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# what determines the max current that can be drawn from a transformer's secondary coil? Answered

how could i figure out what the max current i can get out of a transformers secondary coil? example: a welding transformer

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## 8 Replies

twenglish1 (author)2009-11-21

here are my calculations:

winding ratio:

winding ratio = voltage secondary / voltage primary

voltage primary = 120v
voltage secondary = about 20 or less(i can rewind the transformer if i need a lower voltage)

ratio = 20 / 120

input current:

when i measured the resistance of the primary coil it came out as .5 ohms could this be right?

I = V / R

I = 120 / 0.5

I = 240 - this can't be right, there is no way this transformer is drawing 240 amps from the main. it has to be less than 20 amps cause its only on a 20 amp breaker

twenglish1 (author)2009-11-21

i am going to guess it is maybe 5 ohms because

I = V / R

I = 120 / 5

I = 24 which is alot more reasonable

so about 24 on the primary of the transformer

secondary current:

current secondary = current primary / winding ratio

current secondary = 24 / 0.17

current secondary = about 141 amps

the most the 12 gauge windings can handle is about 40 amps

is 40 amps all it will be able to draw? or will it try and draw 140 and overheat melting the secondary windings?

lemonie (author)2009-11-22

These are inductors - you can't get meaningful values of inductance / impedence by measuring resistance.
If you want heavy-current you'll need a big heavy transformer (in the foot-crushing range of "heavy")

L

twenglish1 (author)2009-11-22

lemonie (author)2009-11-22

You'd still need quite a large transformer to handle 30amps, your reference to overheating in the secondary is appropriate.

L

twenglish1 (author)2009-11-22

how about a microwave transformer?  i have 2 and i can wind about 20 turns on each one, with about 20 volts each, the current has to be about 40 amps cause i was able to weld with both of them hooked together.  both of them together was about 40 volts

NachoMahma (author)2009-11-22

.  Figure out what gauge/gage wire you have. Strip the (enamel) insulation and measure with a dial caliper or similar. Compare measurement to an AWG table.
.  Look up ampacity for that gauge wire. May get lucky and find that figure on same page as above.
.  This will give you a ballpark figure.

twenglish1 (author)2009-11-21

ok the primary voltage is 110/120v.  how would i calculate the primary current? would it be current = volts / resistance of primary coil? the winding ration is secondary voltage / primary voltage right? if so that is about 0.17 and the secondary uses 12 gauge wire