# Transformer help

1. Jul 8, 2009

### pd_du26rt

What should be the maximum and minimum primary voltage of output transformer with 10 turns of 22 SWG primary winding and 500 turns of 34 SWG Secondary winding?

2. Jul 8, 2009

### vk6kro

Depends on the iron core, but as a rough guide take 5 turns per volt.
10 turns 2 volts
500 turns 100 volts

3. Jul 8, 2009

### negitron

It depends on the core AND the supply frequency. You can put more volts per turn on a given piece of iron with a higher frequency.

Assuming 60 Hz, this can be determined experimentally, if you have a Variac. You'll need two meters, one to monitor the input voltage, and one to monitor the input current. SLOOOOOOWLY ramp up the voltage with the Variac, keeping a close eye on the current. When the core begins to saturate, you'll see the start of a sudden, sharp rise in excitation current. At that point, STOP, dial the voltage back a bit and note the voltage. That's your absolute max. There is no practical min, although efficiency will suffer at too low a drive voltage.

You can also roughly calculate your volts per turn figure if you know the core dimensions and permeability. If it's a laminated core power transformer, assuming silicon steel alloy is a safe bet. In that case simply divide 38 (for 60 Hz; for 50 Hz use 45) by the core cross-sectional area (in square cm) to obtain a rough figure.

4. Jul 8, 2009

### uart

Actually that will give you the number of turns per volt (for a nominal 1T peak steady state flux density). If you want volts per turn you'll have to take the reciprocal of this.

5. Jul 8, 2009

### negitron

Right. My error. Oddly, I had it right to start but then I "corrected" it.

6. Jul 9, 2009

### pd_du26rt

Thanks for the quick replies! I am using two ferrite EE cores, dimensions 60x35x8mm size each. The input to the primary is the current-amplified output of a darlington pair (SL-100 and 2N3055)

7. Jul 11, 2009

### Redbelly98

Staff Emeritus