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necroware / ps2-serial-mouse-adapter

PS/2 to serial port mouse adapter
GNU General Public License v3.0
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Maxim MAX232 hot without highside charge pump cap #8

Open hazelutf8 opened 1 year ago

hazelutf8 commented 1 year ago

After assembling board rev v0.6, a MAXIM branded MAX232CPE got hot very quickly after first power on.

Searching online, it seems some MAX232 chips allow low or high side charge pump cap use, but the MAXIM chips require the cap be attached to VCC, and for brand compatibility it is suggested to use a high-side charge pump cap. How to solve MAX232 heating

In order to verify, the C4 ground-side traces were cut, two on both the top and bottom of the board, and a wire was jumped between that cap leg and +5V (using a non-polarized cap for C4). This modification was verified as working with a Microsoft "Mouse Port Compatible Mouse 2.1A" (FCC ID C3KKMP1).

Please let me know if you would like any other FW or HW tests.

necroware commented 1 year ago

Thank you very much for this input, very interesting. I'll try this with different ICs and if everything is alright, I'll change the circuit. In the proposal, it looks like it is ok to use a polarized cap as well. What do you think about it? Would be nice to keep the capacitor types the same if possible.

hazelutf8 commented 1 year ago

I used unpolarized caps since I had them on hand.

From my reading, polarized caps are fine, and should be aligned to whichever power rail direction used. (Your v0.6 design has cap negative leg to ground and positive to the chip, but I used positive to the positive power rail and negative to the chip pin.)

The TI MAX232 datasheet rev M, figure 6, note B, states both polarized and unpolarized are fine.

hazelutf8 commented 1 year ago

The Maxim MAX232 datasheet also allows polarized caps of the same value, but the application circuit uses the opposite power rail for its charge cap. https://www.analog.com/en/products/max232.html

Edit: I mis-read, the positive cap direction is still the chip pin.

jsantcl commented 1 year ago

Hello, I built one using the same max232cpe, got really hot and the mouse is not detected by computer, it's turning the infrared on but no luck with the ctmouse scan. Any troubleshooting instrucctions ? ( I'm no electronic expert just followed the instrucction on your repo) Thanks!

necroware commented 1 year ago

As was mentioned in the first comment in this issue, some max232 are different and seem to be not compatible with the released circuit. max232cpe is such an example, but you can workaround that by removing C4 and soldering it between pin 2 (cap positive lead) and 16 (cap negative lead) of the max232cpe.

stevenhoneyman commented 1 year ago

Hello! Did this hardware fix get tested since? I was going to order some PCBs to build myself one of these but if this enhancement will make it to a v0.8 release of the gerbers then I should probably wait for the fixed version. Thanks

necroware commented 1 year ago

Didn't test it yet, but I made a v0.8 release. Feel free to test and report: https://github.com/necroware/ps2-serial-mouse-adapter/releases/tag/v0.8

jsantcl commented 1 year ago

Hello, did the change for the max232 got very hot and didn't work, then tried with the TI 232 and didn't work either, maybe I've got faulty chips. Is there a way to test the 232 chip ?

jsantcl commented 5 months ago

v0.8 fixed overheating issue, tested with Maxim max232cpe Ti max232n

thanks!