Measuring Resistance of Metal Oxide Power Resistor

[AlTaiRius]

Can I measure a Metal Oxide Power Resistor the normal way?

When I put my fluke on resistance and measure across the legs (I've removed it) I get 22 kOhms. Reading the bands I'm expecting this to be a 330 mOhm +/- 5% resistor.

Am I reading it wrong or is it stuffed?

[eccerr0r]

Re: Measuring Resistance of Metal Oxide Power Resistor

Likely the latter. These are FR resistors and are designed to go south without looking that way...

[AlTaiRius]

Re: Measuring Resistance of Metal Oxide Power Resistor

Likely the latter. These are FR resistors and are designed to go south without looking that way...

Thanks...What is an FR resistor?

Also...should I be able to test normally, just stick my meter on resistance and go across the legs?

[eccerr0r]

Re: Measuring Resistance of Metal Oxide Power Resistor

Flame Resistant (or possibly Fusible Resistor), Neither should burn when they are overloaded, their resistance just permanently goes up.

[R_J]

Re: Measuring Resistance of Metal Oxide Power Resistor

That is likely just a flame proof resistor, it is basically open it should read 0.33Ω not 22k
That is likely the source resistor for the mosfet (next to it) so it is likely shorted and the gate drive pwm ic could also be damaged

[PeteS in CA]

Re: Measuring Resistance of Metal Oxide Power Resistor

Metal oxide resistors can be measured with an ohm meter, same as a metal film or carbon film resistor (or carbon composition, which are going extinct). If measured in-circuit, other circuit paths may have an effect, though unlikely with a .33 ohm resistor.

Concur with R_J's cautionary comment. Low value resistors are often used as current sense in the source circuit of a MOSFET or emitter circuit of a transistor. During the process of the MOSFET or transistor shorting, high voltage can get into the gate/base drive circuit, causing upstream damage.

[AlTaiRius]

Re: Measuring Resistance of Metal Oxide Power Resistor

Metal oxide resistors can be measured with an ohm meter, same as a metal film or carbon film resistor (or carbon composition, which are going extinct). If measured in-circuit, other circuit paths may have an effect, though unlikely with a .33 ohm resistor.

Thanks for confirming this Pete.

I'll look into the other parts that yourself and R_J mentioned.

[redwire]

Re: Measuring Resistance of Metal Oxide Power Resistor

If that resistor has gone open-circuit (high value) then it's very likely the mosfet has shorted and gate-drive circuit is damaged. I would also test that.
I think it's a fusible resistor or at least acted like one. Assuming a resistor can withstand a 10X power overload, that's almost 3A to make it fuse.

[AlTaiRius]

Re: Measuring Resistance of Metal Oxide Power Resistor

I measured across the mosfet and got 2.111 KOhms Gate-Drain & Source-Drain. I got 1.8 Ohms Gate-Source. I'm not very experienced with this kind of thing so not sure if that is helpful without knowing the actual specs of this particular mosfet.

[R_J]

Re: Measuring Resistance of Metal Oxide Power Resistor

I measured across the mosfet and got 2.111 KOhms Gate-Drain & Source-Drain. I got 1.8 Ohms Gate-Source. I'm not very experienced with this kind of thing so not sure if that is helpful without knowing the actual specs of this particular mosfet.

What is the number on the mosfet? 1.8Ω seems to low, are you sure that's not drain to source? either way the mosfet is bad and likely more.

[AlTaiRius]

Re: Measuring Resistance of Metal Oxide Power Resistor

Managed to get the white gunk off. It's a K6A60D. My measurements were across the pins with it still in the board so it's possible I'm not actually measuring the mosfet but something else in the circuit.

[R_J]

Re: Measuring Resistance of Metal Oxide Power Resistor

I would check the gate circuit, it might be the drive circuit failed in some way to just turn on the mosfet and that caused the source resistor to open.
Post a clear picture of the trace side of the board and also post the number from the drive ic if applicable.
The full mosfet number is TK6A60D