Potentiometer Wattage Ratings

[dr.power]

Hi Guys.

I have got several pots. How can I find if they are good enough to work in long term without any problem due to the current or the power passing through them? Unfortuenatley most of small Pots I have seen JUST have the resistance value marked on them but nothging more.

Thank in adavance

 

[alec_t]

Without knowing the physical size and resistance values of the pots, and the currents you intend to pas through them, we can't give you a meaningful answer.

 

[Reloadron]

You will need manufacturers, part numbers and then get data sheets. Pretty much as Alec stated.

Ron

 

[gabeNC]

I was expecting a totally different topic when I clicked on this thread.

 

[BrownOut]

HA! Me too, in fact I was gonna wirte something sarcastic, but you beat me to it

To answer the question; just use an ohmmeter to characterize the pots. If you have a scope, you can try connecting the pots to a DC voltage and looking at the center tap ( also known as the wiper ) while exercising the shaft. If you see smooth transitions, and little noise, the pot should be fine.

If you're worried about maxumum power, try to estimate it from the physical size, as compared to the size of pots of known wattage. Then connect to a supply that gives you the estimated power, and test the temperature. It it heats up, lower the power.

 

[ericgibbs]

Sorry Gabe,
I have now edited the Title...

 

[colin55]

One point you have to rememer is this:
As you turn a pot to its lowest value, the current through the pot increases (in many circuits). The actual wattage dissipated by the pot may not increase, but the current increases. It is the current that causes the problem.
That's why a pot either "burns out" or produces a spot on the track and fails and yet the wattage being dissipated is not very high.

 

[Reloadron]

Sorry Gabe,
I have now edited the Title...

I figured someone fixed it. Should have known.

Ron

 

[dr.power]

Oh so you changed th title name, Right? I forgot the name that I used as the Title here. Is there anybody to remind it to me?lol.

Ok the Pots I have got have no part number or something so, SO I am not able to figure their MAX power.... Thats why I asked the original question here.
The pots that I have got are small, used in most of low paower devices....

 

[MrAl]

Hi,

Oh too bad, i missed the original title Someone fill me in?

You have to be careful when testing pots because they are not always rated for power based on their resistance element, but rather on their contact mechanism. This means the only real way to tell is to read the manufacturers data sheet like Ron pointed out.
If you dont know what it is and it is a 'normal' size pot, you can probably get 1/4 watt out of it or even 1/2 watt, but only some pots can go higher than that.

Aside from the wattage rating, there is another rating that is quite important too. If you intend to use the pot in a typical 'volume' control type application, you'll also have to find the 'maximum number of rotations' specification (actual name of this spec varies). Some pots can last for a large number of rotations, while others are limited to as few as 50 complete rotations back and forth. That means the pot could wear out quite quickly if it's not rated for continuous adjustment. Something to keep in mind if you dont know the original of the junk box pot.

 

[dr.power]

Hi,

Oh too bad, i missed the original title Someone fill me in?

You have to be careful when testing pots because they are not always rated for power based on their resistance element, but rather on their contact mechanism. This means the only real way to tell is to read the manufacturers data sheet like Ron pointed out.
If you dont know what it is and it is a 'normal' size pot, you can probably get 1/4 watt out of it or even 1/2 watt, but only some pots can go higher than that.

Aside from the wattage rating, there is another rating that is quite important too. If you intend to use the pot in a typical 'volume' control type application, you'll also have to find the 'maximum number of rotations' specification (actual name of this spec varies). Some pots can last for a large number of rotations, while others are limited to as few as 50 complete rotations back and forth. That means the pot could wear out quite quickly if it's not rated for continuous adjustment. Something to keep in mind if you dont know the original of the junk box pot.

Hi Al, I will write to you soon just When I noticed that you have more energy for me...
For now Thanks for everything dude.

I think the the title was "Pot" or "Pots" or something like that. Actually I am not a native english speaker, so I do not know what is the problem by the said title?! Maybe it is a damn or something like that? lol.
Any way I am glad if you guys are happy with my posts.

o be on the safe side I assume the power of my pots as something like 100mW. So how many of power is enough for such a pot so that the pot works in long term?

Thanks a lot

 

[OlPhart]

Simple truth I discovered ~1974: a potentiometer is Not a variable resistor. It can function as one, and in theory, do just fine. In practice (as was previously stated) the smaller the setting, the WAY greater the derating for power handling.

A Real variable power resistor is the Old Style wirewound ceramic bar with one side of windings exposed for the "wiper". Even then, you can overpower the wire under power, if it's too short. Proper sizing by value is Real Important.

A cute trick is to use a PWM/MOSFET with a cap for smoothing, but that's Way not trivial... <<<)))

 

[dr.power]

Simple truth I discovered ~1974: a potentiometer is Not a variable resistor. It can function as one, and in theory, do just fine. In practice (as was previously stated) the smaller the setting, the WAY greater the derating for power handling.

Yea, You are right, It makes sense, The less resistance the more current is passing through the pot. But It makes me more worried...

 

[BrownOut]

Don't worry. Just use series fixed resistors to prevent overcurrent. With proper design, you should never need worry about over-powering any vairable resistor.

 

[gabeNC]

Sorry Gabe,
I have now edited the Title...

No need to apologize, that's probably one way to get people to read posts. hahaha.

 

[dr.power]

No need to apologize, that's probably one way to get people to read posts. hahaha.

Actually I did not know that "POT" is a drug! I already used to call it "potentiometer" as it is, But the reading through threads here in this forum changed my taste! I learnt here in this forum that a potentiometer is called "Pot", a microphone is caled "Mike!" and "Mic", a capacitor is called "Cap" and so on. I am not an english native speaker, so I pick up the words the other english speaking guys used to use. Most Of times I am wondering whether the other guys here are able to understand me or not? Anyway thats you huys who learnt me to use HABITED words here. I Never did not gess that POT my refer to a drug, And if so, actually we are in an electronic forum, not in a....

 

[dr.power]

I guess that it is better to mesure or calculate the power and current of the section of the circuit that I want to add the pot, By doing so I have the power of the pot when it is almost short (the worse cause), right?

 

[BrownOut]

I Never did not gess that POT my refer to a drug, And if so, actually we are in an electronic forum, not in a....

Many of us grew up in the 60's when everything was psycodelic ( just another habitated word ) Yeah, the word POT all by itself has a special meaning. But when used in context, such as, "How do I measure the resistance of my pot?" then it has a more 'electrical' meaning. But you're fine, we've just been having a little fun

I guess that it is better to mesure or calculate the power and current of the section of the circuit that I want to add the pot, By doing so I have the power of the pot when it is almost short (the worse cause), right?

Properly designed, you circuit should have no problems when the pot is set to zero ohms. Typically, fixed seried resistors are used.

 

[dr.power]

Properly designed, you circuit should have no problems when the pot is set to zero ohms. Typically, fixed seried resistors are used.

Plz where do I should put the said series resistors? in series with the center pin of the pot or in series with the rest fixed pins?]

For instance I have to use a pot as a voltage divider between the ground and +8V of the power supply, What do I should do? say the pot is a 100 ohms once and 1000 ohms the other once....

Thanks

 

[BrownOut]

Plz where do I should put the said series resistors? in series with the center pin of the pot or in series with the rest fixed pins?]

For instance I have to use a pot as a voltage divider between the ground and +8V of the power supply, What do I should do? say the pot is a 100 ohms once and 1000 ohms the other once....

Thanks

Well, it really depends on your circuit topology. Show me a schematic, and I'll make a recommendation.