power resistors exploding

power resistors exploding

HersirSmiley 02 Sep 2010 01:50

Hello everyone, i was wondering if perhaps someone could tell me what i'm doing wrong. I hook up the power resistors to the thermal core, it heats for a while and suddenly one of the resistors explodes out one side. I originally thought it was a loose connection and i replaced the resistor and checked all connections. Upon running it a second time, the heater ran even further than the first time but again exploded. I am running through the relay board and then to the extruder.

Re: power resistors exploding

bodgeit 02 Sep 2010 13:44

My best guess is that your thermistor/thermocouple is not making a good thermal connection to the block.

This would mean the power to the heater resistors is never getting turned off.

Other reasons could be that relay is not working or not being controlled by the thermistor/thermocouple controll system.

Other reasons might be the value of one the power resistor is not the correct value.

IE If there is one that remains intact each time. Has it a higher value or is not connected properly.

Re: power resistors exploding

HersirSmiley 02 Sep 2010 13:47

Yes so far there is one that is surviving both explosions. maybe it blew and i didn't realize it meaning all the load is on one resistor. also if shutting off is the problem perhaps i need to step to proper temp instead of going in a straight shot.

Re: power resistors exploding

HersirSmiley 02 Sep 2010 14:34

scratch that, both blew this time when i slowly stepped it. got to about 200C and they exploded in quick succession. Both had approx the same resistance as well. So this definitely helped to step them, but still resulted in failure.

Re: power resistors exploding

bodgeit 02 Sep 2010 18:48

What is the rated wattage of the resistors you are using?

I have used the 25W Aluminum clad ones from RS for a heated bed before.

If they are exploding I think you must be exceeding their rated maximum wattage.

A quick calculation should give you the power you are pumping in to them V X I = Watts .

If you can't measure the current then use V^2/R to get Watts (Power)

Re: power resistors exploding

HersirSmiley 02 Sep 2010 20:57

they are rated to 10w. resistance of 5ohm per resistor, for 2.5ohm total when connected in parallel. 57.6W total on a 12V power supply from the makerbot relay board. Which is supposed to be the values of those sold by makerbot. I should note that i did not buy the makerbot mk5 i merely made my own block of same size aluminum, and purchased the resistors from mouser electronics. I am using a thermistor from the original mk4 plastruder. Also the thermistor is in a hole inside the block to get more accurate readings as shown in Nophead's blog from reprap.

Re: power resistors exploding

bodgeit 02 Sep 2010 21:15

I dont know the what Makerbot supply but I would get 25W resistors thou instead of 10W. Seems odd to use resistors with heat sinks… I drilled out holes for ceramic wire wound resistors when I made an Aluminum block extruder I think Nophead did that as well.

Using fire cement to bed the resistors into the holes fire cement as its cheap.

Re: power resistors exploding

HersirSmiley 02 Sep 2010 23:15

thanks for the suggestion, i ordered some other higher wattage ones, i hope that helps to solve the problem. Also thanks for the suggestion of fire cement, i had not thought of that.