asdf53 wrote on Yesterday, 17:53:

The left one (85U03GH) should work fine: […]
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The left one (85U03GH) should work fine:

https://www.alldatasheet.com/datasheet-pdf/do … /AP85U03GH.html

If you want to know if a transistor will work on your PCB, look up the datasheet by googling its model number. It must be an n-channel MOSFET. Then look for one of these graphs:

Gate-to-source voltage vs. Rds(on)
Gate-to-source voltage vs. Drain-to-source current.

At a gate voltage of 4V, you need at least 4A of current, if the resistance of your motor is 1 ohm in the off state. The 85U03GH can do that, and at 4A, it would generate around 0.2W of heat, which is fine. There is a small chance that the switching speed of your microcontroller is not enough for this transistor, but I don't think it will be a problem.

If your microcontroller switches the transistor at a high frequency, the heat generated can be more than 0.2W (can be double of that or more). But it's difficult to calculate when we don't know so many variables. You should test the transistor for possible overheating when the motor is running - put your finger on it or use a thermometer.

So I could try to solder this one?

Yes, it's a good replacement for the old one. Solder it on and then test if it gets hot while the motor is running. As mentioned, it's not possible to calculate the heat dissipation before soldering it, because we don't know the switching current and frequency of the microcontroller.

Okay, I forgot something important in the calculation: The device is battery-powered, so you will have a considerable voltage drop during motor operation, and the battery voltage will drop further as it discharges. This means that the gate doesn't see the full 4V anymore, and the power losses of the MOSFET quickly add up. Assuming the worst case - the controller uses PWM switching at high frequencies and it doesn't use a mechanism to boost the gate voltage - then if the new MOSFET performs slightly worse compared to the original one, it could generate a lot heat and the battery will empty faster.

The best way to know if the replacement is still okay would be if you can identify the model number of the original MOSFET. It starts with STD60N, could it be "STD60NF55L"? That one is almost identical in performance to the MOSFET from your motherboard. The main difference is that the latter has a wider range for the gate threshold (1-3V compared to 1-2V), so there's a slight chance of it performing worse than the old one, but it shouldn't be too significant.

I manged to replace the transistor, but still not working. Now with the new transistor in place, what should I measure again to make sure if it is, or not, the same as before?

I have voltage on both motor wires ~ 4.12V.
Meaning, battery black wire + motor black wire = 4.12V. Battery black wire + motor red wire = 4.12V

How is this possible?

asdf53 wrote on Today, 10:12:

The best way to know if the replacement is still okay would be if you can identify the model number of the original MOSFET. It starts with STD60N, could it be "STD60NF55L"? That one is almost identical in performance to the MOSFET from your motherboard. The main difference is that the latter has a wider range for the gate threshold (1-3V compared to 1-2V), so there's a slight chance of it performing worse than the old one, but it shouldn't be too significant.

It is not possible to read it anymore at all, but I am 99% sure that it wasn't STD60NF55L, because it was something shorter

boby wrote on Today, 14:46:

I have voltage on both motor wires ~ 4.12V.
Meaning, battery black wire + motor black wire = 4.12V. Battery black wire + motor red wire = 4.12V

How is this possible?

This is normal, it happens when the transistor doesn't turn on. No current is flowing, voltage at all points is the same - you're measuring the battery voltage.

What you need to measure now is two things:

With battery disconnected - resistance between pin 1 and 2
With battery connected and switch on - voltage from pin 1 to negative battery terminal

It is not possible to read it anymore at all, but I am 99% sure that it wasn't STD60NF55L, because it was something shorter

Okay, there are some others that start with STD60NF. But let's do that later.

One more thing, can you measure the voltage of the battery when it's not connected to the board? Still 4.1V? What is the nominal voltage written on it?

asdf53 wrote on Today, 15:16:

With battery disconnected - resistance between pin 1 and 2

95.6 kOhm

asdf53 wrote on Today, 15:16:

With battery connected and switch on - voltage from pin 1 to negative battery terminal

4,17V

asdf53 wrote on Today, 15:16:

One more thing, can you measure the voltage of the battery when it's not connected to the board? Still 4.1V? What is the nominal voltage written on it?

4.17V

boby wrote on Today, 16:40:

95.6 kOhm

That's very strange. Could it be that for any of the resistance tests, you left the battery connected? Also for previous tests? Resistance must be measured with all power disconnected from the board. If there is still power, the measured resistance can be much lower than it really is.

If you're still measuring 100 kohm as before, it means that the new transistor's gate is also damaged - extremely unlikely. If you have 100 kohm at a node and remove a damaged part and it tests 100 kohm, the leakage from the node should be gone and you should get an almost infinite reading.

