I have a small lighted billboard powered by a 12VDC transformer. I want to tone down the brightness of the display. If I add a couple of 1k Ohm resistors to the hot side of the circuit do I add them in series or in parallel? Will that accomplish what I'm after?
Bob
Original Post

To use a resistor you need the bulb wattage and have to calculate size and wattage of the resistor used. I dim with diodes,use appropriate amperage diodes. A single string in one direction will drop voltage on DC. Described here for AC again you only need a single string and not paired diodes. each diode in series will drop about .6 volts.

http://www.jcstudiosinc.com/Bl...d=413&categoryId=426

Dale H
Unless it's LED lighting, a couple of 1K resistors will turn it off totally! Dale's suggestion will work for LED's or incandescent lighting.
Assuming this is a few small bulbs directly connected to power, here's a starting point based on Dale's diode method. This shows 3 type 1N4001 diodes where the more diodes the more dimming. I'd guess you'll need somewhere between 2 and 6. The so-called polarity bands of the diodes must be oriented as shown. Radio Shack sells a value-pack of 25 equivalent diodes for \$2.99. Or if you're not in a rush, search eBay for "1N4001" and you can pick up 50 for \$0.99 w/free shipping from Asia.

A single resistor could work and would be positioned in place of the diodes. Per above, it's hard to suggest a value without details. But if your lamps are incandescents, suitable values would be closer to 10 ohms rather than 1k ohms.

Thanks for your help, fellas. Just a clarification - the billboard is new and uses blue/white LED's. So I assume that if I use one 1k resistor that should drop the brightness. If I need more "dimming" than that I just hook a couple together end to end. Is that correct?
Bob
OK, LEDs. A single 1k will definitely drop the brightness and like you say you'll get more dimming with two. No harm in doing so. I'm guessing that 1k may actually be too large in which case take two 1k and tie them in parallel (side-by-side, rather than end-to-end) to effectively make a 500 ohm (=1k /2) resistor. And if that is still too dim, put 3 of the 1k side-by-side in parallel to make a 333 ohm (=1k /3) resistor.
Thank you Stan. That is precisely what I wanted to know. I appreciate your help.
Bob
You also don't have to stick with 1K resistors, if you need a 330 ohm resistor, they do exist.
OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
330-757-3020

www.ogaugerr.com
×
×
×
×