# Need help calculating resistance from specs of an LED

Discussion in 'The Projects Forum' started by boule, Nov 13, 2012.

1. ### boule Thread Starter New Member

Nov 13, 2012
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0
I need to have an LED floodlight parallel to a relay coil, I want to learn to design it correctly and I'm worried about the resistances. They have a 120VAC source. The specs for the relay coil are "Input voltage: 120VAC, Rated load current= 12Amps" and the specs for the light (http://www.ledlight.com/SearchResult.aspx?KeyWords=35644) are "Consumes 10W, powered by 100-240VAC." From my knowledge, the LED current is P/V=10W/120V≈83mA, thus the resistance of the light is 120/.083=1440 ohms. I have no idea if this is the right way to calculate the resistance of my light. Also from my knowledge, the coil is 120V/12A=10 ohms.

*If the lights specs are 100-240VAC, does that mean the voltage is a variable, or is it forced by my 120Vac power source? (Is voltage a changing variable? Which variables change or stay constant)?

*How exactly would I calculate the resistance of the light (on paper, w/out an ohmmeter)?

*If I am correct in my calculations, does that mean I need to add a 1430 ohm resistor in series with my relay coil to make the current flow evenly through the parallel wires?

2. ### crutschow Expert

Mar 14, 2008
23,136
6,862
The 12A relay spec is the contact load rating, not the coil. The coil current will be much less.

If the LED is rated for 100-240VAC then it is likely using some type of constant-current switching regulator that provides 10W to the LED independent of the applied voltage. The current and equivalent resistance you calculated for 120V is correct but, for a constant LED wattage, this current and equivalent resistance will vary with the voltage.

Don't understand you concern about making the current flow evenly in the two parallel wires. Each wire in a parallel connection can carry whatever current it needs. It's only in a series connection that all loads must carry the same current.

3. ### boule Thread Starter New Member

Nov 13, 2012
10
0
Crutschow:
The reason I was worried is because I didn't understand that the "equivalent resistance" and current varied in order to keep the light at 10 watts, I thought the Watts might have varied according to how much current goes through.

Also, I looked @ the relay's spec page, and one of the specs of the "Input: Maximum Consumption AC: 5 VA." Does VA≈wattage, and from what I learned just now, is this what the coil needs to keep constant (and thus varying the equivalent resistance/current?

4. ### Audioguru Expert

Dec 20, 2007
11,251
1,350
The LED lamp is designed to be powered from your 120VAC so why are you worrying about it?

Is the relay designed to have its coil activated from your 120VAC?

What will the relay be used for?

5. ### boule Thread Starter New Member

Nov 13, 2012
10
0
It is designed to have its coil activated from 120V.

Its contact is actually going to be used in series to switch itself and the light on and keep it on. So (switch)---series with--<====(lamp)//(coil)==> and the lamp and coil are in parallel.

Last edited: Nov 13, 2012
6. ### crutschow Expert

Mar 14, 2008
23,136
6,862
You lost me. Can you post a diagram of what you are trying to do?

7. ### boule Thread Starter New Member

Nov 13, 2012
10
0
Crutschow:
http://imgur.com/OaBDY

With this configuration, my question was do I have to worry about the resistances in the coil and light (because there are no resistance ratings for the coil or LED)? And then you said I didn't have to worry about the current since they would carry whatever current they need, so my power source (a regular American outlet 3 prong plug) has enough current? Or am I worrying way too much (since the coil and LED are both designed to run on 120Vac)?

8. ### Audioguru Expert

Dec 20, 2007
11,251
1,350
An American 120VAC outlet can supply 15A so its maximum power rating is 120 x 15= 1800W.
The relay coil uses only 5W and the LED uses only 10W so what are you worrying about?

I don't know why you have the relay.

9. ### crutschow Expert

Mar 14, 2008
23,136
6,862
It's configured as a push-button operated latching relay.

Question for the OP. How are you planning on turning the relay and lamp off once it's on?

10. ### boule Thread Starter New Member

Nov 13, 2012
10
0
It's a time delay relay, turns off by itself after a set time.

Thanks for all of the help!