# building my power resistor

Discussion in 'Electronic Basics' started by Jif, Mar 3, 2004.

1. ### JifGuest

hi

i am trying to wind or build a power resistor that is:
1.6 ohm and the resistor power is > 90 (12V*7.5(i))

what is the formula and what is a good wire to use i have may sizes of enamel copper
wire.

thanks

Jif

2. ### Bill BowdenGuest

Well, there are several possibilities. You could use 1000 feet
of #12 copper wire which would be 1.619 ohms, but that's a
lot of wire. Maybe you could use a smaller gauge like #26
which is .0416 ohms per foot so you need about 38 feet.
At 90 watts you get about 2.3 watts per foot which wouldn't
heat up much assuming you spread out the wire so the air
can cool it off and keep the temperature down. The resistance
will increase a bit with temperature, but I don't have the
numbers.

Various copper wire sizes and resistances are:

Gauge / Turns per enamel inch / Resistance per 1000 ft.

10/9.6/1.018 12/12/1.619 14/15/2.575 16/19/4.1 18/23.6/5.5
20/29.4/10.35 22/37/16.46 24/46.3/26.2 26/58/41.6 28/72.7/66.2
30/90.5/105 32/113/167 34/143/266 36/175/423 38/224/673

-Bill

3. ### CFoley1064Guest

Subject: building my power resistor
Copper wire isn't a very good choice for power resistors, because its
resistance will change dramatically as it heats.

As Don Lancaster has said, the best source of nichrome wire is buying a power
wirewound resistor, gently cracking the coating, and removing the wire that way
by breaking the welds at the end caps or lugs.

A 1.6 ohm, 300 watt Ohmite resistor is available for \$25.00 from Surplus Sales
of Nebraska. Consider twice before re-inventing the wheel.

http://www.surplussales.com/Resistors/WW1-5025.html

Good luck
Chris

4. ### Bob MastaGuest

If you want this for a dummy load or something, you might
consider as a general strategy using series and parallel
connections of other values. Two equal-value resistors
connected in series or parallel will have a power rating
that is the sum of the individual ratings. You can often
find oddball size resistors on sale in large lots, then
play around with the math to get the right series/parallel
combination.

Of course, if you elect to combine 90 one-watt resistors
to get a 90-watt total, you will need to do a lot of soldering.
But having a lot of resistors spread out over a large area
will certainly help with heat dissipation.

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

6. ### JeffMGuest

build a power resistor that is:
Copper wire is for inductors. Nichrome is for resistors.

Easiest way: a 225W resistor with a slider.
You can reuse it for other similar values.

Way most of us do it: junk-box parts series-paralleled like Masta said.

Cheapest way: Keep busted appliances around?
Strip the nichrome out of a junked toaster, toaster oven, iron, or heater.
Making the copper connection to the nickel-chromium metal is the hard part.

7. ### MichaelGuest

I think noone else has mentioned this yet but it might be important in
If you end up making it from wire, and if you wind it, it could end up
having significant inductance depending on how close the turns are to
each other. Winding a layer in the reverse direction of the layer
underneath it can help decrease inductance.

8. ### Gary LecomteGuest

Another alternative is to use a short length of nicrome from an old
hotplate and submerge it in water or oil to keep it cool. Particularly
useful if the load is only used for short periods. Even an undersized
wire can be used.

Gary

9. ### Watson A.Name \Watt Sun - the Dark Remover\Guest

I would use six 10 ohm 20 watt or 15 watt resistors in parallel, and a
39 ohm 5 watt in parallel to bring it down to 1.6 ohms. The 10 ohm
resistors are a standard value and are less espensive than a single
resistor. Another thought: use a 12V headlamp or similar if the
resistance is not critical.

10. ### DarkMatterGuest

Copper wire is rated by ampacity. #26 can only handle one amp
before it heats excessively, and that is for open air. Cramped up on a
tube, and shellacked over, it would probably heat up a lot!

At a one amp max, the resistor would only be able to pass a couple
watts before heating up big time.

First off, the better wire for this is resistance wire, specifically

Short of that, buying two 10% tolerance 3.3 Ohm high power resistors
and putting them in parallel would likely be cheaper both time and
money wise. Especially time wise. My time is worth a lot, especially
my free time.

11. ### DarkMatterGuest

This is not true. If their power ratings are unequal, the bank will
be limited to that power which makes the smaller rated resistor max
out in dissipation. That power level is NOT the sum of the two power
levels of the resistors. It is two times the smallest power value.

Also, one should not dissipate into a power resistor at the rated
power. If one want to dissipate 90 watts, one should get a 200 watt
resistor. At the very least 1.5 times the desired dissipation rate.

12. ### DarkMatterGuest

It's for DC from what I saw. If that is the case, it won't matter.
Particularly since there is no magnetic core involved.

13. ### DarkMatterGuest

A series of oven heating coils. They usually dissipate 1200 plus
watts each.

14. ### DarkMatterGuest

You guys keep forgetting that if he is making 90 watts, he needs a
150 to 200 watt resistor to work this task correctly, and safely.