Resistor gets really hot.. Any suggestions?

Cryogen

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Oct 2005
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We used a 15 ohms/10W resistor to drop down 24V to 13.2V going to a soleniod valve (the valve needs 13.2V at 100% duty cycle... more than the solenoid valve coil gets burned).. The resistor was able to drop down the voltage, but the resistor we used got very hot in just few secs.. It went upto 320 Deg F..

Can anyone suggest a solution/product that will help us achieve this objective? Thanks for your time..
 
A 13.2VDC Power supply and a 24VDC Coil solenoid driver.
Any other approach will still need to dissipate power, whether it be a three-terminal regulator, or a zener diode.
 
Use a 20W resistor or two 30ohm/10W resistors in parallel. Check the specifications of the resistor. Its 10W rating may be at a temperature of 400F
 
My guess is your solenoid coil impedence is around eighteen and a half ohms. That means that your 10 watt resistor is marginal at best, you are right at it rating. You could always use a higher wattage resistor, it won't get as hot to the touch, but you will still be disipating the heat.

Either do as Ron suggested, or replace the valve solenoid coil with one rated at 24V, or learn to live with the power dissipation.
 
Last edited:
Standard voltage regulators will also have to dissipate the same amount of heat.

Only by using a chopper circuit (with a MOSFET output like a IRF540 N-channel, or easier a IRF9540 P-channel) you can overcome the dissipation problem. It should be set to about 50% duty cycle (24V x 50% = 12V) in your case.

Most likely, there's not an off the shelf solution...
 
1) Do you have a spare solenoid? Put it in series.
2) 12 volt, 15 watt lamp (automotive).
3) 12 volt, 10 watt zener diode mounted on a heat sink. I think this will disapate less heat, but not sure.
 
Thanks for all your help..

I do not completely understand how the heat dissipated with be lesser if I choose a higher wattage resistor? Is it because the resistor is not maxed out?

Also, we have to live with this valve :) I do not have an option now to change it to a 24V valve.. The valve resistance is 19.2 ohms..

Thanks again..
 
The power rating of a resistor is the rated maximum amount of power that it can dissipate as heat. The actual amount of power that it will dissipate is the resistance times the current squared. P=R*(I^2)

If you choose a higher rated resistor, it will dissipate the same amount of power at a lower temperature than a lower rated resistor.

$
 
Cryogen said:
Thanks for all your help..

I do not completely understand how the heat dissipated with be lesser if I choose a higher wattage resistor? Is it because the resistor is not maxed out?

Also, we have to live with this valve :) I do not have an option now to change it to a 24V valve.. The valve resistance is 19.2 ohms..

Thanks again..
Click to expand...

Ok you may not be able to change the valve but you can install a new power supply to operate your your valve. Just have your 24VDC operate a slave relay and run your new power supply thru its dry contacts to operate your valve.
 
I had a similar situation in an emergency and used a 100 watt light bulb as the dropping resistor. Seems it was about 13 ohms. And it isn't going to burn out unless there is a lot of vibration. I used a rubber socket (pigtail) and got to be the resident "HERO" for a day or two. FREE lunch would have been better!
 
Cryogen said:
Thanks for all your help..

I do not completely understand how the heat dissipated with be lesser if I choose a higher wattage resistor? Is it because the resistor is not maxed out?

Also, we have to live with this valve :) I do not have an option now to change it to a 24V valve.. The valve resistance is 19.2 ohms..

Thanks again..
Click to expand...


The heat dissipated will not be less. A higher wattage resistor will have a larger surface area from which to dissipate the heat, therefore it will be cooler to the touch. Either way, you are still dissipating the same amount of energy.
 
This might be a stupid question but cant you just get a 24V DC coil for the solenoid valve?

Andybr
 

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