Potentiometers burnin' up!

[Werebear]

Hi All

Usual newbie: I hope this in the right place!

Tinkering around I bread boarded the LM13700 VCO from the datasheet. When I came to produce a control voltage I did the usual trick of using a 100k pot to positive voltage and negative voltage and wiper to cv in on the osc. The pot set on fire. Well smoked and glowed a bit. Oh, says I. Duly found another pot and that too went up.

My power supply is plus and minus 5V from two PP3's and two voltage regulators with caps and diodes. I measure +5.02V from ground and -5.1v from ground, so I'm pretty sure it's not that. I have also used the same trick just from two PP3's.

I am using a 100k resister straight to pin 1 on the LM13700, which is actually using the resister values from Frequency Central's Dalek's Handbag osc.

So is this just bad luck or have I made an obvious mistake?

Bear

[JovianPyx]

sounds like you have power rails on one end of the pot and the wiper.

[Werebear]

Thanks Scott

Hmm... I'll have to check that out but I'd like to think it's not the kind of mistake I'd make twice! Each pot had three diferent colours of wire soldered on and where "pulled" when smoke appeared and then re-attached, but continued to burn up. Then again perhaps that is the kind of dumb thing I do these days.

The only saving grace is that the osc worked for the short time the pot(s) lasted.

Ta

Bear

[JovianPyx]

how many ohms are the pots?

how many watts are the pots rated for?

watts = E * E / R

If you violate the above by going over the pot's watt rating - burning will result.

that and miswiring is all I can think of for smoking pots... oops... that was funny...

[Werebear]

Hi again

well the first one was 100k and the second one was 10k, as they where there already soldered with three leads in my not-quite-junk-yet-box I think I may have used then for a similar experiment when I was trying to build an expo for a 40106 vco. Now that ran off + and - 9V from two PP3's with no regulation. The idea behind using voltage regs on battery supplies is that as the batteries run down I should't have tuning problems until the batteries fall below 5V.

I couldn't say what the wattage of the pots was as it was not a consideration when I bought a couple of handfuls over 18 months ago. Perhaps I need new ones. These have spent two winters in a shed...

The voltage regs I'm using are the L78xx and L79xx types for one amp. Dumb question for somebody who used to work with three phase systems, but they wouldn't bepushing 1A out would they? I'll have to measure that tomorrow now.

Thanks

Bear

[JovianPyx]

LM78xx or LM79xx regulators can source up to 1A if they are supplied as much capacity at their inputs.

Or do you mean 78Lxx and 79Lxx - those are low power regs, not sure what they will deliver, but apparently they can deliver enough to smoke those pots... (I go by the physical evidence that they did )

But the resistance of those pots means that the 10K pot would need 100 volts across it to get to 1 watt. The 100K - even more volts. So let's say they are 1/8 watt, they'd need more than 35 volts across them to exceed that watt rating and most pots are 1/4 watt... And you're voltage is 10 volts, so it's hard to believe without some miswiring or something going horribly wrong with the power applied to the pot.

[Werebear]

Well today is another day so Ill try again and take measurements and try and use pots spiders haven't nested in

Thanks for the input and I'll post again if I've actually discovered cold fusion by mistake.

Bear

[Werebear]

DOH!!!

Yeah of course I had it wrong and I've fried the chip.

Anyhoos, I've given up on that one and am going to use the single amp vco from the data sheet leaving me the other amp for a vca. I need at least three of each for my next project.

Ta for the help and sorry for wasting time.

Bear