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I’m trying to make a heating element from a 4W wire-wound sandbar resistor and datashet 12V power supply. My Ruggeduino is using temperature measurements from a DS18B20 sensor to determine when to switch a transistor controlling power to the the resistor. My problem is that I have burnt out two transistors one C and one H already and I don’t know why.
I have a flyback diode parallel to the resistor to disperse any back EMF the resistor coil generates and a pull-down resistor on the base of the transistor. I can switch an LED on and off with this circuit not at 12V, obviously but when I try it with the sandbar it quickly goes up in smoke.
The grounds are all common. For simplicity I haven’t put the DS18B20 in there. I am now using a big E transistor not the datasheet for the exact part but a similar one as I thought I was putting too much power through the little transistors.
H945 Datasheet, Equivalent, Cross Reference Search
At 12V I calculate that the 4W sandbar will be dissipating all of that, which explains the rapid release of magic smoke from them. Should I be running it with a heatsink?
Why does it get this hot? I thought resistance of transistors, especially larger ones like datazheet, was negligible.
H Datasheet – NPN Silicon Transistors –
I measured the voltage from the basepin on the Ruggeduino and it was putting out 4. Am I doing something wrong?
Is this too low to open the transistor gate sufficiently? Any input will be most gratefully received. I’m rather confused by your trxnsistor. Mike has pointed out some issues and I see more. Hi pwillard, thanks for your input too.
H Datasheet, Equivalent, Cross Reference Search. Transistor Catalog
Here’s a snap of the sandbar: You say I need to be limiting the current from the base of the transistor and the Arduino pin. I hope this should cover it. Thanks for the schematic. You haven’t included a pull-down resistor so I take it you think I don’t need one at all.
I’ll try this and see what happens. Also good to know I can do without the diode. See my drawing above At that kinda current I would start looking into mosfets to keep the heat down, after 1amp heat starts to get in the way with transistors.
After playing around with this for a while I gave up as the big transistor I was using had to be reallocated to another project which is working wonderfully, thanking you. So as I still wanted a heater circuit and I am lazy I bought one of these. You could probably cycle it with the 7v supply, add another ds18b20 easily and when it reaches max temperature, kill it and when it drops a bit turn it on it has enough thermal mass to keep things warm in between.
Hi trajsistor, thanks for your datasheft. Because the heater is just stuck on the bench underneath the fermenter, with a little insulation around it, thermal transfer to the vessel is poor at this stage. This is something I’m looking at improving, probably with an immersion heater, but the point I’m trying to make is that you need the heater at a much higher temperature than the desired fermentation temp.
The upshot of this is that switching of the heater, on or off, occurs every few minutes so it would still burn out.
(PDF) H945 Datasheet download
For example, its now running off 6V and I’ve got it running straight off the power supply without an control. I’ve left it running overnight and I expect it will still be at about 20C tomorrow morning as transistro temperatures are chilly. I might set the relay up again tomorrow. The irony is that the DC power supply I’m using kicks out about as much heat as the resistor does. Unfortunately its too big to fit under the fermenter! Here’s pics datashewt the Arduino and the setup.
The fermenter is covered by a black bag.