Use a used resistor

[uligli]

Can i use a used resistor?

[PCBONEZ]

Re: Use a used resistor

yes

[stj]

Re: Use a used resistor

yes - *IF*
it still reads correctly on a meter.

i'v had resistors change value without looking any different on the outside.

[uligli]

Re: Use a used resistor

yes - *IF*
it still reads correctly on a meter.

i'v had resistors change value without looking any different on the outside.

I'm trying to find out the problem on a Hyundai LCD's invt/psu board, one of the smd resistors reads different values when i switch the - & + probes on it, and i don't think it's normal, should i pull it out of circuit to retest?

[182] should be 1.8k ohms, i get 1740 omhs(value going up slowly on the DMM) and 192 omhs.

[momaka]

Re: Use a used resistor

yes - *IF*
it still reads correctly on a meter.

I agree - definitely check the resistor first with a multimeter before using it. Note the tolerance on the 4th band and see if the resistance is within its range. So for example, if your 1800 Ohm resistor has a Gold band (5% tolerance), then its value can be +/-90 Ohms of that (1800 * 0.05 = 90) - i.e., as low as 1710 Ohms or as high as 1890 Ohms.

should i pull it out of circuit to retest?

If you want accurate measurements, then YES.

Otherwise if you know how the resistor is connected in the circuit, you may be able to deduce a range of acceptable values for what it will read on the multimeter.
For example, the resistor you have in your circuit may be connected in series with a diode and then have that connected across the pins of a transformer - in which case the transformer will act as a short between the diode's other end and the resistor and the result will be the same as if you had a diode and a resistor in series in a closed loop. Basically what will happen then is the resistor will read normal value in one direction and much lower in the other due to the lower resistance of the diode (diodes usually have 100-1000 Ohms resistance).

Resistors also tend to go open circuit rather then short when they go bad. Thus, when measuring a resistor in circuit, *generally* a lower resistance than the stated one is acceptable (since there may be other components in the circuit that could render this situation). A higher resistance that is out of tolerance, however, is not acceptable and the resistor should be pulled out of the circuit and checked.

[kc8adu]

Re: Use a used resistor

you are forward biasing a semiconductor.
resistor is ok.
as for age some age better than others.
used is fine if in tolerance.i very rarely will find a noisy carbon comp but its not unusual for me to use a 50+ year old resistor in a repair.mostly in vintage gear to preserve the appearance.

[MXM]

Re: Use a used resistor

^ nice inputs.

i would use a used resistor, tipically if i cant find the same value on a new one, i just make sure matches properties, also giving correct values in the mm and thats it.... (just on praxis i try to avoid it but yeah if theres no other way.... wel... wth )

jm2c

peace.

[uligli]

Re: Use a used resistor

I agree - definitely check the resistor first with a multimeter before using it. Note the tolerance on the 4th band and see if the resistance is within its range. So for example, if your 1800 Ohm resistor has a Gold band (5% tolerance), then its value can be +/-90 Ohms of that (1800 * 0.05 = 90) - i.e., as low as 1710 Ohms or as high as 1890 Ohms.


If you want accurate measurements, then YES.

Otherwise if you know how the resistor is connected in the circuit, you may be able to deduce a range of acceptable values for what it will read on the multimeter.
For example, the resistor you have in your circuit may be connected in series with a diode and then have that connected across the pins of a transformer - in which case the transformer will act as a short between the diode's other end and the resistor and the result will be the same as if you had a diode and a resistor in series in a closed loop. Basically what will happen then is the resistor will read normal value in one direction and much lower in the other due to the lower resistance of the diode (diodes usually have 100-1000 Ohms resistance).

Resistors also tend to go open circuit rather then short when they go bad. Thus, when measuring a resistor in circuit, *generally* a lower resistance than the stated one is acceptable (since there may be other components in the circuit that could render this situation). A higher resistance that is out of tolerance, however, is not acceptable and the resistor should be pulled out of the circuit and checked.

Very good explanation thank you