Invertor 48V /220V

[Jane]

The invertor has  2 air fans that start only  when quite a high electricity consumption is needed . Otherwise the fans are  quiet. It worked that way for about 2 years. Now,however, the  fans work all the time.

On the enclosed pictures you can  see   a   region   where fans are connected. It seems quite simple. Anyone knows the principle , how the  fans are controlled?
Thank you

[Kwakerman]

Looks like you have a temperature sensor attached to the heatsink (first picture top heatsink).  Might be for overtenp protection or fan control (or both).  Suggest you try and find a circuit drag for it.

[Jane]

Thank you for the reply.
Can you please mark in the picture where do you think there is the sensor?

[voltsandjolts]

It's here:

[johansen]

the traces on the board run all the way to it.

looks like the fans get power from a rectifier and aux turns on the transformer.

[Vovk_Z]

Yes, fans get power from a main transformer. The higher load the higher FAN voltage. But there possibly is BJT transistor which controls swit hing on and off of a FAN. And possibly it became shorted.

[coromonadalix]

yep i would have said the same thing,  come from the main ac voltage then rectified as DC, a crude way to make them work ... and they are 2 wires only ...

the thermal sensor should be used in the main controller section

[BrokenYugo]

From the pics I would guess transistor Q33 is either shorted or whatever can turn it on is.

[Jane]

Finally I checked Q33 and Q31 transistors and they are good. I measured the temperature sensor, which is NTC termistor. It has only about 5kOhm. Is it not small value and because of that it is the culprit why heatsink turns all the time?