new power transformers 230-120 N.America conversion, polarity and fuse location - Page 1
Forum section: electronics / beginners

author: cod65
author_rank: Contributor
timestamp: January 14, 2025, 04:16:55 am
content: So i am converting this synth to U.S. mains power . I've got the new transformers in place(using two separate instead of original bespoke multi singular , which is unobtainable).Ive tested on a variac all the secondaries and they seems to be in range .The 10vac rated secondary on T1 measured about 13vac, 36 volt rated secondary on T2 about 44-46 vac with no load attached. I'm wondering if 44vac from 36vac rated is something to be concerned about? I'm feeding it into an obscure tat7179P dual regulator that is expecting anywhere from 18-30vDC, so I calculate:23vrms:Vpeak = VRMS x 1.414 = 32.5VDCminus 2 diodes in rectifier = 32.5 - 1.4 = 31.1VDCthis is no-load and there must be other drops, correct ? The schematic called for a 20-0-20 to 23-0-23 transformer so i was concerned that 18-0-18 would not be enough -although the service manual and schematic suggest 23v("DC"? - makes no sense to me) secondary.I'm inclined to ease it up on my variac and see when the regulator is working correctly but I don't know if this is the best test or way to proceed.Also an oddity in the schematic - shows terminal 10 as connected to fuse F2, but PCB layout shows fuse connected to terminal 11 ? I know this is AC at the secondaries, but to what extent does polarity matter at this juncture ? 13 volts AC at this point on secondary- I dont think either lead can be considered a 'hot' vs a 'neutral' (the way I understand fuse placement on the primary side it is essential to place fuse on the 'hot' side so circuit wont be potentially energized in fault state, but it seems that because 10 and 11 are 'floating' does it matter if F2 is on either ?Ive gone the distance to make sure the 2 transformers Im using are in phase at primaries an secondaries, but im trying to understand how important it really is - what is considered 'in phase' if one of my secondaries is a 9vac and other is dual output 18-0-18 ?

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2025, 04:27:48 am
content: Quote from: cod65 on January 14, 2025, 04:16:55 amHot and neutral designations only apply toe the primary side (mains). If one side of your AC secondary was grounded in a way that you couldn't put a fuse in there that would disconnect it from ground, then perhaps you'd want the fuse on the other side. But if the fuse is the first thing that the secondary goes to, it really doesn't matter which lead is it on. This is all assuming a relatively simple secondary circuit.

author: cod65
author_rank: Contributor
timestamp: January 15, 2025, 01:52:08 am
content: thanks, yeah the way I interpret the transformer T1 that provides 10vac secondary there is nothing but 2 leads going to rectifier inputs, which are of course not polarized.my second transformer T2 has a center tap as reference 0 , but again no other connections before rectifier inputs. So I am seeing the secondaries and their phases as essentially interchangeable. I still tried to keep the phase between the two aligned, though it doesn't look like it matters , correct ?