Question about transformer secondary specs - Page 1
Forum section: electronics / beginners

author: JuiceKing
author_rank: Regular Contributor
timestamp: August 02, 2012, 12:57:12 pm
content: Hi,I'm building an octopus curve tracer and am looking at on-line sources for a power transformer. I'd like to get one with multiple voltage choices: 24, 12, 6 or so would be good.I'm having trouble interpreting transformer specifications. When a manufacturer, such as Hammond, says that a transformer is 24V CT, does that mean that it has 24V with a 12V center tap? Or, that it is 48V with a 24V center tap? Nowhere do they make this clear, so they must assume it's common knowledge. Which I don't have.Also, I'm having trouble finding a transformer with multiple taps on the secondary. Hammond has single center tap models and dual secondaries, but it looks like the voltages of the dual secondaries tend to be the same as the center taps, so they do me no good. Any suggestions of where to look for what I'm after?Finally, is it wise to fuse this on the mains side? If so, what size/type fuse should I use? My circuit will draw no more than 0.25A on the 24V secondary, so that would be 1.25A on the primary. What value fuse would be appropriate? Should I use slow or fast blow in this situation?Thanks!- Ken

author: Monkeh
author_rank: Super Contributor
timestamp: August 02, 2012, 01:33:39 pm
content: 24V center-tapped means the center tap will be at 12V. Odd voltage taps are normally custom. You can get toroidals with just the primary wound and wind your own secondaries. Hell, you could just strip an old toroidal and do that.Quote from: Electroalek on August 02, 2012, 01:11:16 pmYou haven't seen many transformers, have you? Transformers with four and five taps, as an example, are quite common. They're just not off-the-shelf parts. An SMPSU is probably not appropriate for what he's making.

author: JuiceKing
author_rank: Regular Contributor
timestamp: August 02, 2012, 01:46:30 pm
content: Quote from: Monkeh on August 02, 2012, 01:33:39 pmThanks. I'm thinking now to make the octopus with two built-in voltage selections (24 and 12), plus one external in. That gives me convenience and flexibility. (My function generator maxes out at 10V RMS, so it can fill in the lower voltage range.)Any thoughts on the fuse?Thanks,- Ken

author: Monkeh
author_rank: Super Contributor
timestamp: August 02, 2012, 01:50:47 pm
content: Well, you got the fuse calcs backwards. 250mA on the secondary will be 50mA on a 120V primary!A 3A fast blow will be fine to protect the primary side from shorts. You can protect the secondary seperately if you think it'll be needed.

author: T4P
author_rank: Super Contributor
timestamp: August 02, 2012, 02:26:25 pm
content: Quote from: Electroalek on August 02, 2012, 01:11:16 pmSURE What about unknown mfr 24VCT xfrms ... SMPS? Have you made one?24VCT means 12-0-12

author: G7PSK
author_rank: Super Contributor
timestamp: August 02, 2012, 03:07:49 pm
content: Look for surplus supplies, I have a transformer that has seven taps on the secondary and will give up to 80 volts at 700VA and it only cost me 50 pence. And its brand new unused. I have not yet decided what I will do with it but at the price I could not walk by.

author: JuiceKing
author_rank: Regular Contributor
timestamp: August 02, 2012, 03:21:42 pm
content: Quote from: Monkeh on August 02, 2012, 01:50:47 pmHeh. Yup, got it backwards. Thanks!- Ken