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HP HSTNS-PL11 psu Over Voltage Protection voltage increase?

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I am trying to figure out how to increase the Over Voltage Protection voltage on this supply as I want to modify it for running at 13.8V with quickly varying loads (transmit/recieve) for ham radio operation and I do not want to trigger the OVP.

I have the schematic and a pic of the mainboard, both are attached.

Others have modified this for higher voltage, and that seems pretty straight forward from the schematic. However, they can't reach 13.8V for actual use under quickly varying loads without the OVP kicking in,  They end up dropping the voltage down some to like 13v to keep it from happening.  it would seem there is a little overshoot when the load drops quickly. This would seem to be resolvable if the OVP voltage could be raised a little.

I've been studied the schematic for the way the supply does OVP is not apparent to me.  Anyone have an idea of how it works?

I have looked through the schematic and I may have good news for you.

Apparently the power supply is using a microcontroller to monitor the +12V voltage level via its ADC input to provide the OVP function. In the attached photo, the +12V is divided down to below 3.3V and fed into the uP2 pin15. Thus reducing R901 would do the trick. You can do this by parallelling another high value resistor across R901 via trial and error.

A better method is to measure the pin15 voltage along with the actual +12V output voltage and work out the divider ratio. Then it is a simple matter to increase the OVP to say 14.5V or 15V by changing the divisor ratio.

Initially I couldn't find the MC9S08QG8 on the circuit board but then I noted from a chinese website that it is listed together with MQ64CDTE so they are equivalent. Hope all these helps in solving your problem. Do let us know of the result.

eblc1388,  Thanks, The Chinese version of the uP is a great find, I was assuming that was a uP but couldn't prove it out.

I also saw that voltage divider going into the uP after i posted.  That makes sense and I agree thats likely the place it gets detected, and glad that you think the same. I think that is the right track to go for. it also matches the flow diagram for what they call uP1

There are 2 uP's on the board, one MC9S08QG4 and one MC9S08QG8.  From the information you found, the MC9S08QG4 is the one we can see in the picture and the MC9S08QG8 is the one that the voltage divider of interest is connected to seems to be on another board in the PS that i do not have a picture of.  I belive i found the board and it looks like that board is hard to see without removing the PS completely from the chassis.  So that will be my next step.

I've also made some sense of the labeling on the schematic of things like MB1 SB1
SB2 etc. those indicate the board the components are on and the letters may mean the side of the board so SB1A is one side and SB1B is the other.

The MC9S08QG8 would then be on board and side SB2A which judging form the number of components on the board would be that small board i don't have a picture of yet. unfortunately the boards are only labeled that way on the schematic and not the silk screen as best i can see. I did further conform the small board by finding the programming header jpm900 which is visible with a silk screen label on the that same small board.

This looks very promising, I'll post my progress.

hi, i have same issue, but mine different MQG4CDTE

did you OVP voltage ?

The location of uP2 in my Reply#1 is wrong. The correct location is as follow. Noted that these power supplies have had many revisions and the actual position or MCU type might be different. Mine is PIC16F883 while others have reported PIC16F886. even getting the correct part numbers are difficult as most are blocked by other components.

Once you have found this MCU, then you can carry out the OVP mod as in my reply#1.


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