[sdiy] Stupid resistor tricks?

[Magnus Danielson]

From: Bobinvertedsdiy at aol.com
Subject: [sdiy] Stupid resistor tricks?
Date: Fri, 4 Apr 2003 12:02:44 EST

> Mail order is my only real option for parts. I often find myself wanting to 
> experiment, but not having the right stuff on hand. I'm wondering if I need, 
> for example, a 1.2k 1watt resistor could I get by with a 1k 1watt and a 200 
> ohm 1/4watt resistor in series?

Since P=UI=I²R we get for the total amount:

1 = I² * 1,2k => I² = 1/1,2k

For the 1 kOhm resistor we get

P  = I² * 1k = 1/1,2 = 5/6 W
 1

For the 200 Ohm resistor we get

P  = I² * 200 = 0,2/1,2 = 1/6 W
 2

Since both resistors is within spec (P1 <= 1W and P2 <= 250 mW) your home free.

Notice that I didn't bother actually finding out what I was, only what I² was.

> Also would two 1/4 watt resistors in parallel be an appropriate substitue for
> a 1/2 watt resistor?

Yes, if they have the same size. That operates by current-sharing. The
tolerances of the resistors decides how well-balanced the current-sharing is,
but yes, it works.

> Works in my brain, real life is probably a different story! :(

If you really is to use components on their limits, be sure to put them
sufficiently apart so that they each have sufficient cooling. You can get more
out of compoents when you do not stress them both thermically and electrically
at the same time.

Cheers,
Magnus