Dropping 20 to 30 volts off the incoming supply

[Jason Hubbard]

Sorry, this is very off-topic as far as audio is concerned, but i thought i'd ask the question as there are so many knowledgeable people out there.

I have some IR lamps for illuminating the drive for the CCTV camera's at night, but the voltage in my area is high due to light local transformer loadings at around the 250v mark. This inevitably causes problems with reduced lifespan of light bulbs (they typically only last half as long as they should) and IR bulbs are expensive.

The IR lamp manufacturer used to make a voltage reduction unit but this is now obsolete.

Any thoughts on what my options might be to drop 20v to 30 volts off the incoming mains (other than simply using a large resistor or a variac)?

I'd already thought of using some kind of SCR circuit, maybe with a slow start option to get a little more life out of the filaments - does anybody have something along these lines or an alternative suggestion?

Blbs are either 300 or 500 watts depending on which bulb is installed - i currently have 300 watts but would like the option of the extra wattage.

regards

Jason

[Guy Grotke]

You could do a number of different things. The easiest is probably to just use a lamp dimmer running at about 90-95%. That won't clip the tops of the waves off, but it will keep the average power in the range you want. For a lamp with a filament, that should work fine. Very efficient.

Other options: Get a transformer that is designed for the job. Not too expensive for 500 Watts, since that is only a couple of Amps. Fairly efficient. If you are the DIY-type, you could even wind your own! Autotransformer (ie. Variac) will be easier to find.

Big resistor! 300 Watts @ 225 volts = 1.333 Amps
25 volts / 1.333 Amps = 18.75 Ohms
25 volts * 1.333 Amps = 33.3 Watts

[Jason Hubbard]

I was thinking that the cheapest way forward is probably just to use a normal domestic light dimmer with a thermistor to limit inrush current.

Thanks for your reply

[Guy Grotke]

So I assume those lamps do have filaments? Then you don't really need to lower the voltage. Limiting the average power with the standard dimmer would be fine.

You don't even need to add a thermister to limit the inrush current: Just turn them on with the dimmer in low position, then turn the knob up to your preset maximum position. The time it takes you to turn the knob is plenty of time for them to warm up.

[Jason Hubbard]

The IR lamps are switched on when light falls by a photocell, so i will not be there to switch them on....