Unmarked Transformers - How do you figure out their ratings? - Page 1
Forum section: electronics / projects

author: PStevenson
author_rank: Regular Contributor
timestamp: May 22, 2013, 08:31:39 am
content: as the title suggests, I would really love to know if there is a way of working out the ratings of a transformer that has no markingsfrom various measurements.I've got a box full of unmarked (working) mains transformers that have gathered over the years and the only thing you can figure out about them is how many voltsthey give out (no load) when you plug them in.hope you can help !

author: dr_p
author_rank: Frequent Contributor
timestamp: May 22, 2013, 08:43:28 am
content: all I can think of is to measure the secondary wire's thickness - roughly 6A per square mm

author: Greg Robinson
author_rank: Regular Contributor
timestamp: May 22, 2013, 08:47:28 am
content: You can estimate the total VA by the size, then from that you can estimate what the current capability is. To be cautious, you should also monitor the unloaded/loaded voltages - you can guess the regulation based on the size/VA estimate. For instance, a 50VA transformer will have ~9% regulation, a 100VA ~8%. If the voltage drop matches roughly with your current capability estimates, you should be fine.You can get more complicated and progressively load and measure quiescent temperature/estimate core temperature, but that's a lot of effort. Much easier to design conservatively with unmarked transformers.It gets much more complicated if you have multiple secondaries of different voltages - such transformers should be run even more conservatively if you do not know it's origins.Good luck!

author: firewalker
author_rank: Super Contributor
timestamp: May 22, 2013, 08:49:38 am
content: I believe there is a method to extrapolate the power using the size of the core the resistance and the output voltage on specific load. Actually the reverse method used to design one.Have a look to the link bellow.http://www.giangrandi.ch/electronics/trafo/trafo.shtmlAlexander.

author: poorchava
author_rank: Super Contributor
timestamp: May 22, 2013, 11:03:05 am
content: Quote from: firewalker on May 22, 2013, 08:49:38 amThis is based on an assumption, that manufacturer/designer of the original thing the transformer came from didn't undersize it to save costs or space. Quite often not the case with cheap stuff and consumer product.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 22, 2013, 11:35:10 am
content: In that case it will give you the "real" ratings

author: firewalker
author_rank: Super Contributor
timestamp: May 22, 2013, 11:56:52 am
content: Another method some people use is to connect a variable resistive load to the secondary. When the voltage decreases by 10% relatively to the open circuit voltage, the result V^2/R is the output power of the transformer.Idon'tknow if this is asafeor accurate method. Test it with couple known transformers perhaps?Alexander.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 22, 2013, 12:17:45 pm
content: heypleas read that thread :https://www.eevblog.com/forum/projects/transformer-calculations-and-equations-in-here/msg216687/#msg216687If you got any questions please PM me or post them in herePS: Dave please sticky that threadCheers

author: PStevenson
author_rank: Regular Contributor
timestamp: May 23, 2013, 04:39:59 pm
content: many thanks for your answers.my current method is to guess current by its size and wire width/resistance along side a known one and then lower my guessed ratings to be safeand hope for the best.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 23, 2013, 05:12:33 pm
content: You could try the Calvin and Hobbes method