Hi all,

We are preparing for a competition and we needed a resistor in series with an electromagnet, paralell to a digital screen. We initially decided to go for 6V because the digital screen only works above 5V. However, we had to increase the voltage to 8V.

Once we had soldered the circuit together, we put electrical tape on top of the resistor and it started smoking. We assessed the resistor and the tape had melted. We removed the tape and re soldered the circuit. We bought some heat proof pieces to go beneath the resistor and tested it again. When testing it, the circuit worked well. There were heat proof pieces covering the resistor.

However, when testing today, it began smoking again and when we assessed it, we realised some of the heat proof material had gotten black (the pieces on top). So we realised the heat proof material was smoking (not the resistor).

We have decided to go with a lower voltage and will switch off the power supply as soon as possible. It has been working well so far. It only started heating up when it was on for a long time.

My question is, is this circuit safe and what can we do to reduce the heating effect? Also, we were wondering if increasing the turns on the electromagnet would reduce the effect of current on the resistor.

 

hi,
Welcome to AAC.
What is the resistance value and the current thru the resistor.?
E

 

There are a number of things that you may be able to do to stop the resistor from over-heating:
Will the electromagnet still do what is required at a lower current?
If so, you could increase the value of the resistor to lower the current. Then the resistor would not be dissipating so much power.
If not, you could use a resistor that has the same value but a much higher power rating. It will have more surface area to radiate the heat so the temperature will not be so high.
You could add a heatsink to the resistor to help dissipate the heat.
If you add more turns to the electromagnet, you will require less current to have the same effect. The strength of it is proportional to the amps times the turns. Then you could use a higher value resistor or a lower supply voltage to reduce the current.
Good luck,
Keith

 

Hi,
Welcome to AAC.
What is the resistance value and the current thru the resistor.?
E

I believe it is about 1.2 ohms. We did the calculations a while back so I don't remember the current rating. I read online that it could catch fire, so I'm a bit worried about it.

 

Will the electromagnet still do what is required at a lower current?
If so, you could increase the value of the resistor to lower the current. Then the resistor would not be dissipating so much power.
Hi, we have tried changing the current however, the circuit doesn't work with a lower current.

If not, you could use a resistor that has the same value but a much higher power rating. It will have more surface area to radiate the heat so the temperature will not be so high.
You could add a heatsink to the resistor to help dissipate the heat.
We added a heat sink but it started to burn. I was thinking of employing a cooling system (like one used in heavy duty PCs) would this be suitable?

If you add more turns to the electromagnet, you will require less current to have the same effect. The strength of it is proportional to the amps times the turns. Then you could use a higher value resistor or a lower supply voltage to reduce the current.
If we changed the number of turns, would we require a new resistor?

Good luck,
Keith

 

We used this resistor.

 

We used this resistor.

That resistor is rated at 25 Watts. You are drawing an awful lot of current to make it that hot!
Increase the number of turns on the magnet as much as you can. See how this affects the required volltage. If the resistor still gets hot, bolt on to a heat sink or a piece of thick aluminum or copper sheet.

 

That resistor is rated at 25 Watts. You are drawing an awful lot of current to make it that hot!
Increase the number of turns on the magnet as much as you can. See how this affects the required volltage. If the resistor still gets hot, bolt on to a heat sink or a piece of thick aluminum or copper sheet.

Thank you, what type of heat sink would you recommend?

 

Thank you, what type of heat sink would you recommend?

An old microprocessor heatsink or any piece of aluminium extrusion that has a flat area to attach the resistor. Most old 12V computer fans will run at 8 volts. That would help a lot too.

 

Lower the current / voltage in the coil or put more turns on it, the resistor is just wasting power...