Re-calibrating bench PSU...

On 7/17/2020 11:03 PM, bitrex wrote:
On 7/17/2020 10:56 PM, sea moss wrote:
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?

My guess is the second trimpot is used to calibrate the current limit
knob.  Try applying a short to the output through an ammeter and see
if turning the trimpot changes the display.


Yep that is right. Schem:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


Display reads the correct voltage but wrong current. The voltage across
current sense output cap C10 never budges much no matter how long I spin
that trimpot so looks like I\'ve also got a shorted pot, there.

There are two more pots on the control board to adjust the current and
output voltage limits of the knobs so I guess you could \"mod\" this into
a higher voltage supply straightforwardly, so long as you stayed within
the limits of the associated components and tweaked the current limit
down to keep max output power limit the same.
Click to expand...

They\'re not using low-offset op amps or precision resistors here as far
as I can tell so getting the display readout as good as it can be is I
think one of those things like intonating a guitar, tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can
 
On 7/17/2020 11:03 PM, bitrex wrote:
On 7/17/2020 10:56 PM, sea moss wrote:
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?

My guess is the second trimpot is used to calibrate the current limit
knob.  Try applying a short to the output through an ammeter and see
if turning the trimpot changes the display.


Yep that is right. Schem:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


Display reads the correct voltage but wrong current. The voltage across
current sense output cap C10 never budges much no matter how long I spin
that trimpot so looks like I\'ve also got a shorted pot, there.

There are two more pots on the control board to adjust the current and
output voltage limits of the knobs so I guess you could \"mod\" this into
a higher voltage supply straightforwardly, so long as you stayed within
the limits of the associated components and tweaked the current limit
down to keep max output power limit the same.
Click to expand...

They\'re not using low-offset op amps or precision resistors here as far
as I can tell so getting the display readout as good as it can be is I
think one of those things like intonating a guitar, tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

No mechanical limits, but yeah I should check if I haven\'t just spun it
out of the active range by accident
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

No mechanical limits, but yeah I should check if I haven\'t just spun it
out of the active range by accident
 
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(
Click to expand...

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
 
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