6 ms·

Unless you know what you're doing (and I'm not saying that you in particular don't) this is incredibly dangerous. The capacitors in A/C units can store enough e

Unless you know what you're doing (and I'm not saying that you in particular don't) this is incredibly dangerous. The capacitors in A/C units can store enough energy to kill you, even if the A/C has been unplugged for hours or days. I would not try this at home.

hourislate 5y ago
It's actually a simple fix. You can buy the capacitor on AMZN, kill the main breaker in the box near the AC unit. Get a screw driver and disconnect it and if you want you can then cross the leads with the screw driver to make sure it's drained. Install the new one and flip the breaker and your set to go. Cap costs around $20 and you you saved yourself a $100 service call.
- ericbarrett 5y ago
  Shorting a capacitor of sufficient size can send molten metal bits flying around as the leads melt under high current, or even make the capacitor explode. I’m all for DIY repairs, but if you’re going to do this at least wear safety goggles. Also, short the capacitor after turning the power off and before unscrewing it—that’s the time you’re most likely to complete the circuit with a fleshy part. Edit: There might also be other bits in the system with residual power that refill the capacitor even after unplugging the mains, so a voltmeter is a good idea as well.
  - Faaak 5y ago
    I'm sorry, but you speak like you're living in a movie. Shorting a starter cap won't do anything of what you said. These caps are actually small (couple hundred uF). At the worst you'll produce a small spark. nothing more.
    - ericbarrett 5y ago
      If you think you know exactly what's going on with a strange cap and want to take that risk, go for it. Me, I like my forearms unscarred.
  - Dylan16807 5y ago
    > There might also be other bits in the system with residual power that refill the capacitor even after unplugging the mains, so a voltmeter is a good idea as well. Like what, when it was already turned off? And especially, what do you imagine recharging a significant capacitor to more than 50 volts?
coupdejarnac 5y ago
It's not a big deal as long as you're aware of the danger and discharge the capacitor before removing it. The capacitor is usually the culprit, especially in hot environments like where I live.
- tylerhou 5y ago
  That falls under "if you know what you're doing."
userbinator 5y ago
It sounds like this is a motor start capacitor, which is going to be connected to the motor and discharged if the power is off. Do the research instead of spreading paranoia and FUD.
- tylerhou 5y ago
  Even if motor start capacitors are safe to touch, if you don't know what you're doing you might accidentally make contact with a dangerous capacitor. And the parent comment didn't clearly state to only change motor start capacitors -- they seem to endorse changing all capacitors without any warning. Please don't accuse others of spreading FUD when this type of repair is legitimately dangerous.
  - CamperBob2 5y ago
    You might get run over by a bus on your way to the store to buy the capacitor, too. Don't overlook that possibility while you're jumping at shadows.
  - Dylan16807 5y ago
    Capacitors such as what? A normal AC system is basically two fans, a compressor, and a bunch of pipes, right?
    - pwg 5y ago
      A motor capacitor: https://en.wikipedia.org/wiki/Run_capacitor https://en.wikipedia.org/wiki/Run_capacitor. Both the fan motor (external and internal fans) and the compressor itself will each likely have one. And should they fail, the motors will not start, which gives the appearance of a failed motor for the fans or the compressor, when what needs to be changed is a $5 capacitor.
      - Dylan16807 5y ago
        No, that's my point, they were arguing that there were other kinds of capacitor to worry about.
breput 5y ago
It is theoretically possible, but if your start/run capacitor has failed, it is not going to be dangerous and it is designed to be replaced. Wear (rubber) gloves, short circuit anything you're going to touch with an insulated screwdriver, and you'll be fine. A new capacitor is about $20 vs. $250+ for any service visit.
deleted 5y ago
[deleted]
petre 5y ago
Come on, it's an AC single phase assist cap, not a DC cap of >1000 uF like those found in camera flash units. Just short it with a load before you touch it to be 100% sure.
- grumpy-de-sre 5y ago
  Come to think of it are medium sized high voltage capacitors really that much of an electrocution risk in the first place? I mean sure as anything they can throw you back into your seat and cause a relatively decent burn but in practice are they really an electrocution risk? You're not going to get trapped and unable to let go. Edit: Looks like the biggest risk from a high voltage DC discharge is myocardial depolarization, and that's dependent on the current path and the timing of the heart rythym, but that's typically at energies an order of magnitude more than a charged typical motor capacitor
