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schematic

simulate this circuit – Schematic created using CircuitLab

I'm currently working on a project where we use a Raspberry Pi to print receipts via a receipt printer. I would like to use the same power supply for the Pi and receipt printer so we don't need to worry about staff at the restaurants that will be using this accidentally unplugging one component and not knowing about it. I tried to solder two wires onto the power supply input (from an AC to DC converter 24V, 2.5A out) and then step down the voltage through a voltage regulator. With this method I ended up burning the microcontroller in the receipt printer seemingly when I connected it to the Pi's USB ports. I have a few ideas why this might have happened but I'm not sure, could someone point me in the right direction?

Possible Problems -The voltage regulator is drawing more then 24V which would force the receipt printer to the same voltage -The voltage regular draws too many amps and few are left for the receipt printer -An issue with shared grounds between the pi and printer

More info on the printer (I don't think the datasheet has been released) http://hoinprinter.com/en/download LM2596 datasheet https://www.ebay.com/itm/NEW-LM2596-DC-DC-adjustable-power-step-down-module-NEW-GOOD-QUALITY-M13-/351493700220?_trksid=p2385738.m2548.l4275 LM2596 implementation board https://www.ebay.com/itm/NEW-LM2596-DC-DC-adjustable-power-step-down-module-NEW-GOOD-QUALITY-M13-/351493700220?_trksid=p2385738.m2548.l4275

This is an image of where I soldered the voltage regulator leads to the board. (These wires go to IN the the above schematic) enter image description here

Printer Boardsenter image description here

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  • \$\begingroup\$ Do you have a datasheet for the receipt printer? \$\endgroup\$ – The Photon Nov 10 '17 at 22:45
  • \$\begingroup\$ I don't think the company gives that out. These documents are as close as I've gotten: hoinprinter.com/en/download \$\endgroup\$ – Trent Walls Nov 10 '17 at 22:48
  • \$\begingroup\$ Does the "voltage regulator" module (datasheet plz) have an isolated output? If so, how are you grounding it to the 24V supply? \$\endgroup\$ – Brian Drummond Nov 10 '17 at 23:16
  • \$\begingroup\$ After looking it up the correct name is a step down regulator the data sheet can be found at: onsemi.com/pub/Collateral/LM2596-D.PDF I'm using a board from eBay with the chip pre-implemented on it ebay.com/itm/… . I am not doing anything special to ground it. I had assumed connected the negative pin on the out of the regulator chip would ground it. \$\endgroup\$ – Trent Walls Nov 10 '17 at 23:28
  • \$\begingroup\$ Did you measure the output voltage from the regulator? And also does the Pi still work? \$\endgroup\$ – awjlogan Nov 10 '17 at 23:32
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It happened when you plugged in the USB? That seems to suggest it created a short to me. You can probably test that with everything powered down. Plug everything together without the DC input and check with your multi-meter (although, you probably need to test with a board that isn't burnt out).

In that case you need to swap out that buck converter board for an isolated one. Really you should be isolating the Pi from the other board any way. Having them on a common circuit and then making another connection between them (USB) strikes me as pretty risky, especially without having any idea how the printer circuit is designed.

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From reading the printer manual and translating it http://hoinprinter.com/en/download , I do not see any supply spec near 27V. How did you determine this or was this a supply that came with the printer?

  • HOP-E58 58MM
    • Power Adapter Input: AC 110V / 220V, 50 ~ 60Hz
    • power supply Output: DC 12V / 2A
    • Cash box output DC 12V / 1A
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  • \$\begingroup\$ I'm using the 80mm version of the product which required 24V (hoinprinter.com/en/products/show/Thermal-Receipt-Printer-2#p1). I also tested the power input with my multimeter and it is in the 24.X range when I measure it with no load. \$\endgroup\$ – Trent Walls Nov 11 '17 at 0:55
  • \$\begingroup\$ You may have to insert a small series R to test and measure surge startup currents that may affect transient overvoltage. Can you see any cause for Latchup? external voltage applied before DC in? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 11 '17 at 1:08
  • \$\begingroup\$ There is a chance voltage could be coming from the USB before the printer is powered on via its "ON" switch. Although this should be a fairly regular occurrence so I would assume its designers would have accounted for it. \$\endgroup\$ – Trent Walls Nov 11 '17 at 1:12
  • \$\begingroup\$ Are there any R-Pi peripherals with different grounds? Is there an earth ground anywhere? YOu ought to measure the current thru a sense R on ground or V+ to each DC supply to diagnose the problem \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 11 '17 at 1:17
  • \$\begingroup\$ There are no other peripherals. I will try to set up an R test on the board to see what I'm getting. \$\endgroup\$ – Trent Walls Nov 11 '17 at 1:35

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