4,17V

Okay, this was just to confirm that you have no short circuit on the board.

asdf53 wrote on Today, 15:16:

With battery connected and switch on - voltage from pin 1 to negative battery terminal

If you are now getting 4.17V at this point, that's a very good sign - it was 0V before. This means that now the microcontroller is sending a voltage to turn the transistor on.

With the switch on, while there is 4.17V at pin 1 of the transistor, measure the voltage from pin 3 to pin 2 of the transistor. If it's lower than 4.17V, it means that some current is flowing.

If the voltage is still high, it means that no current is flowing. Remove the battery and measure the resistance from the positive battery terminal to pin 3 of the transistor. If you get a high resistance, it means that either the diode or the motor is blocking the current.

boby wrote on Today, 16:40:

95.6 kOhm

Back to this reading: if you are sure that the battery is disconnected, and you still get ~100 kohm from pin 1 to pin 2, do the following: Measure the voltage from pin 1 to the negative battery terminal. If it's > 0V, then short-circuit pin 1 to the negative battery terminal to discharge any capacitors, then measure resistance again.

If it is still 100 kohm, there is another possibility. Switch multimeter to beep mode and see if there is a connection from transistor pin 1 to any 100 kohm resistors on the board (they have 104 written on it). If yes, then your reading was correct, and there is a 100k pull-down resistor at the gate to turn the transistor off when the power switch is pressed.

asdf53 wrote on Today, 16:55:

That's very strange. Could it be that for any of the resistance tests, you left the battery connected? Also for previous tests? Resistance must be measured with all power disconnected from the board. If there is still power, the measured resistance can be much lower than it really is.

Yes it is with disconnected battery.

asdf53 wrote on Today, 16:55:

If you are now getting 4.17V at this point, that's a very good sign - it was 0V before. This means that now the microcontroller is sending a voltage to turn the transistor on.
With the switch on, while there is 4.17V at pin 1 of the transistor, measure the voltage from pin 3 to pin 2 of the transistor. If it's lower than 4.17V, it means that some current is flowing.

Sorry my bad. I get that voltage from pin1 to motor black wire. Sorry, got confused from so many things.
Still voltage between 2 & 3 is also 4.17V

asdf53 wrote on Today, 16:55:

If the voltage is still high, it means that no current is flowing. Remove the battery and measure the resistance from the positive battery terminal to pin 3 of the transistor. If you get a high resistance, it means that either the diode or the motor is blocking the current.

It is 10 Ohm

asdf53 wrote on Today, 16:55:

Back to this reading: if you are sure that the battery is disconnected, and you still get ~100 kohm from pin 1 to pin 2, do the following: Measure the voltage from pin 1 to the negative battery terminal. If it's > 0V, then short-circuit pin 1 to the negative battery terminal to discharge any capacitors, then measure resistance again.

It is 0V

asdf53 wrote on Today, 16:55:

If it is still 100 kohm, there is another possibility. Switch multimeter to beep mode and see if there is a connection from transistor pin 1 to any 100 kohm resistors on the board (they have 104 written on it). If yes, then your reading was correct, and there is a 100k pull-down resistor at the gate to turn the transistor off when the power switch is pressed.

It beeps with one resistor marked with 104 and also one marked with 100

Is it possible that when you measured the resistance from pin 1 to pin 2 after you cut off transistor leg, you put the probe on the pin that was still on the board, and not on the pin that's on the transistor? The purpose of isolating pin 1 was to see if the 100 kOhm leakage comes from the board or from the transistor. Take the old transistor that you removed and measure again pin 1 to pin 2, it's probably much higher than 100 kOhm and the transistor wasn't damaged at all.

In any case, now we know where the 100 kOhm comes from, it's from the resistor which is normal. But the problem is not solved, there is still no voltage at the gate, meaning the problem is at the microcontroller or a part near it.

I remember this measurement from earlier:

Switch OFF Switch ON Pin1: 4.11V | 0.06V Pin3: 2.03V | same Pin9: 4.14V | 3.94 Pin10: 4.09V | 3.94 […]
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Switch OFF Switch ON
Pin1: 4.11V | 0.06V
Pin3: 2.03V | same
Pin9: 4.14V | 3.94
Pin10: 4.09V | 3.94

This is interesting, because with the switch turned on, there is a quite substantial voltage drop at pin 9 and 10. For such a large voltage drop from 4.14V to 3.94V, it could mean that a large current is flowing, indicating a possible short circuit. The first thing you could try is to turn the switch ON and touch the microcontroller and any parts around it with your fingers. For example, the smaller three transistors above the microcontroller (Q2, Q3 and the other one), and the diodes. See if anything gets hot. Are pin 9 and pin 10 directly connected to the battery positive terminal?

Use beep mode of the meter to identify all points on the PCB that are connected to pin1 of the transistor. Where else is it connected to?

The next step would be to connect your multimeter in current mode to truly see how much current is flowing with the switch turned on. But that's a bit more complicated and possibly dangerous, so we'll do that another time.