  - deleted 5y ago
    [deleted]
  - petre 5y ago
    >75 mA across the heart might be fatal. An AC motor capacitor would probably just give you a short shock since it's only used to introduce a phase shift on the auxiliary phase in single phase AC motors (what the OP is talking about) or for power factor correction in 2/3 phase AC motors. AC switches every 20 ms from positive to negative so the cap only stores very little power. I'd be more weary of capacitors in DC circuits.
barbegal 5y ago
All modern appliances should have a bleed resistor fitted to drain the capacitor within a few seconds of the power being switched off.
mdip 5y ago
Ah, yes. This is a point I really should have made. If you've been "bit" by a small cap on a circuit board, imagine one as big as an adult's fist. I don't recall the output of this thing but one needn't read the label to know that if I can't completely enclose this thing in my hands, it's probably not something one should...enclose in ones hands? It is also in a nice metal container that's probably under more pressure than it should be; mine were always bulging. With board mounted caps, a lot of this stuff lands in the "leaving pizza out overnight" category. I've only had one (small one) truly explode[0] due to age/being defective; several that I blew up in electronics class when the teacher's head was turned, but hey. And about a flobbidy-gillion that bulge, you could lick the leads (that's sarcasm, don't actually do this). Now, fan caps/mostors like this -- I've been bit by (a 400V cap that was "not dead yet") -- I don't know that I actually touched the leads so much as some hair on my exposed arm made contact (it's why I wear my long welding gloves!). ...There's that moment well after you've registered the burning sensation and smelled the hair that you say a prayer of thanks to God while panicking that the business end is not yet any safer than it was a second ago. The (somewhat unsafe) procedure I used was: Cut the power, pop the fuses, pulled the wires in the order I had in my notes from last time (with thick welding gloves[0]), pre-cut electrical tape to cover the three leads (model has lots of distance, it's a big cap). The cap is connected by three wires, far from the rest of the electronics, easy to get to, easy to remove/re-secure the wiring, admittedly -- easy to get a little too comfortable. Carefully handle those leads; hopefully goes without saying -- even if the cap isn't bulging -- avoid heat-based treatments (hot glues/heat shrink). I tape the hell out of the big ones. I've only had to personally handle a small number of these, but there's a TV repair shop in the area that does free electronics recycling. I told the puzzled 15 year old at the counter what it was, he looked puzzled, so I explained that it can explode like a hover-board battery and he shrugged. I guess...good enough. [0] In a manner that might have been a fire hazard, and were it exposed, could have taken an eye out. In my case, a moderate "pop", a black spot on the board, smoke, and shrapnel throughout the small case.
- mdip 5y ago
  Just to disclaim a little more -- I definitely would do this at home, and the first time I did something like this was before I started doing any real circuit work at home. I take into account the typical audience of the site, though. Most of us tinker/work with highly complex things that require good research skills even if they don't touch electronics. The basics of how a fan capacitor works, how to handle one safely -- including testing, and discharging, can be done for less than $100 in parts including the required resistor/multi-meter -- in my case. But after inspection and planning, I opted for skipping some precautions and relying on "being careful"[0] which I don't recommend, but how many deaths is that, really, if we all try it? (/sarcasm) In the case of my unit, there was maybe three seconds where -- if a reasonable enough charge were present (and I had every reason to believe it held enough to hurt ... a lot ... which is motivation enough) -- I was in danger of receiving it if started pouring on me spontaneously on a clear day -- I didn't have remove the cap from the unit to secure the leads. I suspect a lot of A/C units are similar, but YMMV and I'd be willing to bet, again, most readers of this site possess the wisdom to judge the danger for themselves after suitable reading/watching. [0] Common sense which starts with Rule #1: Always have a physical witness if you're working on something that can kill you and ends with #2: Use them to double-check your research if it isn't obvious stuff (or my personal favorite, have them handle the dangerous stuff).

