Can I safely ignore this transformer's rating? - Page 1
Forum section: electronics / beginners

author: PeteAU
author_rank: Contributor
timestamp: November 08, 2012, 08:01:56 am
content: If I only use the 20V tap, can I draw 1.5A? And if I use only the 15V tap, 2.0A?Or, is the equation not linear?

author: Rerouter
author_rank: Super Contributor
timestamp: November 08, 2012, 08:11:34 am
content: you can pull 10A through it if you wish, however the transformer will have the heat from the coil dumped into it,equally the increased voltage droop may require using a higher tap,

author: Kremmen
author_rank: Super Contributor
timestamp: November 08, 2012, 08:13:08 am
content: 2 factors come into play: the current rating of the windings and the flux density in the magnetic path. The second one should not be a problem since it scales more or less proportionately to the amp-turns and should not saturate. The windings however do not necessarily work as you assume. The label says 1 A without specifying anything else and that is a big hint in the direction that all windings are made with the same size magnet wire. You will get away with short term overloads but for continuous use my guess is, no you cannot _safely_ ignore it.

author: mazurov
author_rank: Frequent Contributor
timestamp: November 08, 2012, 07:11:22 pm
content: It is not clear to which winding 1A rating belongs to, could very well be primary. You should be able to see the wires, just look at them and make a guess - there are diameter to current tables available from many places on the net. Also, you can just connect the primary to mains, load the secondary to 1A and observe for 30 minutes. If nothing happens, add another amp and observe for another 30 minutes. Repeat until your desired current is reached. Stop if excessive temperature is reached.

author: SeanB
author_rank: Super Contributor
timestamp: November 08, 2012, 08:05:20 pm
content: 1A will always refer to the secondary. You can only draw 1A, irrespective of the tap you choose. Drawing more from a tap will burn it out with time

author: PeteAU
author_rank: Contributor
timestamp: November 09, 2012, 06:51:13 am
content: I should have thought about this a little more, it seems obvious now.1.0A * 30V = 30 Watts1.5A * 20V = 30 WattsBut in the second case, you have a shorter winding, hence more concentrated heat?

author: David_AVD
author_rank: Super Contributor
timestamp: November 09, 2012, 07:06:30 am
content: The secondary winding's wire would have a maximum current rating.Just because you're using 2/3 of the secondary doesn't mean the wire in that section can tolerate more current.

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2012, 07:59:13 am
content: You can push a transformer past its rating quite a bit more than you can push most other electronic components.If you doubled the current rating on other components, (transistors, lightbulbs, resistors etc) they will smoke themselves pretty fast.Pulling 2A out of 1A transformer will (in general) make it very warm, hum louder and probably start to yellow the plastic/paper with extended use, but in general it should hold up more or less.Especially if it's only used for a short time with breaks in between.I have a few oil filled mil-spec transformers rated at 2A and i've pulled 10A out of them all day long. They get barely warm.But that's a special case, milspec stuff is seriously underrated.Safety Note: I'm NOT recommending everyone go and pull huge overcurrent out of their transformers. There's always some risk with overspecing stuff and i don't want you to go burn your house down.Remember, if the transformer fails its going to fail from heat and heat = fire.But on the other hand, overspecing a transformer is something i do sometimes for a quick hobby project after checking temps and seeing how well a transformer seems to handle it on a case by case basis.

author: SeanB
author_rank: Super Contributor
timestamp: November 09, 2012, 02:44:16 pm
content: Most transformers made these days are specced to barely meet the ratings. The core is designed so it is either just starting to saturate or is actually in saturation, and the windings are so thin that they run very hot at full load, thus needing class H insulation. The Milspec transformer will have at best a class C or D rating, and will have a core run far below saturation, so that it will survive a 100% overvoltage or overcurrent. You always derate transformers if possible, as they are designed down to the bare minimum these days.

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2012, 10:27:55 pm
content: Yeah, that's a good point. If its a real cheap china 1A transformer it's probably in reality a 500mA transformer

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2012, 11:51:59 pm
content: I remember as a youngster when I went dumpster diving I would look for the big heavy transformers in old valve equipment with their beefy valve filament secondaries. I used to like shorting those out to melt paper clips and nails, and the transformers would take that abuse quite happily for some time before the windings got too hot and needed time to cool down again.As a matter of interest some transformers are actually designed this way when the expected load is intermittent. For example the substation transformers supplying third rail traction current to trains in the south of England. Each substation supplies a short isolated section of track and is only loaded when a train is traveling in that section. So the transformer gets hot when a train goes past and cools down again in the rest period between trains.This theory broke down a bit when newer trains were introduced with more powerful motors for faster acceleration and air-conditioning for passenger comfort. Suddenly the old transformers couldn't cope with the higher frequency of more power hungry trains and had to be upgraded (at great expense, not properly accounted for by the planners).

author: Psi
author_rank: Super Contributor
timestamp: November 10, 2012, 02:20:27 am
content: Some transformers, eg old neon sign & MOT (some of them) have a built in magnetic shunt to limit the current.So you could short them 24/7 and they would only draw their rated current.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 12, 2012, 07:15:33 am
content: If you're using it for a DC supply, you could use the higher voltage tap then stick it through a buck converter?

author: M. András
author_rank: Super Contributor
timestamp: November 12, 2012, 09:22:06 am
content: Quote from: SeanB on November 09, 2012, 02:44:16 pmis this apply for costum made transformers from companies who make them for a living? or they would meet their specs and well over their rating would still work. specced in the initial query it will be running at full power 24/7?

author: Psi
author_rank: Super Contributor
timestamp: November 12, 2012, 09:25:45 am
content: na, its just the dodgy brands that get spec'ed to within an inch of their life.Good quality manufacturer's should be fine.

author: David_AVD
author_rank: Super Contributor
timestamp: November 12, 2012, 10:48:24 am
content: Most things are "value engineered" these days. Stay within the ratings I say.

author: amyk
author_rank: Super Contributor
timestamp: November 12, 2012, 11:27:56 am
content: Quote from: Psi on November 09, 2012, 07:59:13 amThey can take momentary surges a lot better too, because of the thermal mass. I still wouldn't recommend going beyond the ratings continuously.

author: Psi
author_rank: Super Contributor
timestamp: November 12, 2012, 11:48:48 am
content: Yeah, it's never a good idea to exceed the ratings on any device.