Electronics help required with Sofar ME3000SP

[Swwils]

Have you confirmed that opto is still OK?


That 4R70 might be knacked.

[Marcus]

Ok so there's a nice 5v square between pins 2 and 3 on three opto input, ( good). But there should be a nice square on the opto output , pins 6, 7 , swinging between local (i.e.gate drive) 0v (pin5) and local supply (pin8).

Begs the question: when you probe the gate voltages, what are you connecting the scope gnd to? (Should be pin5 on the opto or, better: the source pin of the fet - although they're probably common if I'm guessing correctly)).

[Tinbum]

Begs the question: when you probe the gate voltages, what are you connecting the scope gnd to? (Should be pin5 on the opto or, better: the source pin of the fet - although they're probably common if I'm guessing correctly)).

Yes source pin of the FET (battery negative)

[Tinbum]

Ok so there's a nice 5v square between pins 2 and 3 on three opto input, ( good). But there should be a nice square on the opto output , pins 6, 7 , swinging between local (i.e.gate drive) 0v (pin5) and local supply (pin8).

The trace is exactly the same as the gate trace.

[Tinbum]

Have you confirmed that opto is still OK?


That 4R70 might be knacked.

I'll have a look at 4R70.

It's so hard to test things without removing from the board and smd is not easy to work with. (Magnifying goggles on my birthday list).

Why are SMD parts so hard to identify? I'm trying to find diode 58.



I tried putting my meter across it and it seems to show the same continuity either way. (D51 on the bank of FETs above shows no continuity one way).

[Marcus]

Ok so there's a nice 5v square between pins 2 and 3 on three opto input, ( good). But there should be a nice square on the opto output , pins 6, 7 , swinging between local (i.e.gate drive) 0v (pin5) and local supply (pin8).

The trace is exactly the same as the gate trace.

The horrid wiggly one? : that could be either because the opto is faulty, or if its output is shorted. Is the voltage between pins 5 and 8 (gate drive supply) stable? If the pin8 voltage drops when the opto turns on (or off) that suggests there's a short somewhere.

It's hard to see, but it looks like there are a couple of transistors/fets between the opto and the fet gates. I'm guessing it's a totem pole drive to provide more powerful switching drive to the fet gates?

[Marcus]

Have you confirmed that opto is still OK?


That 4R70 might be knacked.

I'll have a look at 4R70.

It's so hard to test things without removing from the board and smd is not easy to work with. (Magnifying goggles on my birthday list).

Why are SMD parts so hard to identify? I'm trying to find diode 58.



I tried putting my meter across it and it seems to show the same continuity either way. (D51 on the bank of FETs above shows no continuity one way).

Could be the diode: are you measuring with the diode test function? What is the reading you get? I would expect 0.3 - 0.7 v one way and much higher the other. but i would try and trace out the gate drive circuit on paper to see what the diode is across. Personally i would expect a semiconductor to fail before a resistor or cap, but you never know. With resistors you should get a reading in circuit of <= stated value; if it's higher its definitely suspect.

[Marcus]

I.d.ing smd is a nightmare - sonetimes you have to work out how the circuit is supposed to work , then work out what the devices have to be to do that (n fet / p fet for e.g.), then look for devices with the same physical characteristics until you find one that's the right sort, and has the right case style and the right markings. To make matters worse the marking are not unique. Not easy.

I got lucky with immersun #2 in that there were some tiny 6 leg devices that had to be dual mosfets, but due to the circuit configuration they had to be one n type and one p type - quite unusual. I eventually i found one with the right 3 letter i.d. and the other two letters were a date code that broadly matched the manufacturing dste of the immersun. They worked anyway.

[Tinbum]

Could be the diode: are you measuring with the diode test function? What is the reading you get? I would expect 0.3 - 0.7 v one way and much higher the other. but i would try and trace out the gate drive circuit on paper to see what the diode is across. Personally i would expect a semiconductor to fail before a resistor or cap, but you never know. With resistors you should get a reading in circuit of <= stated value; if it's higher its definitely suspect.

No hadnt done that but have now. 0v one way and 0.6 the other- same as the diode on the circuit that works.

Is the voltage between pins 5 and 8 (gate drive supply) stable? If the pin8 voltage drops when the opto turns on (or off) that suggests there's a short somewhere.

Great pointer- thanks.

5 to 8 - Voltage on the good one is about 13v, voltage on the bad one is 0.6V.

Getting one on order.

[Marcus]

Ok, so something's shorted- but before you order stuff I'd separate the suspected component and see if the short goes with it (and the voltage comes back up). It could be the opto, the totem pole drivers, the diode or (if they're still there) the fets themselves. Or you could replace everything just to be sure - if you can work out what they are.

Edited to add that we can't assume the gate supply itself isn't the fault - until you isolate the fault(s).