what is VA rating of step-down transformer?

[jhundt]

I run my 117V US Fender amps in Holland using a step-down transformer (220VAC - 120VAC). There is a rating 250 VA on the label, I think this is a current rating. But I can't remember how it works, how to convert to amps. Can anyone explain this to me?

[mark norwine]

that unit is rated for slightly {very slightly} more than 2 amps.....marginal for all-but the smallest fenders...

[jhundt]

thanks for your reply. I will use that transformer for my wife's sewing machine then. But can you please explain briefly the relationship between VA and amps or watts (consumption, not output watts)?

[mark norwine]

breifly...and without a lot of theory that can derail this....

if "simplicity" is the goal, then VA can be though of as being the same as watts....

there are a million reasons why this {technically} isn't so, but for our lesson here, let's discard those reasons. discarding them won't negativly effect your real-world outcome.

so....if your transformer is now thought of as having a wattage rating of 250W, then we can use ohm's law to solve for current:

the tranny delivers 120V, with a max current being drawn such that 250W isn't exceeded...

Let:

E = voltage
I = current
P = watts

And ohm's law tells us that:

P = E * I, therefore,

I = P / E

so if we know P and E, we can solve for I:

I = P / E
I = 250 / 120
I = 2.08 or:
I = "About 2 amps"

Most medium-sized fender amps are fused for 2A and draw a bit less, so your transformer is *probablly* OK, but more "fudge factor" is certainly a good thing in applications like this. Personally, i'd like to see a 5A transformer {or bigger} here...

hope this helps,

mn

[jhundt]

So, 250VA / 120 output volts = 2.08 amps. Great, thanks a lot. Now I'll have to check the three other transformers that are laying around.

Here's a quiz for you : the amp works fine, voltages read correct; why do I end up with 115VAC between the chassis and ground - in this case the radiator that I touched unsuspectingly... ouch!

I think I figured this one out myself , and I see why you could easily kill yourself using your US gear in Holland if you aren't aware of this problem with common step-down transformers.

It has to do (I deduced) with the type of transformer. In most cases these are autotransformers, and the way I read it, these are polarity-sensitive (or whatever the proper electrical term is), unlike an isolating transformer.

[mark norwine]

use only a grounded isolation transformer....

autotransformers are not apropriate for this kind of aplication