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**Wilder Amplification** · 03-29-2010, 10:09 AM

Originally posted by BiBi View Post

Sorry if this is a dumb question...
When purchasing a step down transformer for say 220VAC to 110VAC is it necessary to calculate the current used in the filaments as well as the high voltage section of the circuit?
It's something I don't really understand about fusing as well for that matter-- why an amp that draws maybe 100mA for the high voltage but almost two amps for the filament supply doesn't blow a fuse of say 650mA on the primary.
Obviously I am missing something about the operation of transformers.
Thanks for helping me out with this one!

That's because power is the product of voltage & current (P = I x E). In order to keep the power in = power out relationship the same, when current is high/voltage is low on one side of the tx, it's opposite on the other side (i.e. low current/high voltage).

So say you're pulling 8 amps @ 6.3 volts off your heater winding -

6.3V x 8A = 50.4 Watts

Now divide this power by your mains voltage -

50.4 Watts / 120VAC = 0.42A or 420mA

On the HT winding it's backwards. Since voltage is high/current is low on the HT side, it has to be the opposite on the mains side (lower voltage/higher current). A 100mA draw on an HT winding of 350VAC with a 120 mains translates to -

350VAC x 100mA = 35 Watts

35 Watts / 120VAC = 0.29A or 290mA

For every 1mA drawn on the HT in this scenario translates to a 3mA draw on the mains side. Yet every 1 amp draw on the heater winding translates to a 52-53mA increase in current on the mains winding.

**BiBi** · 03-29-2010, 10:48 AM

Thank you very much sir! That's well explained.

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