# DC and AC ripple question



## Big_Steve (Feb 21, 2016)

I have an Athern DC power supply. It is spitting out around seven volts AC with the throttle all the way up. I didn't test it when it was new, so I don't know what it started at. My Kato DC engines were running fine. My Spectrums suddenly don't want to run. They herk and jerk, if they do move it's slowly, and they stop also. I'm assuming I have a bad power supply. The Spectrums run a good bit better on my Kato power supply. But at full throttle the KATO is also putting out about 4 volts AC.

Before I started looking at the power supply, I cleaned the spectrums and made sure all the electrical points were good. No problem there.

So how much AC ripple on a DC controller is acceptable? And why would the Katos not care, but the Spectrums hate it? It's just a DC motor....

Scratching my head...

Thanks for any info...


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## gregc (Apr 25, 2015)

even 4VAC seems high.

assume you're measuring this with a meter (not a scope). what is the DC reading? 

I would say there is a problem if the DC value is less than the AC value, otherwise the motor is alternately being forced forward and reverse which can build up heat that damages the motor. 

i've seen designs for model RR power supplies that intentionally use unfiltered half or full wave rectified power at low speeds that gradually become more DC at higher settings. The voltage should never go below zero and it should be pure DC at the highest setting.

an SCR based design might have a lot of AC at lower speed settings


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## fcwilt (Sep 27, 2013)

Hi,

How are you measuring this "AC"?

It could be the power pack uses pulsed DC.

Frederick


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## Big_Steve (Feb 21, 2016)

DC readings are normal. Kato is something like 14vdc max, the Athearn is slightly higher.



gregc said:


> even 4VAC seems high.
> 
> assume you're measuring this with a meter (not a scope). what is the DC reading?
> 
> ...


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## Big_Steve (Feb 21, 2016)

A normal Fluke digital multimeter. It is not a true RMS meter.



fcwilt said:


> Hi,
> 
> How are you measuring this "AC"?
> 
> ...


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## fcwilt (Sep 27, 2013)

Big_Steve said:


> A normal Fluke digital multimeter. It is not a true RMS meter.


Hi,

The only way to be sure of what the pack is putting out is with a 'scope.

Frederick


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## ncrc5315 (Jan 25, 2014)

I personally would like to see less than 2.5% AC ripple. I have a MRC power pack around here someplace, tomorrow night, I will try to set it up with the Oscope, and see what it looks like.


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## Big_Steve (Feb 21, 2016)

ncrc5315 said:


> I personally would like to see less than 2.5% AC ripple. I have a MRC power pack around here someplace, tomorrow night, I will try to set it up with the Oscope, and see what it looks like.


Thanks. If you happen to have a digital MM, if you could see what it says on AC, I'd appreciate that also. I don't have a scope, so I'm real curious as to what another digital meter says.


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## ncrc5315 (Jan 25, 2014)

This was kinda surprising. I hooked up my Fluke 87V, to a MRC 1300. ~17VDC full throttle, no load, the AC ripple was 8.95VAC. Under load, (1 loco) at ~12.5VDC, the AC ripple was 4.85VAC, above 12.5VDC, there is ripple, but the DMM can't read it. Now when I put the Scope on, will I have no idea as to what is going on. I will try and post some pictures of what I saw, but as far as the DMM goes, the ripple is surprisingly high.


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## Big_Steve (Feb 21, 2016)

Thanks for checking.





ncrc5315 said:


> This was kinda surprising. I hooked up my Fluke 87V, to a MRC 1300. ~17VDC full throttle, no load, the AC ripple was 8.95VAC. Under load, (1 loco) at ~12.5VDC, the AC ripple was 4.85VAC, above 12.5VDC, there is ripple, but the DMM can't read it. Now when I put the Scope on, will I have no idea as to what is going on. I will try and post some pictures of what I saw, but as far as the DMM goes, the ripple is surprisingly high.


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## fcwilt (Sep 27, 2013)

Hi,

To get an accurate picture (with your 'scope) you should first place a purely resistive load on the outputs of the power pack and then check the waveform.

Then you can replace the resistive load with an actual loco and observe any changes.

Frederick


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## ncrc5315 (Jan 25, 2014)

fcwilt said:


> Hi,
> 
> To get an accurate picture (with your 'scope) you should first place a purely resistive load on the outputs of the power pack and then check the waveform.
> 
> ...


That's what i was thinking, may stop on my way home tomorrow, and pickup an incandescent light bulb, I think you can still buy those.


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## J.C. (Dec 24, 2016)

Big_Steve said:


> I have an Athern DC power supply. It is spitting out around seven volts AC with the throttle all the way up. I didn't test it when it was new, so I don't know what it started at. My Kato DC engines were running fine. My Spectrums suddenly don't want to run. They herk and jerk, if they do move it's slowly, and they stop also. I'm assuming I have a bad power supply. The Spectrums run a good bit better on my Kato power supply. But at full throttle the KATO is also putting out about 4 volts AC.
> 
> Before I started looking at the power supply, I cleaned the spectrums and made sure all the electrical points were good. No problem there.
> 
> ...


what I would ask is what model is it how old is it ?


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