Hello,

New here and not as savvy as most on here, I’m sure. This is my shot in the dark. Hopefully someone might be able to point me in the right direction.

I have an air purifier that is quite pricey which I am trying to save in an effort to help my little one breathe better. Our office got a few in due to the pandemic. After about 2 minutes of running it fresh out the box “pop”! The thing is brand new, and was given to me when my office was going to throw it out after it “blew”. I take it home and open it up and find a 200mfd 25V capacitor blown on the circuit board. Everything else looks intact and there is no other damage that I can see. Easy enough, I have basic soldering skills and was able to get the specs off the shell of the blown capacitor which was still rattling around inside the unit. I order replacements. I get what’s left of the old one off the circuit board, clean up the old solder left over, verify my polarity and solder the new one in. I run it and after about 2 minutes, “pop” again! Same exact one. I figured that the board itself might (maybe?) have the polarity marked backwards? So I install yet another replacement capacitor in with the polarity reversed and now it won’t even come on. Oh, almost forgot to mention… before I ever opened it up to investigate what had blown after the very first time we tried to run it in the office when brand new, it continued to run what I would describe as just fine. No whining or straining sound, no abnormal conditions, everything seemed fine. Had I not observed it blow with my own eyes, I would never be able to tell something was wrong with it.

I’ve done a little research on start and run capacitors, so I know there’re differences. Not sure which type this one may be. I know the device powers on just fine and runs with the capacitor blown. Not sure how safe it is to continue to do so.

I’m running it straight from a wall outlet with no extension cords, with its proprietary power adapter. Any ideas why it may continue to blow the same capacitor?

Please excuse my electronic ignorance in advance. Thanks for your time and attention!

Regards,
Joe

 

Welcome to AAC!

Without a schematic, we can only guess at what's going on.

A picture of the original capacitor might be helpful. What used to be labeled mfd is now labeled as uF. Was the replacement marked as mfd?

 

Is the capacitor in question polarized?

Best guess lacking additional information is that something in the circuit is breaking the cap. For example: this is fed by a rectifier(s) one of which may be shorted.
As @dl324 mentioned, a schematic would help a lot.

 

Welcome to AAC!

Without a schematic, we can only guess at what's going on.

A picture of the original capacitor might be helpful. What used to be labeled mfd is now labeled as uF. Was the replacement marked as mfd?

I unfortunately don’t have the technical drawing/schematic for the board. I can take pictures of if it, not sure if that would help.

As for the capacitor itself, it is indeed labeled uF (original and replacement).

 

Is the capacitor in question polarized?

Best guess lacking additional information is that something in the circuit is breaking the cap. For example: this is fed by a rectifier(s) one of which may be shorted.
As @dl324 mentioned, a schematic would help a lot.

Thank you for responding. Yes, the original was polarized. The replacements were also polarized. I made sure to check and appropriately place it with its correct polarity to the board as marked. I will be opening it up again tomorrow and can take a couple pictures of the board. It seems like a pretty simple board, to be honest. Take that with a grain of salt from someone with my (very low) electronic knowledge.

I truly appreciate your time in responding. Thank you.

 

I can take pictures of if it, not sure if that would help.

Pictures of the board won't be much help.

If the unit operates without the cap, and you feel confident enough in your abilities, you could remove the cap and measure the voltage that the cap is across. If it's higher than 20V or so, that would indicate problems upstream of the cap.

If you don't feel competent to probe a line powered device with the covers off, don't do it.

As for the capacitor itself, it is indeed labeled uF (original and replacement).

You should use specify the markings verbatim. There's a factor of 1000 difference between 200uF and 200mF.

 

Are there rectifiers located near the capacitor in question? If so, check them to see whether they are shorted. Measure right across each rectifier with an ohmmeter (if an digital multimeter, use the diode test setting). A reading near zero means something, perhaps the diode, is shorted.

 

Pictures of the board won't be much help.

Actually we've diagnosed many problems for visitors to this board from good quality hi-res pictures of both sides of the PCB. It doesn't hurt for the TS to post some pictures.

 

Three things will make a capacitor pop, the first is wrong polarity, the second is excessive voltage, and the third is excess current. My first guess is that a rectifier diode has failed shorted, the second is that a voltage regulator has failed shorted.
Did thhis device ever work correctly for a reasonable time, such as several hours. If yes, then the problem is not backwards polarity. But more information is needed to make any reasonable guesses.

 

Three things will make a capacitor pop, the first is wrong polarity, the second is excessive voltage, and the third is excess current. My first guess is that a rectifier diode has failed shorted, the second is that a voltage regulator has failed shorted.
Did thhis device ever work correctly for a reasonable time, such as several hours. If yes, then the problem is not backwards polarity. But more information is needed to make any reasonable guesses.

Hi. Thank you for responding. The device never passed 2 minutes of run time before the capacitors popped (when brand new, as well as after replacing the blown capacitor with an equal rated brand new one). Although it does continue to run after the capacitor blows just as it did beforehand, if that makes any sense. I have unfortunately not been able to open it up again for some good pictures to post of the board in lieu of schematic/diagram of it. I hope to be able to do so tomorrow. Thanks again very much for your time in responding. Much appreciated.

 

OK, so the appliance was bad out of the box! Very interesting. When you say it still runs, that must be the fan running. So the capacitor is for some other function that has not been mentioned yet. Some "purifiers" use a UV light to kill germs,others use an ozone generator, and some claim an "ionizer." So the cap would seem to be part of that additional function, whatever it is claimed to be. Knowing what the function is can help us guess what the cap may be part of.

 

Is it powered directly from AC mains or is it powered from a plug in power supply or transformer. (Sometimes called a wall wart.)
Pictures of the inside of the unit would be helpful so we had some idea of what is inside the unit. Pictures of the component side and etch side of any circuit boards would be useful. (Focused well enough to read component part numbers.)

Les.

 

Is it powered directly from AC mains or is it powered from a plug in power supply or transformer. (Sometimes called a wall wart.)
Pictures of the inside of the unit would be helpful so we had some idea of what is inside the unit. Pictures of the component side and etch side of any circuit boards would be useful. (Focused well enough to read component part numbers.)

Les.

Mostly, because they all include a blower, these devices are mains powered because an AC motor is by far the best choice for this size of device. Even a cheap BLDC motor would cost a lot more that an induction motor of similar power level.

 

I was thinking it MAY use a small computer type fan as we have not even been given any idea of the size of the unit.

Les.

 

I have an air purifier that is quite pricey which I am trying to save in an effort to help my little one breathe better. Our office got a few in due to the pandemic. After about 2 minutes of running it fresh out the box “pop”! The thing is brand new, and was given to me when my office was going to throw it out after it “blew”.

If it blew 2 minutes into operation - return it for a refund OR for a replacement. If it was bad from the factory then there's no telling what other issues the unit may have. It could even start an electrical fire.

 

The first thing to do is to measure the voltage at the capacitor terminals, before replacing that capacitor. If it is AC, then probably a diode has failed short. If the voltage is DC, and the correct polarity, then the voltage may be too high, either initially or after two minutes.