Help identifying a motor driver board component?

[SpannersToSparks]

Hi

Our boiler decided to stop cooperating, and the technician replaced the whole fan unit due to a dodgy control board. This now means I have a motor and board to play with





However the whole motor board is coated in schmoo making it impossible to read the numbers on the components. I believe (but may be wrong) that this is a two coil BLDC (papst calls it an EC motor). It takes 230V AC line voltage and drops that to 14V DC via a full bridge rectifier and I THINK - a voltage regulator. A pin on the underside of the board going to the 'mosfets' says +14V.

My guess was that the "regulator" is the failed part - there appeared to be a darkening of the schmoo around it like it had overheated, and a lot of brown staining on the part itself (pics are from web and are of a good part)





As its a two coil BLDC, I am assuming that only 4 MOSFETS would be needed, meaning the lookalike sibling is not likely to be a mosfet. So I am assuming is a voltage regulator. To check I have done tests for mosfets (charging and discharging pins to check for LOW and HIGH), transistors (checking with DMM diode mode on each pin combination) and voltage regulators (wired it up to see if output pin produces a voltage), and none of them is a "PASS", so my detective attempts suggest to me that this is a regulator passing 14V DC to the mosfets.

Would you agree that's what this is, and perhaps suggest a replacement part? I don't intend to replace it in the boiler, but would like to see if I can get the motor and fan to run

[bktemp]

Using 14V for the mosfets makes no sense for a 75W motor power from 230V.
But 14V makes sense for the control circuit driving the mosfet.
What is the voltage rating of the large capacitor? Does it charge up to >300V?
Is there any voltage on the 14V rail?

[NivagSwerdna]

I actually have a spare one of those!  For the boiler I no longer have!

https://www.keeptheheaton.com/products/worcester-8-716-116-067-0-fan-assembly

What on earth do you plan to do with it?

[SpannersToSparks]

Using 14V for the mosfets makes no sense for a 75W motor power from 230V.
But 14V makes sense for the control circuit driving the mosfet.
What is the voltage rating of the large capacitor? Does it charge up to >300V?
Is there any voltage on the 14V rail?

Hi, thanks for the reply! I've not plugged the thing in as yet given the mains voltage, however what you are saying makes sense. The capacitor is rated to 400V and is 68 uF.

Now I look at the board, there does appear to be a line coming from the rectifier and straight round to the capacitor and on to provide a common header for the MOSFET pins.

Is it using rectified 14V for the drive circuitry, with the mosfets acting as a gate for rectified 230V? Can a voltage regulator actually step 230 down to 14?

[SpannersToSparks]

I actually have a spare one of those!  For the boiler I no longer have!

https://www.keeptheheaton.com/products/worcester-8-716-116-067-0-fan-assembly

What on earth do you plan to do with it?

Haha not sure, at the moment I would like to just make it run, but I may have some uses if I can convince that blower to put out a bit more than the 0.2 bar dp its capable of now

[bktemp]

Is it using rectified 14V for the drive circuitry, with the mosfets acting as a gate for rectified 230V? Can a voltage regulator actually step 230 down to 14?

Yes. Rectified mains voltage at around 300V is used for the motor and 14V for the drive circuitry controlling all mosfets.
There is a 8pin ic on both sided of the mosfets. Those are probably half bridge mosfet drivers, each controlling two mosfets for one motor phase.

It is impossible to see on the pictures how the regulator is connected to the mains input.
Often a capacitive dropper circuit is used for generating a low voltage supply from mains. But in this case this probably doesn't work because of the bridge rectifier.
There are some special voltage regulators capable from generating a low voltage from mains voltage (either a high voltage linear regulator or a phase control circuit using only part of the mains cycle). But it could also be a mosfet acting as a linear voltage regulator (there is a diode next to the regulator, it could be a zener diode).
Trying to read the markings on that part is the easiest way. Otherwise you need to reverse engineer the circuit. Look at the connections between mains, the regulator and the 14V rail to get an idea how the voltage regulator works.
Working on such circuits is difficult and dangerous, because everything is connected to mains. I recommend using an isolation transformer when working on the circuit.
I would start with verfifying all voltages (+320V on the large capacitor, +14V on the small one next to the regulator.
If both voltage are ok, then I would probably replace the small electrolytic capacitors.

[SpannersToSparks]

Is it using rectified 14V for the drive circuitry, with the mosfets acting as a gate for rectified 230V? Can a voltage regulator actually step 230 down to 14?

Yes. Rectified mains voltage at around 300V is used for the motor and 14V for the drive circuitry controlling all mosfets.
There is a 8pin ic on both sided of the mosfets. Those are probably half bridge mosfet drivers, each controlling two mosfets for one motor phase.

It is impossible to see on the pictures how the regulator is connected to the mains input.
Often a capacitive dropper circuit is used for generating a low voltage supply from mains. But in this case this probably doesn't work because of the bridge rectifier.
There are some special voltage regulators capable from generating a low voltage from mains voltage (either a high voltage linear regulator or a phase control circuit using only part of the mains cycle). But it could also be a mosfet acting as a linear voltage regulator (there is a diode next to the regulator, it could be a zener diode).
Trying to read the markings on that part is the easiest way. Otherwise you need to reverse engineer the circuit. Look at the connections between mains, the regulator and the 14V rail to get an idea how the voltage regulator works.
Working on such circuits is difficult and dangerous, because everything is connected to mains. I recommend using an isolation transformer when working on the circuit.
I would start with verfifying all voltages (+320V on the large capacitor, +14V on the small one next to the regulator.
If both voltage are ok, then I would probably replace the small electrolytic capacitors.

Do you think its the capacitors that have failed?

Sadly I cannot read the component at all due to the conformal coating. I even tried sanding it off, no luck! This is why I am trying to find out what on earth it is

I found this youtube video for mosfet testing



and it seems to indicate the part is in fact a MOSFET. The only anomaly is that in the 'on' state, it shows high forward voltage with red on drain and black on source (0.430), not far off the voltage with red on source and black on drain (0.530). The video and others like it indicate that in the former condition, the forward voltage should be positive but low, like 0.004

I believe I have an isolation transformer, will have a think about whether to push on further!

[bktemp]

You probably need to desolder the regulator, because other components in circuit can result in false readings.

Mosfets often fail short circuit. Therefore mains connected mosfets will either explode or at least blow the fuse. Since both doesn't seem to be the case (you have checked the fuseable resistor?), I don't think the mosfet is bad (if the regulator is a mosfet).
Capacitors instead fail quite often and can cause all kinds of problems.

A tester like this one can test capacitors and transistors, mosfets and diodes:
https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/

Before you start desoldering components, start with measuring voltages to see if the 14V supply is working or not.

[Bendba]

Acetone usually remove that coating, beware of the other plastics on the board.

[Alone GREYWOLF]

DC312V to DC14V POWER