member
New to the forums and hoping this is the right place to discuss this.

Here’s the scenario... we have a dynamometer with a DC output voltage that’s adjustable, anywhere from 90 volts to 400 volts. This comes off of a full wave rectifier at 120 hz. At our 90 volts DC, we are still seeing 45ish volts AC, and this number rises as the DC output is increased.

There’s a lollipop style capacitor after the rectifier, I believe the rating was 1-2 Nano farads. It measures ok on a multi meter, but obviously isn’t filtering the circuit as it should.

We hooked up the circuit to an oscilloscope and it appears to be getting rectified as it should. Any suggestions on what might be happening? Any suggestions on how to properly size a capacitor for this scenario?

Thank you for any insight provided.
 

ronsimpson

Well-Known Member
Most Helpful Member
We need to know what the load current is. Or what the load is. The capacitor is very small.
 

MrAl

Well-Known Member
Most Helpful Member
New to the forums and hoping this is the right place to discuss this.

Here’s the scenario... we have a dynamometer with a DC output voltage that’s adjustable, anywhere from 90 volts to 400 volts. This comes off of a full wave rectifier at 120 hz. At our 90 volts DC, we are still seeing 45ish volts AC, and this number rises as the DC output is increased.

There’s a lollipop style capacitor after the rectifier, I believe the rating was 1-2 Nano farads. It measures ok on a multi meter, but obviously isn’t filtering the circuit as it should.

We hooked up the circuit to an oscilloscope and it appears to be getting rectified as it should. Any suggestions on what might be happening? Any suggestions on how to properly size a capacitor for this scenario?

Thank you for any insight provided.
Hi,

As Ron said, we need more information.
The AC ripple depends on several factors, and knowing the operating conditions helps to figure out what the best way to reduce ripple is. It also helps to know the application.

For example, sometimes just increasing the cap value helps, but that has other side effects too.
 

AnalogKid

Well-Known Member
Most Helpful Member
A capacitor that small probably is functioning as a noise filter, not a bulk filter to reduce ripple.

Is this a new dynamometer, or have you had it for a while? If the latter, hasn't the ripple always been there? Why is it an issue now?

ak
 

DerStrom8

Super Moderator
A capacitor of that value sounds more like a Class X/Class Y capacitor, which is used for EMI filtering. It will be utterly useless to filter 90+ volts off of a rectifier. You need a large bulk capacitance (usually in the realm of thousands or tens of thousands of microfarads - not nanofarads - about 1-10 million times larger) to filter out the ripple.
 

unclejed613

Well-Known Member
Most Helpful Member
A capacitor that small probably is functioning as a noise filter, not a bulk filter to reduce ripple.

Is this a new dynamometer, or have you had it for a while? If the latter, hasn't the ripple always been there? Why is it an issue now?

ak
AK is right, if the dynamometer has always had this as an output, then it's meter is calibrated for that input waveform. if you filter out the ripple from that waveform, it affects the meter reading, and the machine is no longer accurate.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
The first post doesn't make a lot of sense to me as it's stated, especially the adjustability part.

Dynamo or Dynamometer.
See: https://en.wikipedia.org/wiki/Dynamo and https://en.wikipedia.org/wiki/Dynamometer

Wikipedia suggests that there may be confusion.

An output of 1/2 the value may suggest other things going on like it has two windings out of phase and one is broken or a bad diode assembly It would be nice to know the shape of the ripple.
 
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