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Testing Components

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I have taken two power components out of a circuit to check them. I think it's unlikely that they are defunct but I am reluctant to throw the circuit away without checking first. I know how to check basic components like diodes and transistors with a multimeter, but these are too sophisticated for me. They are:
  • MOSFET P75NF75
  • Schottky rectifier ???2045CT - I think the ??? are just the manufacturer's code
I imagine it would be easy enough if I knew which combinations of pins should be open or closed circuit.
 

audioguru

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A Mosfet needs a voltage higher than is produced by your multimeter to turn it on. You could measure an infinite resistance between its gate and source and between its drain and source when it is off (gate connected to source), then connect a 9V battery between gate and source to turn it on then measure the on-resistance between drain and source with your multimeter.

A Schottky rectifier is a diode with a forward voltage lower than a silicon diode, about 0.15V.
 

spec

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A Mosfet needs a voltage higher than is produced by your multimeter to turn it on. You could measure an infinite resistance between its gate and source and between its drain and source when it is off (gate connected to source), then connect a 9V battery between gate and source to turn it on then measure the on-resistance between drain and source with your multimeter.

A Schottky rectifier is a diode with a forward voltage lower than a silicon diode, about 0.15V.
I'll give it a go thanks. The Schottky rectifier is a sort of dual one - three pins with common
anodes cathodes, so I'll just need to do the same tests twice.
 
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spec

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I'm a Bristol kiddie.
I'm a Weston super Mare old man and go to Bristol often- my favorite city, especially the pubs.

spec

PS: if you put 'Bristol UK' next to 'Location' on your user page, it will display in the box at the left of your posts.
 
I'm a Weston super Mare old man and go to Bristol often- my favorite city, especially the pubs.

spec

PS: if you put 'Bristol UK' next to 'Location' on your user page, it will display in the box at the left of your posts.
I'm not that keen on personalisation thanks. By the way if you spell "favourite" in the British way it will help people realise that you aren't from the USA:)
 
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spec

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I'll give it a go thanks. The Schottky rectifier is a sort of dual one - three pins with common anodes, so I'll just need to do the same tests twice.
OK, I will draw up a test circuit.

Yes, the diodes are a pair in one case with their cathodes connected together, a common configuration.

spec
 

spec

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2016_10_27_iss01_ETO_DIODE TESTS_1.png
DIODE TEST 1: The ammeter shall indicate 50mA
DIODE TEST 2: The ammeter shall indicate 50mA
DIODE TEST 3: The ammeter shall indicate 0mA
DIODE TEST 4: The ammeter shall indicate 0mA
 
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spec

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2016_10_27_iss01_ETO_NMOSFET TESTS.png
NMOSFET TEST 1.1: The ammeter shall indicate 0mA.
NMOSFET TEST 1.2: Close the switch. The ammeter shall indicate 50mA.
NMOSFET TEST 2: The ammeter shall indicate -50mA.
 
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tomizett

Active Member
I usually test power MOSFETs just using the "diode" mode on a multimeter, looking for the following readings:
G to S in both directions: open
G to D in both directions: open
D to S (drain +): open
S to D (source +): a few hundred mV, like a large diode (this is the body diode if the FET conducting).

Vertical FETs that you commonly find in switching power supplies in TO220 and similar three-pin surface-mount packages are (nearly?) all pinned G, D, S from left to right, with D (centre pin) being the tab. Lateral FETs that you may encounter in older audio amplifiers generally have D and S swapped relative to the above.

FETs nearly always fail short circuit (less than a few tens of ohms) between D and S - a quick check with a multimeter will tell you if this has happened. Very often they will have also shorted to the gate, in which case the gate circuit has come into contact with the voltage being switched and I'll check and repair the gate drive circuit accordingly.

Of course I'm sure there are more subtle failure modes, and spec's circuits above are the only way to test if it's still really a working transistor, but I've found that if a switching FET in a power circuit has failed it's usually pretty unambiguous.

Hope this helps.
 

Mikebits

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NMOSFET TEST 1.1: The ammeter shall indicate 0mA.
NMOSFET TEST 1.2: Close the switch. The ammeter shall indicate 50mA.
NMOSFET TEST 2: The ammeter shall indicate -50mA.
What was the purpose of test 2? With the switch open your just testing the diode, was that your intent? Just wondering if I'm missing something as test 1. seems adequate.
 

spec

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What was the purpose of test 2? With the switch open your just testing the diode, was that your intent? Just wondering if I'm missing something as test 1. seems adequate.
Yes that is correct.

spec
 
I've tested the Schottky rectifier with the multimeter set to "diode" and as I would expect the two diodes conducted in the forward direction but not in reverse. I also tried the circuit above with an oldish 9V battery that gave 8.55 volts open circuit (but probably less under load), and got about 25mA in the right tests. Looks alright to me.

I have tried tomizett's tests on the MOSFET and it passed. I have tried spec's tests on the MOSFET and it passed. Well 20mA anyway, but it was the same oldish 9V battery.
 
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Thanks for the advice on how to test these components. You might be interested to know what circuit they came out of!

The circuit is an electric bicycle controller. There is a main circuit board and a small one with some infrared sensing components on - the pedal assist sensor (PAS) which checks whether you are doing any work:) I was fairly convinced that the problem was on the latter, but due to nasty mechanical design I wasn't able to remove it without damage, so I can't be sure. Having checked both semiconductors and found them to be sound, I think it is unlikely to be a problem with other components on the main circuit board and I wouldn't be able to test or fix it if it was. It's most likely to be related to the sensors on the secondary board.

I thought the problem was probably caused by a loose solder joint on the PAS board. You will be familiar with a mouse wheel sensor - a rotating disk with radial slots between an infra-red transmitter and an infra-red receiver. There are two of these. The left-hand pedal crank is attached to the crankshaft. The right-hand pedal crank is free to rotate on the crankshaft, but is attached to a disk with springs which bear against a similar disk on the crankshaft so that it rotates a few degrees before pushing the crankshaft. One slotted disk is attached to the crankshaft and one to the right-hand pedal, so both slotted disks rotate when the left-hand pedal is pressed and they rotate a few degrees relative to each other when the right-hand pedal is pressed. The infra-red transmitter and receiver pairs are fixed to the frame. I can upload photos of the arrangement if anyone is interested.

I was hoping that it worked by matching the pulses from the two sensors when the right-hand pedal moved a little under pressure. I therefore wired the sensors so that both sensors conducted, but the motor gives a kick then nothing, so I conclude that the sensors have to be 'flashed' on and off as well, perhaps so that the controller can check the speed. It's not practical to re-attach the slotted disks, so I would have to have a flasher circuit to simulate that - and I'm not quite sure what I need to simulate. All too much bother, so I'll chuck it and buy something else. It was worth a try though as it's daft to chuck something out that might work, so thanks again for your help.
 
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