72v to 12v 10A output is high 19.6v

[marce002]

hello, this is from electric moto scooter... something happened and blown an onboard fuse in the converter box..replaced it and also replaced a bulged cap 100v 100uf ... now works but get 19v in the output instead of 12V...
what else to check please!! this is the board:

[R_J]

It looks like the feedback voltage on pin2 is too low, check the components around the optocoupler and the opto itself, you can check the voltages on the opto as well, check the two pins on the primary side using pin5 of the KA3845, as ground, and check the two pins on the secondary side using the black wire as ground. post the optocouplers number as well
Looking at the circuit, the vcc pin is also connected to one pin of the opto on the primary side, so one pin should be around 19v. the pin next to it that goes to pin2 of the ka3845 (via a couple components I can't see in the picture due to the wires) should have around 2.5v.

You could also just check the voltage across the two pins of the opto, check across the two pins on the secondary side (led) and across the primary two pins (photodiode)

[R_J]

You likely need a load on the output and it would drop down to 12v, Without a load the output cap can charge to the peak voltage of around 19vdc

[marce002]

that is the first i check... it still is 19.34v with lights on and all turned on... besides the audio center and BT device does not work due to overvolt... what else to try?
note the 3 resistors 1kohm near the transformer seems to be quite hot to touch, but maybe normal

[sam_sam_sam]

What is your battery voltage at the battery

[R_J]

Post a clear picture of the back of the board so we can trace the circuit.

[marce002]

battery is 84v ... pictures:

[R_J]

What is that solder blob on pin 5 of the ic?
There is an optocoupler off the output as well as a glass diode, the diode may be a zener diode? There are a couple components under the output wires I can see. It may be that the opto is not providing feedback so the power supply can't see the output voltage and is running full output

The three 1k resistors seem to be in parallel across the output.

[sam_sam_sam]

Is this an electric car or something else

[truclacicr]

This appears to be the main controller (UC3845D):

https://www.mouser.com/datasheet/2/149/uc3844-309655.pdf (datasheet)

https://www.ti.com/lit/ds/symlink/uc3842.pdf (datasheet and application circuit)

AIUI, the VREF = 5V and VFB = 2.5V when the IC is regulating properly. If these voltages are correct, then I would be looking at the resistive divider that is connected to the output voltage that is being sensed.

[R_J]

Yes, it is an ICF KA3845D, the ic is working, but seems to be lacking proper feedback

[marce002]

The values measured are (no load )

[R_J]

You need to use pin 5 (ground) of the ic for reference when checking the ic voltages. what is with that solder blob I asked about in post #7
The primary and secondary grounds are isolated by VD1 (can't see its number) likely a dual fast recovery diode

[sam_sam_sam]

Here is the data sheet for this KA3842B

https://www.mouser.com/datasheet/2/1...iRhhm50qRK_C0_

[marce002]

sorry, the blob is ok, just me replacing a zener but was not shorted and the other blob on + is a wire + used with my power supply to test also ok.

i will recheck and report back Volts, because ic gnd was not my reference, used wire conecror gnd, my fault.thks

[marce002]

hi the values from GND pin 5 with out load (blue) and with lights (green) on are:... and just in case the diode im VD1 and the other one on left V1 are: VD1= MBR20100CT and V1= HY1710

[R_J]

It looks like the feedback voltage on pin2 is too low, check the components around the optocoupler and the opto itself, you can check the voltages on the opto as well, check the two pins on the primary side using pin5 of the KA3845, as ground, and check the two pins on the secondary side using the black wire as ground. post the optocouplers number as well
Looking at the circuit, the vcc pin is also connected to one pin of the opto on the primary side, so one pin should be around 19v. the pin next to it that goes to pin2 of the ka3845 (via a couple components I can't see in the picture due to the wires) should have around 2.5v.

You could also just check the voltage across the two pins of the opto, check across the two pins on the secondary side (led) and across the primary two pins (photodiode)

[marce002]

Thanks a lot T J!! it was a 320 ohm resistor close to the opto, whose value was wrong and turned out to be burned in the hidden bottom side. tks now output is 12.8V the end