# Resistors in parallel

#### stellanhaglund

Joined Mar 31, 2017
23
Hi!

I'm trying to figure out a specific resistance of at least 36ohm that can dissipate about 220W.

I have done some calculations using 100ohm 10w resistors but if I put them in parallel to get 220W I will just get 100/22ohms right?

So I'm wondering if there is a good way to figure out my options for resistors or do I have to try every combination I can come up with?

Not that experienced yet so please be humble!

#### Dodgydave

Joined Jun 22, 2012
8,469
Correct, with 10W resistors, you will get 22 resistors in parallel @ 4.5 ohms 220W..

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#### dl324

Joined Mar 30, 2015
9,121
What are the resistors being used for?

#### crutschow

Joined Mar 14, 2008
23,500
How about 3 rows of eleven 10W, 10Ω resistors in series, with the three rows tied in parallel.
That will give a total resistance of ≈36.7Ω with a total power rating of 330W for a good 1/3rd power margin.

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#### shortbus

Joined Sep 30, 2009
7,078
Hi!

I'm trying to figure out a specific resistance of at least 36ohm that can dissipate about 220W.

I have done some calculations using 100ohm 10w resistors but if I put them in parallel to get 220W I will just get 100/22ohms right?

So I'm wondering if there is a good way to figure out my options for resistors or do I have to try every combination I can come up with?

Not that experienced yet so please be humble!
Is this still for your WEDM? This is one of the things that is covered pretty good in the Langlois book. Personally for mine I bought what is commonly called corrib resistors, while they a pretty expensive new, they are found on Ebay real cheap. One other nice thing is they are adjustable, so you can fine tune the needed resistance if needed.

That link I gave you to the other WEDM build also gives some limited information on how to figure the resistors too. Though the actual build uses light bulbs. But that is what was used in the original Russian EDM circuits, so don't rule that out. Russians were the originators of EDM after all.

If you didn't decide to buy the book, there is a way to get it from Scribed.com if you sign up there.
https://www.scribd.com/document/244272841/Build-an-EDM-by-Robet-Langlois

#### WBahn

Joined Mar 31, 2012
24,850
Hi!

I'm trying to figure out a specific resistance of at least 36ohm that can dissipate about 220W.

I have done some calculations using 100ohm 10w resistors but if I put them in parallel to get 220W I will just get 100/22ohms right?

So I'm wondering if there is a good way to figure out my options for resistors or do I have to try every combination I can come up with?

Not that experienced yet so please be humble!
There are certainly lots of potential ways to do this, but let's go for low hanging fruit.

Simplest: A square matrix of equal resistors in which you have N strings in parallel, each containing N strings in series. The resulting equivalent resistance is the same as the value of the individual resistors, so here you would use 36 Ω resistors. If you allow a power rating margin of 100%, then you want the ability to dissipate 440 W. You will be doing that pretty evenly over N² resistors. If each resistor can dissipate 10 W, let's say, then you need

N = sqrt(440 W/ 10) = , so you would need N = 6.63. Normally you would round that up to 7 which would be 49 resistors giving you a power rating of 490 W. But if we round down to 6 that would still give you a power rating of 360 W, which is likely enough (you have to make that call).

This is simplest because you only have one free parameter -- the power rating of the individual resistors.

Next Simplest: A rectangular matrix of equal resistors. Here you have three free parameters, so there are a lot more combinations to consider.

Regardless of how you end up arranging things, one thing to keep in mind is that the power rating of a resistor makes certain assumptions, most notably that it is operating in an environment in which the background temperature is ambient. With resistor arrays like this, the heat getting dumped into each resistor isn't only coming from the power being dissipated in that resistor, but also from the heat being dissipated by other nearby resistors unless you ensure that the heat from each resistor is carried away without any significant coupling to the surrounding resistors.