CMMI-8PCA help needed....

[therecordingart]

I'm using this transformer in a 312-ish card and I'm only getting an input impedance of about 36R. Shouldn't it be like 150R?

 

[emrr]

you mean with a multimeter? a 150 ohm rating will never measure 150 with a multimeter.

 

[therecordingart]

Maybe my newb is showing, but I was under the impression that I should be able to measure an impedance of 150R across pins 1 and 4 of the transformer. After looking at the data sheet for this transformer it appears that I forgot to add the two 75R resistors. If I were to add those resistors I would measure about 186 ohms (within the tolerance of the resistors).

So now my question is....

Should I try to get the impedance across pins 1 and 4 as close to 150R as possible or is 186R ok?

 

[keefaz]

I think you're measuring transformer DC resistance, the impedance is AC resistance, that means that resistance varies with the frequency of the AC signal, I think you need to set a frequency signal test at first to measure impedance, but I am newbie myself and I don't know how to measure exactly the transformer impedance...

 

[therecordingart]

Yeah, and I think I'm just a little lost. On the data sheet for the transformer maybe those two 75R resistors are representing a microphone and not resistors? I sent an email to cinemag.

 

[emrr]

they represent a test condition for establishing a 150 ohm source while testing. nothing to do with operation. AC impedance and DC resistance are completely different things. At face value, your multimeter is useless. It's a good bit more involved than that. You are getting a reading that corresponds roughly to the majority of 150-600 ohm rated windings.

 

[therecordingart]

[quote author="emrr"]they represent a test condition for establishing a 150 ohm source while testing. nothing to do with operation. AC impedance and DC resistance are completely different things. At face value, your multimeter is useless. It's a good bit more involved than that. You are getting a reading that corresponds roughly to the majority of 150-600 ohm rated windings.[/quote]

Ok cool. I really appreciate the help. I'm going to look through the internet for a little transformer 101.

 

[therecordingart]

I got an email back from David at Cinemag and he said you shouldn't measure DC resistance on the transformer because there is a chance that you can magnetize the core.

What are the odds that I screwed up my transformers by doing this? I was measuring the resistance using a digital multimeter...does that make a difference?