Replacing 75 ohm heating coil with resistors?

Question

I am trying to remove the thermal fuse, thermostat, 75 ohm, and 150 ohm heating elements from this diagram:

This is the schematic for a popcorn machine that has two heating coils and a DC fan to blow over the coils: http://www.sweetmariascoffee.com/forum/viewtopic.php?f=14&t=5761. Basically I am trying to create a 115 V AC circuit that powers the 18 V DC motor through the rectifier bridge.

I imagine I need to replace the 75 ohm heating coil with some kind of resistor for this circuit to work correctly. Can I simply replace the 75 ohm heating coil with 75 ohms worth of resistors that I typically use in my DC only circuits?

Answer 1

NO! You buy a step-down transformer with a 12vac output that also isolates your fan from the AC line. You need to check the motor current rating and buy a transformer rated 150% of the motor current. Should not cost much. Connect the 12 VAC leads to the input of the rectifier bridge. The motor should be getting at least 16 VDC to 18 VDC.

Do not bypass the thermal fuse. It can be physically mounted to the transformer to kill the power if the transformer overheats-but there should be no reason for it to do so.

The thermostat and resistors can be discarded.

Answer 2

Sparky256 has already told you what you should do, so I'm going to explain what will happen if you do it the way that you suggested...

As given, your design uses that resistor to drop the voltage from 115VAC to 18VAC, so a voltage drop of 97V, which is then rectified into 18VDC to power the motor. From the resistor value of 75 ohms, we can calculate the power dissipated by Power = Volts x Amps, or rewritten, Power = Volts^2/Resistance. With a 97V voltage drop, we find that 97^2/75 = 125 Watts dissipated.

This is bad news. Not only do you have to pay for this wasted power, it will also destroy most 'normal' resistors. Standard little round electronics resistors are only rated to dissipate 1/4 Watt (some are 1/2 Watt). Above that rating, they die. Horribly. You would need to split that 125 watts evenly over 500 resistors to avoid destroying them.

Alternatively, you could get power resistors, but these are huge and expensive. I have some 25W power resistors, and they are the size of my entire thumb and can only handle 1/5 of what you need.

Conclusion: Get the step down transformer! Not only is it a better solution, it will save you money in both power and materials.