Using AC resistors for a DC circut

Thread Starter

81vee

Joined Mar 31, 2018
4
G'day guys,

I have recently scrapped an old cassette deck that wasn't worth fixing, and I grabbed the old switches and variable resistors off of it. I was wondering if it would be hazardous to use those resistors as a sort of dimming switch for a DC system- just imagine one or two small light bulbs per resistor- something with low draw. What do you guys think?

-81vee
 

MaxHeadRoom

Joined Jul 18, 2013
19,028
As long as the current (wattage) rating of the resistor falls within the design criteria, AC/DC does not matter.
Max.
 

WBahn

Joined Mar 31, 2012
24,692
I have no idea what an "AC resistor" is (as separate from a "DC resistor").

What will determine if it is safe to use is whether the values and ratings are appropriate for the circuit you are using them in.
 

MaxHeadRoom

Joined Jul 18, 2013
19,028
One difference can occur in resistors that are constructed with zero or low inductance for some H.F. circuits.
Max.
 

Thread Starter

81vee

Joined Mar 31, 2018
4
I put "AC resistor" because the unit was powered by 120v from the wall. Unfortunately, I don't know the wattage rating of what I pulled- the unit is from the mid 70's and there's virtually no documentation on it. I was able to pull out the manufacture comany as well as an ohm value of "10kΩ", which I figured to be the max resistance. I figure the wattage rating can't be that great. Looks like no thicker than 20 gauge wire right now, which I could definitely change.

Thanks,
-81vee

Edit:
The lowest resistance of the unit is near-zero
 

WBahn

Joined Mar 31, 2012
24,692
So, for your intended use, is a 10 k ohm pot a reasonable choice? If not, then everything else is a moot point.
 

Tonyr1084

Joined Sep 24, 2015
3,578
With a tape deck from back then the most common size resistor is likely half watt. You may have some larger resistors, which would be as much as 1 watt, but they're twice the size of half watt resistors. Another common size is quarter watt. Knowing that small light bulbs typically run anywhere from 3 watts to 35 watts (I'm talking dash bulb lights and side marker lights on a car, not the turn signal / parking lights) They can be closer to 50 watts and headlights are commonly 55 watts.

Wattage is a measurement of how much voltage and current are running through a circuit at any given moment. Also, light bulbs have a peculiar habit of reading very low resistance (ohms) when cold (not lit) and changing to much higher resistance when lit (at full brightness). So if you use a bulb rated at 35 watts and 12 volts then you're looking at 35 ÷ 12 = 2.9 amps. 12 ÷ 2.9 = 4.14Ω. So, when lit, your bulb resistance is 4.14 ohms and is running 2.9 amps through the circuit. If you add a resistor to dim the bulb to half the brightness (half the current) you would need a resistor value of 8.3Ω and sized to handle 17.5 watts. That's a HUGE resistor and you're not going to find such a resistor in your scrap parts. Given that you need a 17 watt resistor, good practice would be to use a resistor wattage rating of AT LEAST 150% of the expected normal operation, meaning you need a resistor rated to handle over 26 watts. Many people, since resistors typically are so small, quarter watt size resistors are typically twice the size needed. And if you NEED a quarter watt resistor, a half watt resistor takes up hardly any more space, so doubling the size of a resistor is no big deal when dealing with such low wattage circuits. But to dim a light bulb using a resistor, you'll need a haunkin' big resistor.

It's not likely you'll find much useful in the way of dimming a light bulb out of an old tape deck. You COULD, however, likely find what you need to power an LED as a source of light. Or a few LED's for more light. SuperBright LED's are - well - super bright. And can even be hard on your eyesight if you stare straight into one. Not advised! Also, it's not likely you have any super bright LED's in your old tape deck. Especially if it is 70's made.
 

Tonyr1084

Joined Sep 24, 2015
3,578
Using a 10KΩ pot - you won't like the outcome. If you try to push that much wattage through it, at 10K the bulb isn't going to light. But when you start to get around the point where the filament begins to glow - it's likely your pot will glow too. For a moment. Then be done.

Do you know any of the specs on the bulb you want to use? That'd be a useful point for us to start with. Voltage and wattage and we can figure the rest.

Now, if you want to get fancy and have a truly dimmable high wattage lightbulb then the way to go is with PWM (Pulse Width Modulation). Or an autotransformer (which doesn't mean it's from a car).

Maybe you can use the pot and some transistors. The pot will control the transistors and the transistors will control the power. Provided you don't exceed their wattage capability.
 

Thread Starter

81vee

Joined Mar 31, 2018
4
Well, Tony, you've helped me out bunches. I was actually considering using it for a dome or map light or something from my imagination for an old pickup i'm fixing up. If I were to use it, it would definitely have to be LED, which seem to draw about 0.5 to 1W depending on the LED.
 

WBahn

Joined Mar 31, 2012
24,692
Well, Tony, you've helped me out bunches. I was actually considering using it for a dome or map light or something from my imagination for an old pickup i'm fixing up. If I were to use it, it would definitely have to be LED, which seem to draw about 0.5 to 1W depending on the LED.
There are much better ways to control the current to a power LED than just using a potentiometer.

But even if that weren't the case, let's look at some numbers. If your power source is 12 V (nominally) and your LED was about 3.3 V, then at 1 W you would need about 300 mA (and at 0.5 W you would need about 150 mA). To drop the remaining ~9 V your series resistor would need to be either 27 ohms or 54 ohms. That is so far from the 10 k ohms that your pot provides that there's no point considering it any further. But if we did, then that pot would have to dissipate a few watts of power, and it is almost certainly not intended to do that (especially at that total resistance value).

Instead of using a single LED, use three of them in series run at something around 0.25 W (though that might be too bright for your application -- I don't have a good feel for that). There are then a number of ways to drive it that trade off simplicity for efficiency.
 

Thread Starter

81vee

Joined Mar 31, 2018
4
Thanks for the informative reply, WBahn. In that case, I may not use it in the long run, but the thing is, they have LED's that use 12V electrical systems, like that of a car. Then I woudn't have to dissipate 9V?

BTW here's an image of what I pulled.

I noticed that the rating on the side is stated a little differently. It goes like this:
"B10kΩX2"

It's original intended purpose was to control stereo input when recording as well as the mater volume of the input to the tape.
 

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Tonyr1084

Joined Sep 24, 2015
3,578
go to the junk yard and pick up a dash board light dimmer module. They're most always PWM. Probably get it for a buck. And it will control LED OR Incandescent bulbs.
 

WBahn

Joined Mar 31, 2012
24,692
Thanks for the informative reply, WBahn. In that case, I may not use it in the long run, but the thing is, they have LED's that use 12V electrical systems, like that of a car. Then I woudn't have to dissipate 9V?

BTW here's an image of what I pulled.

I noticed that the rating on the side is stated a little differently. It goes like this:
"B10kΩX2"

It's original intended purpose was to control stereo input when recording as well as the mater volume of the input to the tape.
If you use an LED module intended for 12 V operation, then if you want to dim it you need to be sure that it is dimmable and then you need to dim it using the method intended for it.

BTW, the fact that those pots came out of something that was powered by 120 VAC does not mean that they ever saw anything close to that voltage. They were almost certainly used in low-voltage, low-power signal circuits.
 
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