Can you add the schematic, mosfet and motor details?
Hi 3dgeo,
the SOT23 markings seems 3B. These are likely to be the good old BC856B (PNP transistor). Unfortunately I cannot read the marking on the leftmost, maybe you can provide another picture.
Do not use IRFZ44N for switching an high inductive load such a motor powered with 50V. These MOSFET are 55V and even with a diode paralleled with the motor they will die fast.
Best,
0xfede
The diagram lacks the freewheeling (synchronous) diode to the motor. It's extremely important.
Otherwise, given usual things such as layout and proper power bypass capacitors, no problem in concept.
So it seems you are just measuring the DC link voltage. Either a buffer opamp, or a 100n ceramic cap right next to the MCU input pin is needed to provide the ADC low AC impedance it needs.
Edit: since the picture doesn't match the first post description at all, it's hard to say. I based my answer on the picture, but where's the second motor, transformer, etc?
Than You for Your reply, 0xfede,
sorry for being dummy– I should attach marking on them too, I don't know why I did not do that, here it is:
I had a feeling that IRFZ44N would be "too weak" so I ordered IRF3710PbF, they look great on paper, I hope 100V will be enough. I would like to drive IRF3710PbF with simple PC817 optocoupler. I already tested PC817 with 31kHz, it seems to work just fine. The main question now is what PWM frequency is ideal for STP80NF70 particularly in this circuit?
Finally made more detailed blueprint (MOSFET should be IRF3710PbF, 100V/57A/23m?):
Diode next to capacitor is B350B (Surface Mount Schottky Rectifier) https://www.vishay.com/docs/89122/b360b.pdf
If it hasn't been mentioned you need a resistor from gate to source. More likely, the opto feeds the voltage divider to the fet.
Hello again, Mr 0xfede,
thank You for Your help, I really appreciate it.
Yes, actually it's 1BW, sorry for misleading – it's very hard to see it.
Already ordered thru-hole variance of these transistors (BC556 and BC546), will try to replicate that circuit and see what will happen, all I need to find out now is those caps capacities, is there an easy way to do that without special equipment?
Did I mentioned this is for sit stand desk?
Motors will be running very rarely, at most few times a day for like 30s max.
I would like to continue logic part of the circuit till I get new mosfets and I was wondering if shortly (5-10s while testing code) running these motors at 50V with practically no load will damage them? I don't know if my assumptions is correct but I think motor should take ~6A while running on 50V with no load, it's still less than 12A I calculated so I should be ok, am I right?
What comes to running mosfet, yes, I forget pulldown resistor, thats not a biggie...
I can't understand why optocoupler shouldn't be safe in this situation – voltage divider should protect it by reducing voltage to 10 V. At this point I'm ok with the idea that I have to kill few components to figure this out
What comes to CAP - ok, I will move it in to "gray" position.
Well, I ordered this cheep DSO138 oscilloscope, with it I hope to monitor what happens in circuit.
If it hasn't been mentioned you need a resistor from gate to source. More likely, the opto feeds the voltage divider to the fet.
The PC817 transistor, depending on production variant and production year may withstand only 35V so it is not recommended to place the divider after the optocoupler.
..... Who knows what this was or what you are designing.
I just know this circuit is wrong as shown and the FET will turn on and probably never turn off as you have shown it.
The gate has to have a resistor to the source.