Why place power switch on secondary side of transformer? - Page 1
Forum section: electronics / beginners

author: Koldman
author_rank: Newbie
timestamp: December 31, 2016, 04:58:30 am
content: I've notice in the pile of cheap guitar amps I've been pulling apart that the power switch on almost all of them switches the secondary side and not the mains side. I've failed to find an answer on the google machine. A small amp I have sitting on the floor here has switched secondaries and the transformer gets very warm when switched off, its drawing almost 4 watts. Seems wasteful?

author: Inflex
author_rank: Regular Contributor
timestamp: December 31, 2016, 05:19:02 am
content: Extra safety I suppose. If anything goes wrong with the switch you'll only be at risk of the secondary voltage and isolated from the mains.Yes, transformers tend to be a little wasteful with losses even when 'idling' ( which was always one of the good reasons to turn off equipment at the GPO ).With the advent of high efficiency SMPS wall-warts things are a lot nicer now without all those heavy transformer style units (and cheaper too, I remember agonising over which transformer to get as a kid for my CB Radio PSU ).

author: Koldman
author_rank: Newbie
timestamp: December 31, 2016, 05:43:40 am
content: Its a peavey rage, the transformer looks like it would disintegrate in a breeze.

author: ProBang2
author_rank: Frequent Contributor
timestamp: December 31, 2016, 04:10:46 pm
content: The quick and dirty reason: A transformer (as any coil), simply switched on the primary side, generates high voltage spikes.In a DC circuit therefor is aflyback-diode.Good luck to do something similar in an AC circuit...

author: vk6zgo
author_rank: Super Contributor
timestamp: December 31, 2016, 04:44:17 pm
content: Quote from: ProBang2 on December 31, 2016, 04:10:46 pmHundreds of thousands of pieces of equipment for many decades had the power switch on the Mains side of the transformer,without any such problems.(Some equipment had switches in both the primary & secondary sides.)The only reason for using a switch in the secondary only is pure parsimony----you can get away with a crappy switch!

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2016, 04:51:06 pm
content: A high voltage would only be produced, if the current in the primary is interrupted when it's at its peak. However, in this case, the secondary has fewer turns than the primary so it will be stepped down and the capacitors on the DC side of the rectifier will absorb any brief transient voltage spike.

author: Koldman
author_rank: Newbie
timestamp: December 31, 2016, 08:56:08 pm
content: Quote from: vk6zgo on December 31, 2016, 04:44:17 pmThat would make sense to me, these are small amps, the switches are front mounted in a small space. I have two physically larger amps that switch mains side. The small amp switches are the type that melt while soldering if you're not careful.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: December 31, 2016, 10:20:57 pm
content: I dont care what country you're in, putting the power switch downstream of the power transformer is WRONG, WRONG, WRONG. That would never pass safety regulations here in Canada. Transformers can fail, too. Having the power switch downstream of the PT would allow it to cook and cause a fire hazard if it failed short circuit.The only thing that should be upstream of the power switch is the fuse. That is the way all equipment is built and safety tested by CSA, UL and the hydro companies here. If Peavey or anybody else built them with the switch on the secondary side of the PT them it would not pass UL and not be allowed for sale in Canada or the USA.If you have one like that and have the opportunity to fix it, you should. When the mains power line enters the cabinet, the first thing it should hit is the fuse then the power switch in that order. And it should tie to the terminal on the fuse holder which is least accessible when opened to reveal the fuse for changing.

author: barry14
author_rank: Regular Contributor
timestamp: December 31, 2016, 10:35:05 pm
content: Although I agree with with basinstreetdesign says, I should note that my Hewlett-Packard 209A Audio Oscillator power switch is on the DC side of the power transformer. I have never liked this design and I don't know why HP did it but certainly HP is a reputable manufacturer and so there must be a good reason. I would suspect there are other HP equipments that also switch on the DC side.

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2017, 12:21:17 am
content: Placing the power switch on the secondary side is wrong in every way."Guitar amplifier", as the OP posted, is the key here.The legendary Marshall amps had their main power switch at the back side of the enclosure. With this switch on, filament power was supplied to heat up the tubes.On the front panel, a "Standby/On" switch turned on high voltage for the anodes/grids, making the amp operational; this one obviously on the secondary side of the transformer.The (probably far east) producer didn't understand this, and placed the switch on the secondary side to provide a "Marshall feeling" for the guitarist, but forgot the main switch at the rear.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 01, 2017, 12:43:08 am
content: The standby switch is another poorly-understood feature. Kevin O’Conner has a fairly good explanation about why this switch should never have been used in the first place:http://www.londonpower.com/standby-switch

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2017, 01:22:56 am
content: Who cares about Fender amps? They were always cr*p.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2017, 01:48:18 pm
content: Quote from: basinstreetdesign on December 31, 2016, 10:20:57 pmWhilst I completely agree with you that the power switch should be on the primary, the transformer should also have adequate over current and thermal protection to protect against fire.