Variable power supply

Discussion in 'Power Electronics' started by flhtci2006, Feb 13, 2013.

1. flhtci2006

2
0
Feb 13, 2013
I need a power supply to operate a 20W halogen bulb at ~ 3.3v, 4.5v and 6v. The three voltages allow for variable brightness of the bulb (used with a microscope). Any ideas?

Also, am I correct that a 20W load (the bulb) at 3.3V would pull ~6A? I ask as I found a power supply that puts out 3, 4.5 and 6 volts but is rated at 2A.
http://www.powersupplieswarehouse.com/mw-powersupply.html

Thanks

2. BobK

7,682
1,686
Jan 5, 2010
The 20W rating of the bulb is at some specific voltage. At that voltage, V, it will draw 20/V Amps. At a lower voltage it will draw less current, but not linearly, because the resistance of the filament is temparature dependent. It will therefore draw more than half the rated current as half the voltage.

Bob

3. flhtci2006

2
0
Feb 13, 2013
Agree....it's 20W at 6 volts which is a 3.4A pull. So, if I had an output of 4V (to make the bulb less bright), it would pull 5A. If so, that means I need a pretty hefty power supply to run that bulb.

The power supply we had was just a transformer with three secondaries, something like a 3.5V, 4.5V and 6V with each tap going to a selector switch allowing us to dial one of the three voltages. The transformer went tits up and I cannot find a replacement. A new power supply is \$250 which seems high to me.

Last edited: Feb 13, 2013
4. BobK

7,682
1,686
Jan 5, 2010
If it is rated as 20W at 6.3V it will not use 20W at 4V, it will use something less, which is why it would be less bright! It will certainly draw less current at 4V than it does at 6.3V. But, as I tried to explain the relationship is not linear as it would be for a resistor.

If a resistor pulled 3.2A at 6.3 volts, then at 4V it would pull 3.2 * 4 / 6.3, which is from Ohms law. But a light bulb is not a resistor, it's resistance increases with temperature, so it would pull more current at 4V than expected by the above equation becuase it is not as hot as it would be at 6.3V.

You wil be safe if you get a supply that will deleiver over 3.2A (say 3.5A) at any of the voltages.

Bob