Forum: Analog Circuits APC SMT3000I - resistor R11 (0R33 5W) smoking

von elk0 (Guest)

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I have two APC SMT3000I which both have the same problem.

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%.

I would be very grateful for any idea.
Greetings Florian

: Moved by User
von K. S. (the_yrr)

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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 [1] 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 
(two current transformer, one 230V 60Hz and one 240V with 1n4148 
switching diodes each, one 100kHz with battery input and 12V/24V output)

[1] https://elektrotanya.com/apc_smart-ups_su2200_3000.pdf/download.html

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von elk0 (Guest)

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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.

von Solomon E. (Company: Munics Wireless Network Limite) (musson)

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Please can you give me the values for the two crystals XT1 and XT600 on 
the main board of the APC smart ups SMT3000I,thanks.

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