# Dumb question

Discussion in 'Electronic Design' started by pmoseley, Dec 3, 2007.

1. ### pmoseleyGuest

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

Light bulb

3. ### Guest

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

M

4. ### Don LancasterGuest

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

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Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

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5. ### pmoseleyGuest

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

6. ### Fred BloggsGuest

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.

7. ### David HarmonGuest

On Mon, 3 Dec 2007 15:42:18 -0800 (PST) in sci.electronics.design,
Just use 20 watts of your lighting, then put the rest on the relay.
Don't waste the 20 watts.

8. ### Guest

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

9. ### Active8Guest

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
auto bulbs are available since, IIRC military
trucks use 2 batts in series for 24 V - can't remember why.

mike

10. ### Active8Guest

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

11. ### pmoseleyGuest

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

12. ### Active8Guest

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

13. ### ehsjrGuest

28.8 watts will make that resistor hot. Use 5 25 ohm