# Dumb question

• posted

I have a landscape lighting unit that can turn on whatever is wired into it for 2, 4, 6, or 8 hours from when the photo cell trigers the unit. My problem is that the unit needs a minimum load ot 20 watts to operate (max 300W). It puts out 12 or 14 volts. I want to use a 14v. relay to turn on a nouber of light which would far exceed the 300W max. If I can figure what the relay draws how do I figure what to add in parallel with the relay to add up to 20 watts to make the unit work. I guess it would be a resistor of some sort. But how do I figure the ohm rating and power requirement?

TIA

Peter Moseley

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Light bulb

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Boris Mohar```
• posted

Could you just put 20 watts worth of lights in parallel with the relay to make the photo cell unit happy?

M

pmoseley wrote:

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There's been an ugly rumor going around that Ohm's Law might be somewhat useful here.

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073```
• posted

OK the way I understand it is as follows: 20W/V=1.429I. And using R=V/I (be gentle with me guys. It's been a long time since I've done this) R=14/1.429=9.8 Ohms. Radio Shack has a 10 Ohm, 10W wire-wound resistor. So, I guess two of these in parallel would do it. Right?

TIA Peter Moseley

• posted

Sounds like you're using what is usually called an "electronic transformer" intended for low voltage lighting applications. That would then be a 12 or 24 volt output selection and not an oddball 14 volts. The minimum loading requirement of 20W seems to be a standard and is something they call a "demand circuit design." If your "unit" is in fact an "electronic transformer" of this type, then it is most likely a high frequency inverter type, producing a low voltage AC at several tens of thousands of kilohertz. This would make sense because high frequency operation makes for much smaller size (volume, "footprint," or what have you) which is what most people are after when they use the "electronic transformer." If your limited concentration span has lasted this far then you need to know that an ordinary low voltage relay will not operate at this of a high frequency, and this aside from the fact that a

14V relay is rather rare.
• posted

On Mon, 3 Dec 2007 15:42:18 -0800 (PST) in sci.electronics.design, pmoseley wrote,

Just use 20 watts of your lighting, then put the rest on the relay. Don't waste the 20 watts.

• posted

Plus, if the original poster uses resistors, depending on how small (physically) they are, they can get pretty hot, dissipating 20 watts. That's a lot of heat, especially if placed in an enclosed space - could possibly start a fire.

Just my 2 cents, and fwiw, I'm not an electronics/electrical expert by any stretch of the imagination

M
• posted

nope. two 10 ohm rs in || would be 5 ohms, consuming 40 W total or 20 W per, and goodby resistors. unless you need a space heater also, forget using resistors, and load it down with some automotive bulbs which handle 13.8 V or so nicely - if, in fact, that thing really has a 14 V option - otherwise, 12 V isn't such a bad choice. I suppose 24 snipped-for-privacy@gmail.com auto bulbs are available since, IIRC military trucks use 2 batts in series for 24 V - can't remember why.

mike

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nope. two 10 ohm rs in || would be 5 ohms, consuming 40 W total or 20 W per, and goodby resistors. unless you need a space heater also, forget using resistors, and load it down with some automotive bulbs which handle 13.8 V or so nicely - if, in fact, that thing really has a 14 V option - otherwise, 12 V isn't such a bad choice. I suppose 24 V auto bulbs are available since, IIRC military trucks use 2 batts in series for 24 V - can't remember why.

mike

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OK. Thanks Mike. How about a 5 ohm 50 watt resistor with the 12V output. That would pull 2.4 amps for 28.8 watts. I'm worried that a

12V lamp would burn out. If the resistor doesn't put out too much heat I can just plug it in and forget it.

peter

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Now you get more heat. At one time some people might not care, but with rising energy costs these days, you might put them in a box for a bun warmer ;) whatever works. Do it if there's nothing better staring you in the face.

Mike

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28.8 watts will make that resistor hot. Use 5 25 ohm 25 watt resistors in parallel instead to spread the heat out over a much larger area.

Ed

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