# How do I control the primary voltage in a transformer?

Discussion in 'The Projects Forum' started by electronice123, Jun 6, 2009.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
How can I control the voltage in a transformers primary winding in order to control the secondary's output voltage?

My primary coil has a resistance of 5 ohms and will be used at 1-12V.

Can I just use a pot as a voltage divider to control the voltage, or will the current be to high (at 12V) and burn up the pot?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
It depends on the pot's power rating.

Another solution is to use an autotransformer (variac) as to be able to adjust the voltage and use another transformer on its output to provide isolation from the main.

3. ### Bernard AAC Fanatic!

Aug 7, 2008
4,238
414
mik3's suggestion realy works great,'use it with hot wire foam cutter for fine current control.

4. ### franzschluter Active Member

Jun 1, 2009
95
0
It'll burn the pot man... You can try using high valued resistor for your voltage divider yes... But once you apply load.. It'll collapse the voltage and an increase in current will appear in your pot hence burn up.. You can try using a Rheostat instead..
But as mike suggested.. A variac is the best solution..

Another funny solution is..
You mount multiple regular diodes since these are dirt cheap... All in series.. Note that diodes can dissipate somewhat heat.. And each diode will have a voltage drop of roughly 0.7V.....Use germanium diodes if you want 0.3V drop. Get the idea?

+12V --[ 0.7V drop ]---[ 0.7V drop ]---[ 0.7V drop ]---[ 0.7V drop ]---GND

So you can build a selector which will go in between these terminals. Hence you have a voltage selector. It's a very cheap solution compared to a variac. But you cannot put higher loads. 1A is already max.

5. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
Thanks everyone for the help!

I guess it's time to find a good variac!

6. ### Bernard AAC Fanatic!

Aug 7, 2008
4,238
414
I believe Variac is a trade-marked name, auto-transformer would generally have one tap, a variable auto-transformer is a generic Veriac.

Apr 20, 2004
15,815
283
8. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
I was thinking about using a household dimmer switch (pot). If I am thinking right they usually have a decent amount of current running through them for the bulbs.

At most I will have 4 or 5 amps load at 12V.

9. ### Bernard AAC Fanatic!

Aug 7, 2008
4,238
414
Most dimmers are for non-inductive loads, and are marked as such. What wattage do you need?