How do I determine what this Transformer is? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 20th March 2003, 04:26 PM   #1
diyAudio Member
Join Date: Feb 2003
Location: Hudson, WI USA
Default How do I determine what this Transformer is?

I picked up this transformer at the surplus store for $9.95.... they had an entire box of them. It was the biggest transformer that they had, but I'm curious what do I have to do to measure what the specs on it are? I'm pretty electronics noob, so let me know what tools I need and measurements I should take.

  Reply With Quote
Old 20th March 2003, 04:37 PM   #2
diyAudio Member
Join Date: Jun 2002
Location: NY Area
Send a message via AIM to nania
Does it have any writing on the label or on its body? If not, you'll need a meter to measure its taps. How much does it weigh? Multiply its weight by 30 to approximate its VA rating. That's where I would start.
  Reply With Quote
Old 20th March 2003, 08:04 PM   #3
diyAudio Moderator
planet10's Avatar
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Originally posted by nania
Multiply its weight by 30 to approximate its VA rating.
Weight in pounds?

wotan... do you have a Variac? I use one of those to figure out power transfomers.

community sites,, ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  Reply With Quote
Old 20th March 2003, 08:15 PM   #4
diyAudio Member
Join Date: Jun 2002
Location: NY Area
Send a message via AIM to nania
Planet 10

Yes, that advice was for weight in pounds. I thought he was from the US so that was my suggestion. I suppose I should have been more specific since kilos times 30 would be too conservative an estimate
  Reply With Quote
Old 21st March 2003, 05:04 PM   #5
diyAudio Member
Join Date: Feb 2003
Location: Hudson, WI USA
It weighs about 6lbs.

I was hoping to use it for a 2channel gainclone, but I could always pick up more of these.... now looking at it it has a %.95 price tag on it, and the shop probably had 100 of them. Could I use something like this?

Thanks, Rob.

This is all that is written on it...
Attached Images
File Type: jpg mvc-011f.jpg (69.0 KB, 136 views)
  Reply With Quote
Old 21st March 2003, 05:34 PM   #6
diyAudio Member
Join Date: Mar 2003
Location: Northwest
Well... if you don't even know if it has a 120 volt primary, proceed with caution. But in general, here's what I do:

Use an accurate ohmeter to figure out the windings. Beware of the connection and lead resistance of your DMM. The higher resistance winding will (hopefully) be the primary. Some transformers have a double primary designed for 120/240 operation.

If what you think is the primary is center tapped, that's not a good sign. Hopefully the secondary IS center tapped if you're building a push-pull amplifier. On some transformers you also have to figure out the phase (direction) of the windings. This can be done using say an audio signal generator (which don't usually mind driving an effective short circuit) set to 60hz and experimenting with series/parallel configurations of windings until you get the desired results.

Once you figure out the windings, connect 120 volts ac (or some lower voltage from another transformer if you're being cautious) to the primary and measure the no-load secondary voltages. This will give you the turns ratio of the transformer.

With the transformer running on 120 volts, load the secondary with a power resistor (or combination of them) that will result in about a 0.1 amp to 1.0 amp AC RMS current. Note the voltage reading on the secondary with this load (make it brief if your resistors cannot handle that much power). It's safe to briefly overload the resistors by a factor of 4X if they're large ceramic power types (and they should be). P = V**2/R (V squared divided by the resistance).

Repeat the above test with a load the produces a current from 1.5 amps to 3 amps. Subtract the two readings and divide it by the difference in current:

R = (V1 - V2)/(I2 - I1)

The resulting value is the effective series resistance of your transformer and can be used to calculate its voltage drop at any given load as follows:

Vout = Vnoload - I*Rseries

You then multiply the Vout value by 1.414 to get your DC supply rail voltage.

What this test doesn't tell you is when the transformer will start to go into saturation (overload). Some people estimate the total VA capacity by weight as suggested above, and others use a formula based on the wire gauge of the windings (which can be hard to figure out with some transformers). It can also sometimes be estimated from Rseries but I don't have those formulas handy.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
How to determine if class G/H? tktran303 Solid State 21 4th September 2008 11:48 AM
"rule of thumb" used to determine xxVA for a power supply's transformer? CopyofAudiófilo Chip Amps 11 22nd July 2008 12:01 PM
How to determine phase of transformer secondaries? Onvinyl Parts 10 7th March 2006 10:53 AM
how to determine P1 from P2 of a Push-pull transformer? up4 Tubes / Valves 10 13th June 2005 10:54 PM
how to determine the pin layout of NPN or PnP? tone Solid State 3 31st December 2002 01:15 PM

New To Site? Need Help?

All times are GMT. The time now is 12:41 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2