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Capacitor Replacement


NathanL

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Hi everyone. I'm pretty new to inverter repair. I have a Firestar (Voltronic from what I can work out) 3000VA 24V inverter that someone kindly connected 48v batteries to. They popped the main capacitors on the inverter. There are 4 of them. They are 4200uf 35V electrolytic caps, which are not available locally as far as I can find. Can I replace them with 4700uF 35V caps available locally? Or maybe 3900uF? Any advise?

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6 hours ago, NathanL said:

someone kindly connected 48v batteries to. They popped the main capacitors on the inverter.

Sadly, they might not be the only things that popped.

 

6 hours ago, NathanL said:

Can I replace them with 4700uF 35V caps available locally?

The exact capacitance is irrelevant, as long as it's in the same vague area, and physically fit. You cannot extend the leads.

The important specifications for the capacitors are

  • Ripple current rating, the higher the better
  • Temperature rating, the higher the better; this will almost certainly be 105°C
  • Life rating at the temperature limit. You want at least 5000 h, preferably 10 000 h at 105°C.

So something you get from a hobby electronics store is unlikely to work long term. Sure, any capacitor that is over 2 200 μF and physically fits will work for a week, but these capacitors protect the MOSFETs from spikes, so cheap capacitors will likely result in a loud bang within a year.

I suggest you study this post, and find a suitable replacement from an online supplier such as Mouser or Digi-Key. [ Edit: the linked post refers to 48 V machines, but the same principles apply. In a 24 V machine, you can probably get away with 35 V rated capacitors, however. ] I don't know which ones are the most convenient to access from Africa; I'm located in Australia.

Good luck. Check out the index of the linked topic (first post, page 1) to read about MOSFETs and their gate drivers. It's unlikely that MOSFETs will blow up from 50 VDC, but it's possible. There are likely several other bypass capacitors on the battery bus; any electrolytics will likely also have blown. You might be able to spot these by inspection. I note that open circuit bypass capacitors can be the source of many unusual and difficult to track down problems.

Edited by Coulomb
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  • 1 year later...
On 2020/01/25 at 11:23 AM, Coulomb said:

Sadly, they might not be the only things that popped.

 

The exact capacitance is irrelevant, as long as it's in the same vague area, and physically fit. You cannot extend the leads.

The important specifications for the capacitors are

  • Ripple current rating, the higher the better
  • Temperature rating, the higher the better; this will almost certainly be 105°C
  • Life rating at the temperature limit. You want at least 5000 h, preferably 10 000 h at 105°C.

So something you get from a hobby electronics store is unlikely to work long term. Sure, any capacitor that is over 2 200 μF and physically fits will work for a week, but these capacitors protect the MOSFETs from spikes, so cheap capacitors will likely result in a loud bang within a year.

I suggest you study this post, and find a suitable replacement from an online supplier such as Mouser or Digi-Key. [ Edit: the linked post refers to 48 V machines, but the same principles apply. In a 24 V machine, you can probably get away with 35 V rated capacitors, however. ] I don't know which ones are the most convenient to access from Africa; I'm located in Australia.

Good luck. Check out the index of the linked topic (first post, page 1) to read about MOSFETs and their gate drivers. It's unlikely that MOSFETs will blow up from 50 VDC, but it's possible. There are likely several other bypass capacitors on the battery bus; any electrolytics will likely also have blown. You might be able to spot these by inspection. I note that open circuit bypass capacitors can be the source of many unusual and difficult to track down problems.


Hey mate, I just found this thread, I was wondering if i could get your advice, my batteries on my 3kva 24V inverter were disconnected while my solar PV (sola) input was active (it smoked), I opened her up cleaned up all the capacitor oil and from a visual inspection only the capacitors have bulged, I looked over the mosfets and IC's and i see no burns. I did some continuity testing on the mosfets and they seem like they're fine. Should i be good if i replace the capacitors? also what would you recommend locally in Australia?

Cheers,  

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On 2021/11/29 at 7:05 PM, S64 said:

my batteries on my 3kva 24V inverter were disconnected while my solar PV (sola) input was active (it smoked),

Yikes. I would not have expected that. I guess that the battery bus voltage spiked very high.

On 2021/11/29 at 7:05 PM, S64 said:

I looked over the mosfets and IC's and i see no burns.

I'm suspicious that nothing else blew. Sometimes the MOSFETs die with only hair-line cracks. But if you can measure the reverse diode from drain to source, and no connection from gate to source (other than the gate pull-down resistance), then you may have dodged a bullet.

On 2021/11/29 at 7:05 PM, S64 said:

I opened her up cleaned up all the capacitor oil

Lovely job 🤮 💩

> Should I be good if I replace the capacitors?

Time will tell.

On 2021/11/29 at 7:05 PM, S64 said:

also what would you recommend locally in Australia?

The only place I know of that stocks the good capacitors is Digi-Key. [ Edit: that was a mis-remembering. It's the factory capacitors that are a special order; the recommended replacements are available from Digi-Key and other suppliers. ] That may have changed, but I doubt it. There is no-where that I know of that you can buy them off-the-shelf (apart from virtual, on-line "shelves").

Edited by Coulomb
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In South Africa, I've ordered a LOT of capacitors from RS electronics.  They have the most decent options (which will be shipped from the UK).  All the other local companies sell only common stuff.  I doubt you'll get the same selection that @Coulombgot from Digi-key but at least it'll be half decent.

It takes a really long time selecting capacitors (for me personally) because you need to balance specifications as @Coulombmentioned and price.  With RS there is no rhyme or reason for the price.  Some good quality things will cost a lot less than some less high quality products based purely on volume I guess.  So I spend a lot of time going over pages and pages of capacitors sorted by price and looking at specs until I find one that works.

My 2c.

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11 hours ago, Coulomb said:

Yikes. I would not have expected that. I guess that the battery bus voltage spiked very high.

Probably should have mentioned it was an off grid inverter, which i assume has something to do with it, I have a feeling the I simply overcharged the capacitors.

 

11 hours ago, Coulomb said:

The only place I know of that stocks the good capacitors is Digi-Key

The capacitors in the inverter are 4200 uf at 35V should I get the same or could i get an improvement

 

11 hours ago, Coulomb said:

I'm suspicious that nothing else blew. Sometimes the MOSFETs die with only hair-line cracks. But if you can measure the reverse diode from drain to source, and no connection from gate to source (other than the gate pull-down resistance), then you may have dodged a bullet.

Went over them as ya said they're fine, not sure about what the pull down resistance is but I am getting drain to source

Also thanks a ton for ya time

Cheers,

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11 hours ago, S64 said:

The capacitors in the inverter are 4200 uf at 35V should I get the same or could i get an improvement

There is more to capacitors than their capacitance and voltage rating. There is ESR, temperature rating, and rated hours of operation. In this application, the actual capacitance is largely irrelevant. They are there to absorb spikes of switching energy.

Since this is a 24 V model, the ones I usually recommend for 48 V inverters aren't suitable. So you'll have to select your own. But there may well be another model in the same long life range that can be used.

My colleague Weber did most of the work selecting the replacement capacitors for 48 V models. You can read about Weber's selection process here: http://forums.aeva.asn.au/viewtopic.php?p=64358#p64358 and the link about a third of the way through the post to the original post two years before that one.

Ah. On skimming it, I see that it's the factory capacitors that are a special order. Weber's states that his recommended capacitor is available from other suppliers. My misremembering. So that's good news.

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