# Power Resistor

Discussion in 'General Electronics Chat' started by Weasel, Dec 23, 2002.

1. ### WeaselNew Member

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Hi all,

Just found this forum and it looks excellent.

I am just starting in electronics so apologies for the daft questions in advance.

I am trying to measure the resistance of several aluminium cased power resistors, but they are all reading "10" on my digital multimeter (selector set to 2000M). If I set the meter to any other setting I just get "0".

One resistor has the following printed on the casing "Arco/ 87.13 HS25/R27 +- 5%.

I think this is a 27 Ohm 25 Watt 5% Power resistor, but cannot figure out how to check it with the meter.

Thanks, :?:

2. ### mechieNew Member

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Codes

Hiya Weasel, welcome aboard.

The only important info on the resistor you quoted is "R27 +- 5%"

It is normal to use the multiplier (in this case "R") to represent the decimal point. So this is a 0.27 Ohm resistor with a tolerance of +/- 5%.

Its value is therefore in the range of 0.2565 to 0.27135 Ohms

Multipliers used instead of decimal points are :-
R = Ohms
k = kiloOhms
M = megaOhms

u = micro (for capacitors !)

Small capacitors also have a weird system of showing the multiplier as a third digit, for example "103", the 3 means 3 zeros (value in pF).

3. ### kinjalgpActive Member

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I think it is selector set to 2000m i.e 2 Ohms.
M=Mega Ohms
m=milli Ohms

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5. ### WeaselNew Member

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Thanks for that guys.

The multimeter settings for resistance are 2K, 20K, 200K, 2000K, 20M & 2000M. (There's also one above 2000M, small arrow pointing left to right against a bisected line).

I still do not understand why the meter will not measure these power resistors though. I tested it on the small resistors (13k) and it seems to work fine.

Thanks,

6. ### mechieNew Member

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Multimeter Ranges

If you select the lowest range on your meter (2k) and i assume it is a three-and-a-half digit display (maximum indication is 1999) then the smallest it can display is 1 (ignoring zero :wink: )

If we assume the meter's calibration is spot-on, the test leads have zero resistance, and your resistor is 1.5 Ohms then the meter will show either 1 or 2 -- the display's limit of resolution, if we assume it rounds to the nearest integer (rather than just trunkating) it will show 2.

If the resistor is 0.49 Ohms or less then the nearest whole value is zero.

We have made several assumptions here, the biggest being the meter's calibration. In industry it is normal to have periodic cal checks against a Standard (in the UK this is now UKAS or traceable to the same). A cal certificate will then provide guidance on corrections required for the meter.
[ This is fine for work where the employer pays -- my home meter has never been checked ]
Even then you are at the extremes of your meter so you can't expect a good reading.

The display of '10' for the 2Gig (!) range must be a meter fault.

7. ### bogdanfirstNew Member

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u mesured te resistor on a 2000M scale that is 2000000000ohms, if u sesured 10 that means 10megaohms(10 milion ohmas), so my guesss is that it's interupted...
i got a question, does the value fluctuate or is it constant...
also from what u said it is a 0.27 ohm resistor, so if u mesure it on the smallest scale(2K) it will show zero because compared to a 2000 ohm scale(2K) 0.27 ohm is like a shortcirsuit.....
about what u said a scale with an arrow and a veritcal line, something like this ->|-, that;s a diode...it mesures aomething else...
so i hope i helped somehow.... :roll:

8. ### WeaselNew Member

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Thanks again,

Erm.... I think I may have sussed it now!

There was another setting below the 2K.

This one is just 200 (I think 200 Ohm)

When I connect the resistor to this it reads 00.5 and an alarm sounds from the meter (continuous tone)

I have tried it on another two power resistors that are soldered together in series (27R & 30R) and it reads 56.7 with the meter on the "200" setting. I guess the first resistor is definitely a 0.27 Ohm and the meter just rounds up to 0.5

9. ### bogdanfirstNew Member

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ya, i think so....
it shows 0.5 because there is also mesured the resistance of the wires, advanced meters have a setting, u shortcircuit the wires, press a button and the resistance is set to zero, so if u would mesure it it would probably show 0.3 rounding it