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Topic: Wrong labeled resistors? (Read 2264 times) previous topic - next topic

artvolk

Good day!

I'm building this simple LED driver from here (http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/):



I haven't 3.9Ohm, 0.5W resistors, so I used 3 in parallel 12 Ohm, 0.25 resistors at first (12/3 = 4, 1,25/3 = 310mA) . The measured current was about 310mA as it should be.

Next day I've bought 0,5W resistors labeled as (orange/white/gold/gold) -- they should be 3,9Ohm +/-5%, but my multimeter says that they are 4,6 Ohm. I've connected them to the driver and current is about 250mA (close to 1,25/4,6 = 270mA).

Are resistor wrong labeled or I'm missing something?

retrolefty

Hard to say, as most multimeters are not real accurate at such low ohm range. And there is the resistance of your meter leads to consider and factor for.

artvolk

Accuracy of multimeter was one reason I was thinking off. If I connect two resistors in-series it gives me 8,3 Ohm (~4,2 each), if three resistors -- 12,2 (~4 Ohm each)...

Is there any other way to check?

retrolefty

#3
Feb 02, 2011, 09:18 am Last Edit: Feb 02, 2011, 09:22 am by retrolefty Reason: 1
Quote
Is there any other way to check?


A few maybe. Most meters are more accurate measuring voltage and current then ohms. So if you wired a series 100 ohm resistor (any size to lower the series current) with your low ohm resistor(s) and wire it to a stable regulated DC voltage of 5vdc. Then measure the voltage drop across the low ohm resistor(s), and then measure the current in series with the voltage source, you can then calculate what the low ohm resistor is, R = Vr/I.

Resistors will even change their value a little depending on what temperature they are running at, and at 300ma you are heating them up a bit, no?

Digital multimeters quality and accuracy varies with cost of the meter.

Lefty

artvolk

#4
Feb 02, 2011, 11:03 am Last Edit: Feb 02, 2011, 11:05 am by artvolk Reason: 1

A few maybe. Most meters are more accurate measuring voltage and current then ohms. So if you wired a series 100 ohm resistor (any size to lower the series current) with your low ohm resistor(s) and wire it to a stable regulated DC voltage of 5vdc. Then measure the voltage drop across the low ohm resistor(s), and then measure the current in series with the voltage source, you can then calculate what the low ohm resistor is, R = Vr/I.

Thanks, I'll try this.


Resistors will even change their value a little depending on what temperature they are running at, and at 300ma you are heating them up a bit, no?

I've running the whole thing about a hour and resistors are cold (LM317 are warm, but not too hot).


Digital multimeters quality and accuracy varies with cost of the meter.

Mine is the cheapest one, born in China :)

retrolefty

At least you have a meter and understand the need to utilize it in helping to build circuits. Many new comers here jump right in wiring things to their arduino with no knowledge of the voltage, current or resistances they are working with. Electrons are invisible so we need meters and scopes.  :)



johndimo

A quick way to check the resistance on a resistor that low of value is to first measure the resistor. Then short out your leads to measure the resistance in the leads (most affordable leads will read .3 to .5 on a good meter). Subtract the resistance of your leads from the value you read when you measured the resistor.

Also, remember the tolerance of 5%. For a 3.9 ohm resistor, the value can be anywhere from 3.7 ohms to 4.1 ohms.

artvolk

Thanks all for the responses, here how I solved it -- I just connected resistor directly to my multimeter sockets (without leads -- and they reads from 3.9 to 4.0. The measured resistance of leads are 0.5 Ohm.

Thanks all!

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