hourislate 5y ago

It's actually a simple fix. You can buy the capacitor on AMZN, kill the main breaker in the box near the AC unit. Get a screw driver and disconnect it and if you want you can then cross the leads with the screw driver to make sure it's drained. Install the new one and flip the breaker and your set to go. Cap costs around $20 and you you saved yourself a $100 service call.

ericbarrett 5y ago
Shorting a capacitor of sufficient size can send molten metal bits flying around as the leads melt under high current, or even make the capacitor explode. I’m all for DIY repairs, but if you’re going to do this at least wear safety goggles. Also, short the capacitor after turning the power off and before unscrewing it—that’s the time you’re most likely to complete the circuit with a fleshy part. Edit: There might also be other bits in the system with residual power that refill the capacitor even after unplugging the mains, so a voltmeter is a good idea as well.
- Faaak 5y ago
  I'm sorry, but you speak like you're living in a movie. Shorting a starter cap won't do anything of what you said. These caps are actually small (couple hundred uF). At the worst you'll produce a small spark. nothing more.
  - ericbarrett 5y ago
    If you think you know exactly what's going on with a strange cap and want to take that risk, go for it. Me, I like my forearms unscarred.
- Dylan16807 5y ago
  > There might also be other bits in the system with residual power that refill the capacitor even after unplugging the mains, so a voltmeter is a good idea as well. Like what, when it was already turned off? And especially, what do you imagine recharging a significant capacitor to more than 50 volts?

coupdejarnac 5y ago

It's not a big deal as long as you're aware of the danger and discharge the capacitor before removing it. The capacitor is usually the culprit, especially in hot environments like where I live.

tylerhou 5y ago
That falls under "if you know what you're doing."

userbinator 5y ago

It sounds like this is a motor start capacitor, which is going to be connected to the motor and discharged if the power is off. Do the research instead of spreading paranoia and FUD.

tylerhou 5y ago
Even if motor start capacitors are safe to touch, if you don't know what you're doing you might accidentally make contact with a dangerous capacitor. And the parent comment didn't clearly state to only change motor start capacitors -- they seem to endorse changing all capacitors without any warning. Please don't accuse others of spreading FUD when this type of repair is legitimately dangerous.
- CamperBob2 5y ago
  You might get run over by a bus on your way to the store to buy the capacitor, too. Don't overlook that possibility while you're jumping at shadows.
- Dylan16807 5y ago
  Capacitors such as what? A normal AC system is basically two fans, a compressor, and a bunch of pipes, right?
  - pwg 5y ago
    A motor capacitor: https://en.wikipedia.org/wiki/Run_capacitor https://en.wikipedia.org/wiki/Run_capacitor. Both the fan motor (external and internal fans) and the compressor itself will each likely have one. And should they fail, the motors will not start, which gives the appearance of a failed motor for the fans or the compressor, when what needs to be changed is a $5 capacitor.
    - Dylan16807 5y ago
      No, that's my point, they were arguing that there were other kinds of capacitor to worry about.

breput 5y ago

It is theoretically possible, but if your start/run capacitor has failed, it is not going to be dangerous and it is designed to be replaced. Wear (rubber) gloves, short circuit anything you're going to touch with an insulated screwdriver, and you'll be fine. A new capacitor is about $20 vs. $250+ for any service visit.

deleted 5y ago

[deleted]

petre 5y ago

Come on, it's an AC single phase assist cap, not a DC cap of >1000 uF like those found in camera flash units. Just short it with a load before you touch it to be 100% sure.

