APC Smart 700 high absorption currents in start up (power on) only

[Anonymous user]

Hi, I have 2 (two) UPS Smart 700, but only one of the two work bad (the problem happens) in start up only. Anyway, in the event of a power failure, both units work regularly and deliver energy perfectly. Both units are powered by a magnetothermic switch (circuit breaker 6A) which opens only when the unit (presumably faulty) is turned on, then consequently this unit starts to power from its own battery. By restoring the power supply i.e. by closing the circuit breaker the unit goes into normal operation. In my opinion, It looks like this Smart unit some problem during the (check) routine at start up only. I did a thorough internal visual inspection but I don't seem to have seen anything damaged or "diseased" by excessive heating. I apologize if I haven't better described the problem. Please, let me know your opinions and suggestion aimed to solve the problem. I am able to do more in-depth checks such as measurements, checks and parts replacements. Thanks a lot.

[BillP]

Hello, 

These units are 15+ years old now, my personal experience with them shows that suprisingly lot of problems can be solved or better yet, avoided by replacing small electrolytic capacitors on the PCB. If my memory serves right, SU700 PCB has 1x33uf, 1x100uf and 6x22uf capacitors (not entirely sure though, there can be different revision PCBs around with some differences). These can be bought cheap and is always considered good practice to replace them because the ones in the PCB are usually aged beyond spec by now. The use of higher capacitor voltage ratings are recommended, eg. 50v instead of 16v for 22uf capacitors. 

Take the unit to a service center, or attempt repair only at your own risk. 

[Anonymous user]

Thanks Wolf for the suggestion. Let me say that I love old PCBs (which are easier to investigate and repair). The capacitors that you have mentioned are labeled on PCB as C40 (33uF), C48 (100uF), C34, C35, C37, C53, C54, C80 (22uF). Seems that C40 (33uF), C37 and C80 (22uF) have a power filtering function, AND the C48 (100uF), C34, C35, C53, C54 (22uF) should play a role in some "time constants". The C53, C54 (22uF) are involved in RS232 connection only. Surely, at the first opportunity, I will replace the capacitors that you have mentioned. Any other suggestion?

Thanks!

[BillP]

First I advise testing the unit after replacing the caps, I did the same with a Smart 700 unit that ran on battery but always sounding constant overload tone, and even had the 30A glass fuses melted indicating high current flow through the MOSFETs. After replacing the capacitors, the unit worked flawlessly. It might be possible that high and low side of the MOSFETs were switched against each other, causing extreme heating, and eventually could have blown the entire inverter had I not replaced capacitors in time. These Smart-UPS units are well made I agree, they just need proper maintenance every 10-15 years 🙂 

[Anonymous user]

Wolf, today I replaced C40 (33uF), C48 (100uF), C34, C35, C37, C80 (22uF) on my Smart 700 unit without having solved the problem. Seems that the cause of the defect during power up is not due to these capacitors. Any other suggestion?

[BillP]

Need some more info on your setup: does the problem happen without having any load connected to the ups? Does it happen if you move the ups to an another power outlet, preferably to one which is on a different circuit? 

Do you mean your own circuit breaker 6A, or the safety pushbutton-type breaker on the back of the UPS? 

If your setup has its own circuit breaker installed in between your power grid and the ups, it is possible that the startup inrush current from the load connected to the ups causes it to trip. 

[Anonymous user]

Sure Woft, hereto is my additional info. The problem don't happen having any load connected, this was a good clue. Move the ups to an another power outlet is irrelevant. The circuit breaker 6A is connected to both (2) UPS Smart units (i.e. Is not the push button-type breaker on the back of UPS). Yes, Wolf, you are correct, the startup inrush current from the load connected to the ups causes it to trip.

But let me say that today I did several very interesting test. I would clarify that the problem (trip) follow/happen only if I connect PC Monitor in any of the two UPSs. Objectively, the monitor/s is the cause the "trip". I apologize, but I would specify that the "trip" happen ONLY alter the UPS was power off, disconnected from the power outlet, and pressed the "off button" on front panel and then waited a few minutes. In any other condition (power off then power on, disconnected and reconnected from the power outlet, etc.) the trip problem does not occur, all work fine, and here is why I was disoriented. Sorry.

I apologize again, but (in your opinion) it's possible that one PC monitor could have inrush current so high? The switching power supplies generally have high inrush currents. I know about to put NTCs in series with the line to limit the inrush currents. What you think about it? Let me know. Thanks.

[BillP · Accepted solution]

Yes it is possible having such inrush current with a single load. There is basically a very short moment of short circuit while the buffer capacitor takes charge at power on. The reason for needing to wait a few minutes having the device turned off is that it is the time required for the capacitor to discharge, and thus to cause inrush again at power on by charging. Most of the switching mode PSUs already have NTCs in them to dampen the current flow exactly because of this reason. However, instead of adding more NTCs, I'd change the circuit breaker to a slower type, yours seems to be too fast acting. Circuit breakers are rated by acting speed alongside amperage similarly to fuses, eg. slow, fast, very fast... etc. 

[Anonymous user]

I would like thank you for the time that you spent to answer me. Yes, surely, I will do so. I will replace the circuit breaker to a slower type. I wish you good things for yourself. Ciao.

[BillP]

I am glad I could help, I wish you the best as well.