Smoking potentiometer

[David55]

Sorry I'm posting this here, I didn't want to start a new thread for this question and I figure you guys will know the answer anyway.

Okay, I am having a problem with a smoking potentiometer. I hooked it up to a power supply to test controlling an LCD backlight with it. I hooked everything up okay and it worked for about half a minute, controlling the backlight brightness. Then it started smoking...
After checking everything, I hooked it up again to see if it still works (I used a multimeter to measure the output voltage). After a few spins and and about a minute it started smoking again.

Do you guys know why this is happening?

David

[Zak698]

David,
My respose would be that the lcd is drawing too much current for that pot, hook up your multimeter and see how much current it is drawing and then compair it to what the pot is rated at....

[David55]

Originally Posted by Zak698

David,
My respose would be that the lcd is drawing too much current for that pot, hook up your multimeter and see how much current it is drawing and then compair it to what the pot is rated at....

That's exactly what I did. I hooked it up to a 5v current from a power supply, and to a multimeter. it looked like it was working fine for about a minute and then is started smoking again.
The pot says 500 ohm on it.

David

[Zak698]

How many Amps is the pot rated for?

[Al Skierkiewicz]

Originally Posted by David55

That's exactly what I did. I hooked it up to a 5v current from a power supply, and to a multimeter. it looked like it was working fine for about a minute and then is started smoking again.
The pot says 500 ohm on it.

David

Does the pot have 500 ohms on it or just 500? 500 indicates a 50 ohm pot, 50 times 10^0. 50 ohms on a 5 volt supply is 100 ma or a power dissipation of 1/2 watt. Add to that the current you are supplying to the LCD and for most pots you have exceeded the power rating. Generally, once a pot has been stressed, it longer remains at the value it began with. Take your multimeter and set it to read resistance and place it across the pot to see what the value is now. Let us know shat you find.
Al

[David55]

Originally Posted by Al Skierkiewicz

Does the pot have 500 ohms on it or just 500? 500 indicates a 50 ohm pot, 50 times 10^0. 50 ohms on a 5 volt supply is 100 ma or a power dissipation of 1/2 watt. Add to that the current you are supplying to the LCD and for most pots you have exceeded the power rating. Generally, once a pot has been stressed, it longer remains at the value it began with. Take your multimeter and set it to read resistance and place it across the pot to see what the value is now. Let us know shat you find.
Al

The exact writing is " b500Ω".

And how exactly do I check the resistance?

This is the multimeter I have:

http://www.tenmars.com.tw/ezfiles/te...3_S_YF3503.jpg

[Al Skierkiewicz]

Originally Posted by David55

The exact writing is " b500Ω".

And how exactly do I check the resistance?

This is the multimeter I have:

http://www.tenmars.com.tw/ezfiles/te...3_S_YF3503.jpg

Dave,
You turn the switch to the lower left corner marked 2000, place a meter lead on each of the outer terminals on the pot and the meter should read 500 +/- 50 if things are OK. Then take one of the terminals and place it on the center terminal (leave the other on an outer terminal) and you should see the meter vary as you turn the shaft on the pot from zero to about 500. The meter should not show an open circuit during any point on the rotation. This indicates a failure of the pot internally. Is there any other markings on the pot? In your design, what does the pot feed?