Transformer Sizing - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: January 06, 2014, 10:13:48 am
content: Happy New Year to all,Just a newbie question. I want to buy an air purifier and I found one at a good price in ebay.The only problem is that it works with 110V instead of 220V which we have here in Greece.I would need a transformer. I already have one from 220V to 110V (100W)The air purifier has a maximum consumption of 80Watts.I guess that since the air purifier has a motor inside, it will have a much larger initial surgecurrent which my transformer will not be able to handle, but my question is if we assume thatthe motor has already started to operate, will this transformer be enough for the specific device?TransformerI am a bit confused at where the 100W rating is assigned to. 220V or 110V?It looks like that the output is 110V / 100Watt, but I just want to be sure...Thank you!

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 06, 2014, 12:01:37 pm
content: 100W at 220V is the same amount of power at 110V, however, generally, it is the transformer that is initially sized to the power, eg, a 100W core, and then the rating of the wire used to wind the windings is chosen to fall within those specifications.Next up, transformers are an interesting thing, they are a fairly analog device; eg, when they hit their rating, they don't just "pop", they get a bit hotter and they hum a bit, and as you progressively overload them they get hotter and hum louder until they pop.Additionally, because they are generally pretty large they have a lot of thermal mass and thus thermal inertia, so they can handle short term overloads just fine.It's just fine for your air purifier.

author: hgg
author_rank: Frequent Contributor
timestamp: January 06, 2014, 12:20:29 pm
content: Quote from: peter.mitchell on January 06, 2014, 12:01:37 pmHi, I don't understand... How is this so?100W = 220V * I => I = 0.45A and100W = 110V * I => I = 0.90AIn my situation the air purifier consumes a maximum of 80W = 110V * I => I = 0.72AIf you have the 100W specification on the transformer, how do you calculate the max currentthat will be available to the load?Thanks.

author: mrkev
author_rank: Regular Contributor
timestamp: January 06, 2014, 12:40:15 pm
content: Quote from: hgg on January 06, 2014, 12:20:29 pmIt's still 100W... Same power, different current...Anyway whole this depends on manufacturer of this transformer. They usually claim output power, so the 100W is for the 110V part, at the input, there will be 100w+trans. looses.I have seen transformer with claimed max. input power (and max. output was therefore by about 20% lower), I think it's not that common and 20% overload for short time is not a problem (in most cases).

author: hgg
author_rank: Frequent Contributor
timestamp: January 06, 2014, 12:47:00 pm
content: Great!It looks like that I could use it for that purpose then.Thank you very much!George.

author: Unknown
author_rank: Unknown
timestamp: January 06, 2014, 05:14:19 pm
content: Quote from: hgg on January 06, 2014, 12:47:00 pmAlthough I would be inclined to keep an eye on it as it is likely to get quite warm and possibly not last all that long.

author: hgg
author_rank: Frequent Contributor
timestamp: January 06, 2014, 06:06:23 pm
content: Hi, what minimum wattage would you suggest for the 80watt load ?

author: Unknown
author_rank: Unknown
timestamp: January 06, 2014, 06:15:10 pm
content: Quote from: hgg on January 06, 2014, 06:06:23 pmIf it was running for extended periods of time I'd probably double it if you stuck to the cheap autotransformer style. Depends on the quality & efficiency of the transformer and the duty cycle of the load.

author: hgg
author_rank: Frequent Contributor
timestamp: January 06, 2014, 06:36:22 pm
content: Is there a way to calculate the minimum wattage in order to be on the safe side?Also, what would a maximum temperature be for a transformer working in its safety zone?