Transformer VA question - Page 1
Forum section: electronics / beginners

author: Greg65
author_rank: Contributor
timestamp: June 05, 2014, 03:10:26 pm
content: Dear forum, I have small wall plug in transformer that feeds a small 4 channel audio mixing desk that is not working any more. It is rated at on the label as;Input: 230V / 50Hz / 70mA / 14WOutput: 2 x 18.5v~ / 2 x 250mAI have seen this one below on ebay for sale and I am wondering is the VA output rating good enough to match the the broken one so that I can replace it? or else what would I need to go for in VA, to at least match the old one?Power Rating:: 20VAPrimary Voltage:: 230VRegulation(%):: 12%Secondary Voltage:: 2 x 18VType:: Single Primary, Dual SecondaryThank you, Greg

author: DmitryL
author_rank: Regular Contributor
timestamp: June 05, 2014, 03:24:13 pm
content: It's strange. Transformers usually don't fail, they are quite reliable things. Unless you short the output, it overheats and thermal fuse blows (if there is a thermal fuse in it).Have you checked it with the multimeter ?What is primary/secondary resistance ?On the other hand you can replace it with 2 identical transformers with similar specs (if you can't find one with 2 secondaries), just watch for the secondary current it can provide.ebay search for "18v transformer" gives a plenty of choice.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 05, 2014, 03:45:50 pm
content: Yes, that transformer looks like a suitable replacement. The original is only about 10 VA (18.5 V * 2 * 250 mA) so the 20 VA replacement is actually bigger than you need. That should not be a problem unless it is physically too big.Also, I second that it is weird for a transformer to break. I would double check for blown fuses or for a loose/broken wire somewhere near the power inlet / switch.

author: Greg65
author_rank: Contributor
timestamp: June 05, 2014, 05:01:59 pm
content: Thank you for the reply's folks, I tested the old transformer and its definitely done, I have found the fault was in the three prong plug that plugs into the equipment has a short in it, so it has shorted the secondary of the transformer and caused it to blow.When working out the power rating in VA for the transformer, I use the secondary voltage times the mA to give me the VA result?And what about this transformer that has dual outputs? I'm guessing we simply add the individual VA's up to get the total? yes?

author: Greg65
author_rank: Contributor
timestamp: June 05, 2014, 05:09:10 pm
content: Ok, I've got it now, thanks ejeffrey,

author: DmitryL
author_rank: Regular Contributor
timestamp: June 05, 2014, 09:37:36 pm
content: Quote from: Greg65 on June 05, 2014, 05:01:59 pmSo, what's blown ? If it is primary (measures as open) than you might have a chance to "repair" your transformer.If it is fitted with a thermal fuse, than shorting the secondary causes its overheating and this fuse blows, not letting everything catch a fireIf it is the case, you may try to either short this fuse or replace it with something similar. If there was no fuse and primary is a toast, then you are out of luck.The thermal fuse looks like a small rectangular thing positioned under insulating tape/paper, very close to the winding. You can try acurately cutting the insulating tape/paper (try not to damage the winding below it) and see if you can find this thing under.Measure it, if it is open, you can try to short it and test the transformer.I have "fixed" several good transformers that wayGoogle "transformer thermal fuse" and see the pictures.

author: Fank1
author_rank: Regular Contributor
timestamp: June 05, 2014, 10:21:26 pm
content: Except for an open winding, you can usually check a transformer with your nose.