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Solid State Amplifiers => Amplifier Discussion => Topic started by: syndromet on June 29, 2006, 12:06:25 PM

Title: How to measure a transformer?
Post by: syndromet on June 29, 2006, 12:06:25 PM
Hi.
I have a broken Pioneer amp, and I'we ordered my first amp kit from Joe. I was hoping to use some of the parts of the Pioneer in my kit, mainly the transformer. My question is; how do I measure the secondary voltages on a transformer? And how much power does a stereo LM1875-amp need?

Nice place, by the way.  :tu:
Title: Re: How to measure a transformer?
Post by: syndromet on July 09, 2006, 05:56:58 AM
I guess I was a little short on my last post, since I'm qiute shure some of you know this.
Here are some pictures of the transformer. Can anyone help me find wich side is the primary?
Pic1 (http://www.syndromet.com/bilder/trans1.jpg)
Pic2 (http://www.syndromet.com/bilder/trans2.jpg)
Pic3 (http://www.syndromet.com/bilder/trans3.jpg)
Pic4 (http://www.syndromet.com/bilder/trans4.jpg)

One side is marked with a red dot... Does that mean anything?
Title: Re: How to measure a transformer?
Post by: joecool85 on July 09, 2006, 07:46:29 AM
I would assume that the red dot side is primary...but its not a good idea to assume when you are dealing with more than 9volts lol.  RDV, chris?
Title: Re: How to measure a transformer?
Post by: teemuk on July 09, 2006, 11:52:02 AM
It's never good to assume anything when dealing with mains voltages. Plug it into a low voltage AC source and do some tests.
Title: Re: How to measure a transformer?
Post by: RDV on July 09, 2006, 12:53:20 PM
The red dot may just be polarity marking also.

Here's one way to do it if you have a decent DMM. Set the DMM to measure resistance and on the lowest range available cause you'll be measuring fractions of an ohm.

You should see:

3.3 ohms +/-  between the primary wires.

.8 ohms +/-  between the secondary pairs.


I've tested a few this way and it seems to work pretty consistantly.

HTH

RDV
Title: Re: How to measure a transformer?
Post by: syndromet on July 09, 2006, 01:37:43 PM
Thanx, RDV!
Here is the meaurements:
Side 1
Pin1 and 2 - 8,7 ohm
pin1 and 3 - 1.2 ohm
pin2 and 3 - 9.5 ohm

side 2
pin1 and 2 - 0.9 ohm
pin1 and 3 - 1.5 ohm
pin2 and 3 - 1 ohm


Does this tell you anything? The primary should see 230V, by the way.
Title: Re: How to measure a transformer?
Post by: RDV on July 09, 2006, 03:51:19 PM
Side 1
Pin1 and 2 - 8,7 ohm = primary =Pin 1 = hot?
pin1 and 3 - 1.2 ohm = Pin 2 = common?
pin2 and 3 - 9.5 ohm = primary = Pin 3 =  hot?

side 2
pin1 and 2 - 0.9 ohm = secondary =Pin 1 = hot?
pin1 and 3 - 1.5 ohm = secondary = Pin 2 = common?
pin2 and 3 - 1 ohm    = secondary = Pin 3 = hot?

My guess would be side 1 being primary, with side 2 being CT secondary. When testing this, you really need to be careful

When taking apart the Pioneer amp it would have been useful to have noted where the mains voltage entered the transformer.

Is the side with the dot side 1?

I'd buy a transformer myself.

RDV
Title: Re: How to measure a transformer?
Post by: teemuk on July 10, 2006, 01:56:56 PM
Pin1 and 3 = Secondary ??! Secondary with such a small resistance to primary side - sounds very unreal. More likely the additional pin in the (assumed) primary is a tap for another voltage. i.e. 220 or 240 V. Makes more sense if you think: pins 3-2 = whole winding (9,5 ohms) = smaller turns ratio, pins 1-2 = little less winding (8,7 ohms) = higher turns ratio. After all 9.5 ohms is pretty close to 8.7 + 1.2 ohms.

Edit: So whatever you do, do not connect only pins 1 and 3 as primary (1,2 ohms): This would probably result into a very high voltage at secondary.

Those resistances seem pretty odd since they should be related to each other. Perhaps you did something that interfered with the probes. I know that sometimes measuring taps like that is pretty difficult. If the secondary is center tapped (I guess it is) resistance between pin 1 and 3 should be pin 1 and 2 + pin 2 and 3. Assuming that pin 2 is the CT. The resistances are pretty similar though.
Title: Re: How to measure a transformer?
Post by: RDV on July 10, 2006, 02:14:33 PM
Quote from: RDV on July 09, 2006, 03:51:19 PM
I'd buy a transformer myself.

RDV
Title: Re: How to measure a transformer?
Post by: syndromet on July 10, 2006, 05:45:24 PM
Guess I'll buy a transformer! It seems Pioneer uses this transformer for a AC out,  to supply other stereo components with power.
Now I just have to find the right transformer at the right price. I don't really understand why everything has to be so expensive here in Norway. 50$ +shipping for a regular 24V 200vAC torodial. Guess it is worth it anyhow.

Thanx for the help, guys!
Title: Re: How to measure a transformer?
Post by: joecool85 on July 10, 2006, 07:24:49 PM
Tranformers are always AC out.