[zybar]

Guys,

I was recently told by a vendor that a small issue I am having with their gear could be caused by having DC on my dedicated 20 amp line.  How would I go about trying to validate this?

Is there a way to measure DC using simple tools (such as my volt meter)?

Thanks,

George

[jtwrace]

great question

[Niteshade]

What kind of problems are you having?  Did the person you spoke with suggest what could be injecting DC into a dedicated AC line? I would believe that if something were doing that, you would be having other problems throughout your house.

[zybar]

A transformer is being saturated and buzzing.

George

[Ethan Winer]

Is there a way to measure DC using simple tools (such as my volt meter)?

I'm not a power expert, but I can't see how DC could get into a power line unless the pole transformer is defective. I'd call the local power company and ask them to come check it out. They'll have the tools and knowledge to assess this properly.

--Ethan

[JoshK]

Measuring DC on your mains has been discussed a couple times before iirc, but bears repeating.  Probably the best way is to use an o-scope, which is something most audiophiles don't own.  I'll let a more experienced member comment on how to do that if one does have an o-scope.

There are some DMMs that can measure DC on AC, but they usually are the spendier DMMs like Flukes.  You'd have to resort to the manual for your DMM to know for sure whether yours could or not.  Probably not unless it is a Fluke or hi-end DMM.

The other thing is to cobble together a DC trap as Occam has highlighted a few times.  Try out the DC trap on the suspect piece of equipment and see if that changes anything.  If it does, you likely have your answer, if it doesn't you probably eliminated the DC question.

[Bob in St. Louis]

Would it be as simple as sticking DVOM probes in the outlet with the meter set on DCV?

Bob

EDIT: Josh beat me to it....But I'm still curious if I could use my Fluke 83 set on DCV.

[JoshK]

Is there a way to measure DC using simple tools (such as my volt meter)?

I'm not a power expert, but I can't see how DC could get into a power line unless the pole transformer is defective. I'd call the local power company and ask them to come check it out. They'll have the tools and knowledge to assess this properly.

--Ethan

Intuitively, you'd be right.  However, I think there could be reactive leakages that could cause DC.  Maybe, maybe not, I really don't know.

[JoshK]

Would it be as simple as sticking DVOM probes in the outlet with the meter set on DCV?

Bob

Most DMMs won't give a reliable reading this way from what I've been told.  Mine reads really erractically when doing this.

[Bob in St. Louis]

Josh, you beat be to the punch. i've edited my post since.
I'd be willing to try it out with my Fluke 83 DMM/DVOM as long as it wouldn't let the smoke out. 

Bob

[JoshK]

Josh, you beat be to the punch. i've edited my post since.
I'd be willing to try it out with my Fluke 83 DMM/DVOM as long as it wouldn't let the smoke out. 

Bob

I think you'd be fine trying.  You could check the manual first to be safe, but I've tried it on my much cheaper DMM with no bad effects.

[Bob in St. Louis]

.900VDC here.

[arthurs]

.900VDC here.

What's considered acceptable or normal? 

[Bob in St. Louis]

Not sure, but that's more than I expected to see.
George, how much do you have? (assuming you've got a DVOM).
How about everybody else?

Bob

[zybar]

Not sure, but that's more than I expected to see.
George, how much do you have? (assuming you've got a DVOM).
How about everybody else?

Bob

I don't know...which is why I was asking for a way to measure it. 

If I can measure it, I would tell the vendor what the number is and get their feedback.

George

[Niteshade]

What is the specific problem? Do you have an isolation transformer to try it with?

[TomS]

Is there a way to measure DC using simple tools (such as my volt meter)?

I'm not a power expert, but I can't see how DC could get into a power line unless the pole transformer is defective. I'd call the local power company and ask them to come check it out. They'll have the tools and knowledge to assess this properly.

--Ethan

DC shift on AC line is sometimes caused by asymmetry of the waveform.  That is, when positive cycle and negative cycle are skewed relative to each other such that the voltage over time no longer nets zero volts "average".  Perfectly symmetrical waveforms should yield zero volts when averaged.  If not transformers with big cores can get a bit upset and buzz for you.

That said, I'm not sure how you'd do this without a scope.  I'd be very careful poking a multimeter set to DC ranges since the DC input circuit might not be able to handle 1.414 times 120vac peaks.

I seem to recall my Fluke DMM manual covering some of this, so I'll take a look when I get home.  

Maybe Paul will chime in here and rescue me...  

[ted_b]

Other than the transformer physically buzzing (or amp shutdown, etc.) would DC issues manifest themselves more benignly, like sonically?

[zybar]

What is the specific problem? Do you have an isolation transformer to try it with?

Exactly what I said above...a buzzing transformer.

[JoshK]

I guess a more likely cause of buzz is transformer imbalance.  I think it is convenient for the manufacturer to claim DC on the lines.