idiot needs help with parallel resistance calc

[rotarymike]

And yes, I'm the idiot.

I'm rebuilding an old automobile wiper switch tester that used light bulbs to put a load across the relays and electronics in the switch. Basically, plug the switch into the box, switch wipers into low for example, and a light corresponding to low comes on.

I'm rebuilding it much much smaller and using some surplus 3mm white LEDs I have. They have a Vf of 3.0-3.8 and (it says) Iv of 6000. They came with 680 ohm resistors to use with 12-14.4V, so that tells me realistic forward current is about 20 mA.

I want to put a larger ceramic resistor (10W or so) parallel with the LED/C.L. resistor so most of the current goes through the big R, like a meter shunt, and so the switch under test sees a similar (resistive) load to the motor. The LED circuit is intended to indicate whether the switch is working properly.

I can't remember whether I can treat the LED as not there and do the calcs as two parallel resistors, or what. I can't find my old Air Force tech stuff either (wife probably has it in the attic). I'd done due diligence with Google and not found much. Any help?

[Robert Reed]

Your LED circuit will consume about 1/4 watt. If this load is parrelled by an upwards of 10 watt load, I don't think you need to be concerned about them.

[dyarker]

Agree with Robert. The load is going to be in 100s of mA or Amp range right? Then ignore the 20mA of LED ındıcators.

To answer question ıin subject:

Rparallel = 1 / ((1/R1) + (1/R2) + ... + (1/Rn))

In words: Convert all resistances to conductances (Mohs = 1/Ohms) by inverting, add the conductances, invert again to convert total conductance to parallel resistance.

Cheers,

[rotarymike]

thanks folks. I thought that the 20 mA would be insignificant, but I didn't want to drop an amp through the LED and let out the magic smoke.

[Bob Scott]

I use a really easy way to calculate parallel resistances.
Let's say I have 3 ohm, a 4 ohm, and a 5 ohm resistors in parallel. I just use the following key sequence on my TI-25 calculator:

3
1/x
+
4
1/x
+
5
1/x
=
1/x

With some of those Chinese designed TI calculators you have to use the X^(-1) key instead. My TI-30X scientific doesn't have a 1/X key.

[haklesup]

The formulas are correct and since you seem to be a DIY type work just fine but you can also google "parallel resistance calculator" to come up with a bunch of web pages to do it for you. (just a little shortcut)

[CeaSaR]

With some of those Chinese designed TI calculators you have to use the X^(-1) key instead. My TI-30X scientific doesn't have a 1/X key.

The 30X series is bizarre. My 30XA has the 1/X key while my son's 30XIIS doesn't. Which 30X(?) do you have?

CeaSaR

[Bob Scott]

The 30X series is bizarre. My 30XA has the 1/X key while my son's 30XIIS doesn't. Which 30X(?) do you have?

I have the 30XIIS. I don't like it. You have to enter the argument AFTER a function. You can't just enter a number and press the SIN button to get the sine.

Also, the thing is about 7 years old. A couple of columns of buttons stopped working. The 30IIS has flexible printed circuit pressure sensitive adhesive tape to connect the keyboard to the main board. The press-on connections went bad. I used a sharp needle to poke through the tape into the connections in several places per connection. Now it works again.

It was fun after I accidentally spilled all the keys on the floor, finding out which button went where. I wish they had not stopped making the TI-25. It was designed for humans, not alien engineers.

Please pardon this off-topic digression.