Hello,
I am working on "power module" for Arduino (schema below). Inputs are controlled by Arduino, then separated by optocouplers, which are opening the transistors.
BUT11A for higher current (Fans, water pump) and BD237 for relays.
I am powering the transistors on "POW" connector with 11.1V. When I open the Q1 transistor, current on base is 18mA, which is 1,9A collector-emitter current.
Fans, connected to the transistor take 1,9A at 11.1V, so it should be enough. I dont know why, but the current from transistor is only 0.3A and the transistor is overheating really fast.
Can someone please help me whats going on in the circuit? I couldnt find anything, but maybe I am missing something.
Thank you.
InToSSH:
Hello,
I am working on "power module" for Arduino (schema below). Inputs are controlled by Arduino, then separated by optocouplers, which are opening the transistors.
BUT11A for higher current (Fans, water pump) and BD237 for relays.
I am powering the transistors on "POW" connector with 11.1V. When I open the Q1 transistor, current on base is 18mA, which is 1,9A collector-emitter current.
Fans, connected to the transistor take 1,9A at 11.1V, so it should be enough. I dont know why, but the current from transistor is only 0.3A and the transistor is overheating really fast.
Can someone please help me whats going on in the circuit? I couldnt find anything, but maybe I am missing something.
Thank you.
I would suggest you decrease the base resistor to increase the base current. Current gain in transistors can vary from device to device and with temp, an you are probably not fully turning on the transistor and it's having to dissapate the voltage drop across it. You can check this by measuring the collector terminal voltage when it's in the 'on' condition, voltage should be just a couple of tenths of a volt or less. As you are operating the transistors as switches you want to force the transistor into fully saturated on condition, not try to match current gain to the exact load requirements. Check the maximum rated base current and operate it near but lower to that, to ensure saturation of the emitter collector junction.
The BUT11A devices have a typical gain of 20 at 0.5A collector current, minimum guaranteed gain of 10.
For 1.9A collector currents the gain will be even less. You posting suggests you think the gain is over 100... For that you'd need a darlington or superbeta transistor. The BUT11A's are ultra high voltage switching transistors, not optimal for low-voltage switching.
The BD237's will fare only similarly, gain around 20.
When you use a switching transistor you need to saturate the device, which means feeding several times more current into the base than the gain figures would suggest - this enables the collector-emitter voltage to drop substantially and reduce conduction losses. For the lower current loads you can simply reduce the base resistors by a factor of 10 or so.
Typically bipolar power transistors have poor gain at highish currents and you might want to consider darlingtons, MOSFETs or super-beta transistors instead of the BUT11A's (superbeta's can sustain gains of 100 or more at several amps).
[EDIT: Oh yes, in English usage "open" and "closed" have the reverse sense for electrical switches as for water taps, to avoid confusion best to use "off" and "on"]
retrolefty: Yea I was decreasing the value of the resistor on base, but even when I replace it with piece of wire, it wont still have enough current to feed the base, because the optocoupler is limited with 50mA.
Runaway Pancake: Now I am working with OK1/Q1.. Voltage from OK1 to Gnd is 11.4V at this time (depends on battery charge). Voltage on the resistor R6 is 8.5V.
MarkT: Yea now I see it, I dont know why I was thinking about beta 100, thats for my different transistor. Anyway, when reading your post, now I see I cant feed the base of BUT11A enough with optocoupler, nor even with optocoupler with integrated darlington. Maybe I would have to use some BC* transistor before this one (I would have to remake my circuit board). I have some MOSFETs here, but I dont know if I can just connect them to the circuit board instead of BUT11A. Of course matching gate-base, source-collector, drain-emitter if I am right. I think I have never used MOSFETs before so it will be quite now thing for me.
Ok, so I have put MOSFET IRFZ44N instead of BUT11A, which has the same pinout, just added the 1k resistor between source and gate and now it look just fine, without overheating. I think its the best solution, without need to make new PCB.
Thank you very much for your help
I have some MOSFETs here, but I dont know if I can just connect them to the circuit board instead of BUT11A. Of course matching gate-base, source-collector, drain-emitter if I am right.
No, gate/base, drain/collector, source/emitter. MOSFETs are voltage controlled, not current-controlled. You would normally want a resistor from gate to source to ensure Vgs=0V when you want it switched off (any leakage current into the gate will pull it high since the gate is completely insulated).