grumpy-de-sre 5y ago
Come to think of it are medium sized high voltage capacitors really that much of an electrocution risk in the first place? I mean sure as anything they can throw you back into your seat and cause a relatively decent burn but in practice are they really an electrocution risk? You're not going to get trapped and unable to let go. Edit: Looks like the biggest risk from a high voltage DC discharge is myocardial depolarization, and that's dependent on the current path and the timing of the heart rythym, but that's typically at energies an order of magnitude more than a charged typical motor capacitor
- deleted 5y ago
  [deleted]
- petre 5y ago
  >75 mA across the heart might be fatal. An AC motor capacitor would probably just give you a short shock since it's only used to introduce a phase shift on the auxiliary phase in single phase AC motors (what the OP is talking about) or for power factor correction in 2/3 phase AC motors. AC switches every 20 ms from positive to negative so the cap only stores very little power. I'd be more weary of capacitors in DC circuits.

barbegal 5y ago

All modern appliances should have a bleed resistor fitted to drain the capacitor within a few seconds of the power being switched off.

mdip 5y ago

Ah, yes. This is a point I really should have made. If you've been "bit" by a small cap on a circuit board, imagine one as big as an adult's fist. I don't recall the output of this thing but one needn't read the label to know that if I can't completely enclose this thing in my hands, it's probably not something one should...enclose in ones hands? It is also in a nice metal container that's probably under more pressure than it should be; mine were always bulging. With board mounted caps, a lot of this stuff lands in the "leaving pizza out overnight" category. I've only had one (small one) truly explode[0] due to age/being defective; several that I blew up in electronics class when the teacher's head was turned, but hey. And about a flobbidy-gillion that bulge, you could lick the leads (that's sarcasm, don't actually do this). Now, fan caps/mostors like this -- I've been bit by (a 400V cap that was "not dead yet") -- I don't know that I actually touched the leads so much as some hair on my exposed arm made contact (it's why I wear my long welding gloves!). ...There's that moment well after you've registered the burning sensation and smelled the hair that you say a prayer of thanks to God while panicking that the business end is not yet any safer than it was a second ago. The (somewhat unsafe) procedure I used was: Cut the power, pop the fuses, pulled the wires in the order I had in my notes from last time (with thick welding gloves[0]), pre-cut electrical tape to cover the three leads (model has lots of distance, it's a big cap). The cap is connected by three wires, far from the rest of the electronics, easy to get to, easy to remove/re-secure the wiring, admittedly -- easy to get a little too comfortable. Carefully handle those leads; hopefully goes without saying -- even if the cap isn't bulging -- avoid heat-based treatments (hot glues/heat shrink). I tape the hell out of the big ones. I've only had to personally handle a small number of these, but there's a TV repair shop in the area that does free electronics recycling. I told the puzzled 15 year old at the counter what it was, he looked puzzled, so I explained that it can explode like a hover-board battery and he shrugged. I guess...good enough. [0] In a manner that might have been a fire hazard, and were it exposed, could have taken an eye out. In my case, a moderate "pop", a black spot on the board, smoke, and shrapnel throughout the small case.

mdip 5y ago
Just to disclaim a little more -- I definitely would do this at home, and the first time I did something like this was before I started doing any real circuit work at home. I take into account the typical audience of the site, though. Most of us tinker/work with highly complex things that require good research skills even if they don't touch electronics. The basics of how a fan capacitor works, how to handle one safely -- including testing, and discharging, can be done for less than $100 in parts including the required resistor/multi-meter -- in my case. But after inspection and planning, I opted for skipping some precautions and relying on "being careful"[0] which I don't recommend, but how many deaths is that, really, if we all try it? (/sarcasm) In the case of my unit, there was maybe three seconds where -- if a reasonable enough charge were present (and I had every reason to believe it held enough to hurt ... a lot ... which is motivation enough) -- I was in danger of receiving it if started pouring on me spontaneously on a clear day -- I didn't have remove the cap from the unit to secure the leads. I suspect a lot of A/C units are similar, but YMMV and I'd be willing to bet, again, most readers of this site possess the wisdom to judge the danger for themselves after suitable reading/watching. [0] Common sense which starts with Rule #1: Always have a physical witness if you're working on something that can kill you and ends with #2: Use them to double-check your research if it isn't obvious stuff (or my personal favorite, have them handle the dangerous stuff).