It is currently Sat Jun 23, 2018 8:50 pm

Testing unknown transformers

 
Posts: 4092
Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Testing unknown transformers

Post by Refugee » Wed Mar 18, 2015 12:50 am

I have acquired a 650VA transformer that previously lived in a slot arcade machine that was stripped for spares and scrap. As it is potted it was offered for free on another forum due to this lowering the scrap value. The windings are just marked with voltages and nothing else.

DSCF1833.JPG

DSCF1837.JPG
Saved from a faulty power supply chassis

The testing it a little bit technical as you have to load up the windings with an entire army of dummy loads and do some ohms law.

The trick is to load each winding while monitoring the voltage until it falls from the open circuit voltage to about half way down to the marked loaded voltage remembering that these will drop a little bit when the more powerful windings are loaded.

In this example I started with the biggest dummy load I had on the 11 volt winding as it read so low with a DMM that there was no chance of getting anything useful from the DC resistance.
I had to load this with 2 ohms to get it down from 12.7V to 11.7V.
This must be something like 110VA.
The 9.5V winding came in at about 2A so that is 19VA.
The 7V winding came in at about 3A so that is 21VA.
The 40 volt winding came in at about 6.25A with a nice toasty dummy load at 250VA.
The 50 volt winding rolled in at 5A with another toasty dummy load at another 250VA.

The totaling based on the size and weight of the transformer is:-
11V 110VA plus 9.5V 19VA plus 7V 21VA
We now have 150VA
Add the two powerful windings at 250VA each and we have a reasonable estimate of the individual ratings of all the windings on this 650VA lump.
Attachments
DSCF1832.JPG
DSCF1829.JPG
DSCF1828.JPG

 
Posts: 67
Joined: Sat Sep 20, 2014 6:59 pm
Location: Bedford, UK.

Re: Testing unknown transformers

Post by johntheboffin » Wed Mar 18, 2015 8:18 am

Looks quite a meaty transformer! What are you going to do with it?

I sometimes use old transformers like this as autotransformers to operate 220 V Continental radios from 240 V mains; I am listening to my nice Saba Freudenstadt 8 like this as I type.

John

 
Posts: 4092
Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Re: Testing unknown transformers

Post by Refugee » Wed Mar 18, 2015 1:23 pm

One of the jobs I am looking into is to convert all lighting circuits in the house to a low enough voltage that LEDs can be direct driven without unreliable chopper circuits.
The other use is for a big bench power supply.

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Testing unknown transformers

Post by Terrykc » Wed Mar 18, 2015 4:08 pm

I remember seeing an article many years ago about (re)winding the secondaries of a transformer to give a range of voltages in one volt steps using the minimum number of secondaries by connecting them in a combination of in and out of phase connections.

For example, 1, 2, 7 & 21V secondaries would give 1 - 31V like this:

1= 1
2= 2
3= 2+1
4= 7-2-1
5= 7-5
6= 7-1
7= 7
8= 7+1
9= 7+2
10= 7+2+1
11= 21-7-2-1
12= 21-7-2
13= 21-7-1
14= 21-7
15= 21-7+1
16= 21-7+2
17= 21-7+2+1
18= 21-2-1
19= 21-2
20= 21-1
21= 21
22= 21+1
23= 21+2
24= 21+2+1
25= 21+7-2-1
26= 21+7-2
27= 21+7-1
28= 21+7
29= 21+7+1
30= 21+7+2
31= 21+7+2+1

The range can be increased up to 94V by adding a 63V secondary ...

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Testing unknown transformers

Post by Terrykc » Wed Mar 18, 2015 5:33 pm

I've no idea what the original sequence was - the 1-2-7-21 sequence was the simplest one I came up with when I looked into the idea today.


Return to General Work Shop Discussion



Who is online

Users browsing this forum: No registered users and 3 guests