Hello, I have two APC SMT3000I which both have the same problem. https://www.apc.com/shop/de/de/products/APC-Smart-UPS-3000-VA-LCD-230-V/P-SMT3000I If you turn on the UPS and it performs a self-test it can happen that the load resistor R11 - 0,33Ohm, 5Watt (Only once on the board) starts to smoke and then burn out. This can also happen when switching to battery operation. The probability that the resistance smokes during the self-test is about 50%. If the UPS passes this, the probability is again at about 50% when switching to battery operation with the same problem. The UPS produces then a very unhealthy "screaming" noise (see video). I have already replaced the resistors of both UPSs several times after they were completely blown out. The error message on the display then showed "Overvoltage at the output". As described, it can happen that the UPS easily pass the test and also run smoothly on battery, without the resistance burns through. (Battery calibration has gone through recently without problems) I have the assumption that in both scenarios the power from the batteries is sent somewhere wrong through the board, and produces a short circuit within the board. (Relay not switched ?!) Here is a video with new batteries, 100% charged (self-test passed, even with smoking resistor). : / You can hear how the power can not even be maintained for 250W and the hot air gun is weakening. After 20 seconds, the battery was at 55%. https://youtu.be/y2zbfGiP5Fg I would be very grateful for any idea. Greetings Florian
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elk0 schrieb: > I would be very grateful for any idea. you need the schematic, if you cant find one online you have to at least reverse the circuit around the resistor. if you know where the connections from the resistor lead to, check the parts in its vicinity. if there is a relay connected it could have contact problems. it is very unlikely that this resistor is in the 12V circuit, it could be a current sense for the 230V in/output or an auxiliary power supply. i have found this  but it doesn't have a 0.33Ohm resistor nor does the the whole schmeatic make any sense, the XFRM1/2 labels from the 12V switching transistors aren't found anywhere else, i couldn't identify any hf power transformer or hv DC circuitry, something seems to be missing. (two current transformer, one 230V 60Hz and one 240V with 1n4148 switching diodes each, one 100kHz with battery input and 12V/24V output)  https://elektrotanya.com/apc_smart-ups_su2200_3000.pdf/download.html
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Thanks for reply, I got the confirmation in another contribution too that it's moste likely that the relay is defective what bridges the resistor when the UPS is running on battery / doing a self-test.