logic board repair

i looking at a Emerson ECM for a variable speed 3 phase ac motor. it appears the thermistor that comes off the bridge rectifier had exploded and someone soldered the legs of it together and bypassed it. I'm confused when i look at the circuit board because it looks like the thermistor goes from the ac side of bridge rectifier through a jumper wire and to the positive side of a DC capacitor bank.

From my experience ac on a dc capacitor cause them to explode. My question is if this thermistor is now missing and the legs are just bridged together could this work? no one really has any info on these ECM boards i had to pick all the glue off the board to uncover everything.

sorry i just re read that. obviously bridging the ac side of a power rail to a dc capacitor is not going to work. not only that but that probably not even how its configured, i wish i knew what i was looking at lol

Hello notsolowki,
You asked the mods to close this thread, sorry, there's no such option. You had a problem then realised the solution yourself. That might be useful to someone else.

Thanks.

take a photo, maybe someone has the time

It is usually an NTC thermistor that limits the inrush current into the bus caps at power up. As current flows through the thermistor, it heats up and the current increases until cap voltage reaches peak ac voltage. It’s a common method used in larger switch mode power supplies and smaller motor inverters.

With it jumped out, diode bridge failure and or input fuse blowing is the likely result. It’s all about the peak current that flows at initial power up when the bus caps are at zero voltage.

How should i choose a value for the thermistor? with the thermistor jumped out the board will power up and the cpu is send and receiving data with the furnace logic board. I used a scope to check the signal generator wires to the motor drive chip and there was no activity.

Without the thermistor jumped out the furnace logic board complains no communication with motor ecm. From what i can tell the furnace cpu tells the motor ecm to spin up with motor but there is never any motor drive signal output from the motor ecm cpu so the furnace detects no air pressure and complains fans not running.

The attachment size limit is frustrating

the bottom circle is the two thermistor legs.

If the board powers up with a jumper, shows microprocessor activity but a motor does not run and then without the jumper it’s dead, there is no point in replacing the thermistor.

The problem was probably over-current in the motor circuit which took out the thermistor but didn’t blow the fuse. I’m surprised that something didn’t blow up when it was powered up with a jumper but hey, there’s lots more in play that we haven’t seen.

Honestly, without schematics and parts, which probably don’t exist in the wild, it’s pretty much a lost cause. Those kinds of control boards are cost engineered and not designed to be repaired. Board level repair just isn’t done. It’s replace, not repair in today’s world. Spare board sales are just another revenue stream.

WattsThat:
If the board powers up with a jumper, shows microprocessor activity but a motor does not run and then without the jumper it’s dead, there is no point in replacing the thermistor.

The problem was probably over-current in the motor circuit which took out the thermistor but didn’t blow the fuse. I’m surprised that something didn’t blow up when it was powered up with a jumper but hey, there’s lots more in play that we haven’t seen.

Honestly, without schematics and parts, which probably don’t exist in the wild, it’s pretty much a lost cause. Those kinds of control boards are cost engineered and not designed to be repaired. Board level repair just isn’t done. It’s replace, not repair in today’s world. Spare board sales are just another revenue stream.

i have documentation of all the chips on the board. these boards are actually quite expensive. This ecm has an onboard EEPROM. it has write protections. i know the CPU runs C++/C/Assembly from the docs. do you think the CPU would reprogram a corrupted EEPROM? if it is some error lockout, i wonder if its saved in the EEPROM. if i could clear the EEPROM maybe it would work again?

To me it looks like the jumper wire is bridging the line from 120ac to the dc capacitors? am i crazy is this normal or am i missing somthing

Since I had KiCad open anyway, and allowing for the limitations of your pictures, this is what I think you have with the "jumpers" and capacitors:

I'll leave the explanation to someone else, but you should tell us what is connected to the jumper wires.

That would make a split DC supply, as in +175(?)V 0V -175(?)V
Except that 0V would very much NOT be ground! This is one of those cases where ground and 0V are NOT the same thing and must not be connected together.

i know not enough to fully understand why the DC capacitors oxide layer don't breakdown and short with ac voltage connected. is the only reason they don't explode be because they are operating and much lower than their rated voltage?

arduarn:
Since I had KiCad open anyway, and allowing for the limitations of your pictures, this is what I think you have with the "jumpers" and capacitors:

I'll leave the explanation to someone else, but you should tell us what is connected to the jumper wires.

The Jumper wires go into a 5 pin connector and are connect to each other at the connecter. the jumper wires are connected together

"JUMP3" and "JUMP4" are the selector for 110 Volt operation. The jumper is open for 220 V. :sunglasses:

This was present in all the early PCs.

Paul__B:
"JUMP3" and "JUMP4" are the selector for 110 Volt operation. The jumper is open for 220 V. :sunglasses:

This was present in all the early PCs.

Nice. So really that circuit is just a slightly more luxurious version of a Delon voltage doubling circuit?

I'm guessing the flying tails would have been crimped together according to the instructions by the original installer for the local supply voltage.

Paul__B:
"JUMP3" and "JUMP4" are the selector for 110 Volt operation. The jumper is open for 220 V. :sunglasses:

This was present in all the early PCs.

Hi Paul, is my explanation in reply #12 incorrect? Thanks.

Well, on 110 V it is a split supply, on 220 V you cannot use it as a split supply because only the resistors balance out the charge on the two capacitors so you cannot use them separately - that is why the resistors are there. :grinning:

Ah!
Yes, thank you!