Determine transformer amps? - Page 1
Forum section: electronics / beginners

author: cio74
author_rank: Regular Contributor
timestamp: November 29, 2013, 03:56:30 pm
content: Hello,I dismantled an old Sherwood rd-6106r Av transceiver rated 5 x 65W audio power. It has a nice big transformer with 2 x 32V outputs, thick wires. Plus lots of other small wires on the other end.I can't find the schematic of this transceiver, any idea how I can safely determine the output current?Thanks.

author: Kremmen
author_rank: Super Contributor
timestamp: November 29, 2013, 04:46:02 pm
content: One method is to rely on the fact that iron core transformers are all dimensioned pretty much the same. So simply weigh the xformer and look up what is the average power rating of similarly heavy transformers. From the power and known secondary voltage you can calculate a pretty close approximatiuon for the max secondary current.For multiple secondaries you can estimate the individual currents as fractions of the total based on the wire cross sections. It will not be spot on, but close enough if you are conservative in your usage.

author: Rory
author_rank: Frequent Contributor
timestamp: November 29, 2013, 06:29:01 pm
content: This article says you can determine the approximate VA rating of a transformer just by knowing its weight and primary resistance.http://sound.westhost.com/xfmr2.htm#s113

author: cio74
author_rank: Regular Contributor
timestamp: November 29, 2013, 08:27:21 pm
content: I have weighted it, 3.5 kilos, 8cm height and 10cm diameter. 5 ohm primary resistance and 2 x 0.45 ohm on the secondary windings.Looking into that web page, it's between 300VA and 500VA which makes sense considering the total audio power.Assuming 350VA, 32V in secondary, I am looking to have it correctly operating at about 10A under load?

author: Smokey
author_rank: Super Contributor
timestamp: November 30, 2013, 03:51:30 am
content: This procedure requires two identical transformers.1) Take transformer #1 and hook it up to a current source.2) Keep turning up the current source until transformer #1 smokes.3) Once transformer #1 smokes, turn off the power supply and note current level.4) Discard transformer #15) Use transformer #2 below the current level that transformer #1 smoked.6) Profit!!!!

author: tehmeme
author_rank: Regular Contributor
timestamp: November 30, 2013, 08:22:50 am
content: Quote from: Smokey on November 30, 2013, 03:51:30 am

author: kripton2035
author_rank: Super Contributor
timestamp: November 30, 2013, 09:09:59 am
content: Quote from: Smokey on November 30, 2013, 03:51:30 amlike it too ... is this method in relationship with your nickname

author: Smokey
author_rank: Super Contributor
timestamp: November 30, 2013, 10:10:58 pm
content: We actually did smoke a potted in transformer at the office once. Of all the nasty burnt electronics smells I've smelled, that had to be in the top 3. It took a solid week of airing out before that smell went away.I guess I really should have put a note in my procedure:NOTE) procedure should be performed outdoors or under a fume hood, or in the office of someone you REALLY don't like.

author: cio74
author_rank: Regular Contributor
timestamp: November 30, 2013, 11:10:34 pm
content: Quote from: Smokey on November 30, 2013, 03:51:30 amDo you think you're funny? Really, do you? Your post is so meaningless and stupid, I was asking for a bit of help not for a clown.

author: Skimask
author_rank: Super Contributor
timestamp: November 30, 2013, 11:22:37 pm
content: Quote from: cio74 on November 30, 2013, 11:10:34 pmIt wasn't meaningless.I laughed.

author: c4757p
author_rank: Super Contributor
timestamp: November 30, 2013, 11:28:42 pm
content: Quote from: cio74 on November 30, 2013, 11:10:34 pmWait... how can you concentrate on transformers when you've got that massive stick so far up your ass? You should be in the hospital, not at the workbench...

author: GK
author_rank: Super Contributor
timestamp: December 01, 2013, 12:08:31 am
content: From the transformer size/mass guesstimate the regulation (little <5VA transformers can have as bad as 30%, big >500VA 5% or better). Measure the unloaded output voltage. Load the secondary until the output voltage drops by the estimated regulation %.Alternatively, the transformer may have its secondary output voltage labeled. In that case you do not have to guesstimate the regulation %. Unloaded, you will measure a secondary voltage higher than stated. Simply load the transformer until the secondary voltage drops to the rated value - however for accurate results make sure to measure the primary voltage as well (mains voltage can vary) and take the subsequent scaling factor into account (actual primary voltage / rated primary voltage) if a Variac is not available to adjust it. Also be sure to use a quality meter that measures AC I/V in "true RMS".

author: Skimask
author_rank: Super Contributor
timestamp: December 01, 2013, 01:23:21 am
content: Quote from: c4757p on November 30, 2013, 11:28:42 pmWhere is that damn like button?

author: c4757p
author_rank: Super Contributor
timestamp: December 01, 2013, 01:25:01 am
content: Up his ass along with the stick, most likely.

author: Skimask
author_rank: Super Contributor
timestamp: December 01, 2013, 01:28:36 am
content: Well then...I ain't reaching up there...or is it "down there"?

author: TMM
author_rank: Frequent Contributor
timestamp: December 01, 2013, 02:04:50 am
content: Quote from: cio74 on November 30, 2013, 11:10:34 pm

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 01, 2013, 03:08:18 am
content: Quote from: GK on December 01, 2013, 12:08:31 amThis is my prefered method, however, i like to run them loaded like that for a while and measure temperature rise too, just incase.