# Will this capacitor explode?

Discussion in 'General Electronics Chat' started by Uber_Goober, May 1, 2013.

1. ### Uber_Goober Thread Starter Member

Jan 19, 2013
45
4
I'm back here asking dumb questions again...

I'm rectifying 120v house current to power an HHO generator for an HHO torch I'm building. Anyway, the pulsing DC you get from that is inefficient and detrimental to the plates of the generator, so I am going to put a filter capacitor on it. I'm not a EE and I'm not sure exactly what my target ripple should be. So I figured I'd just put the biggest UF capacitor I could find in the voltage range. So I ordered this capacitor:
Its 35,000uf @150V
http://www.ebay.com/itm/TCI-150v-35...706?pt=LH_DefaultDomain_0&hash=item2ec7c8b73a

I assume these capacitors are designed to work at their max voltage. But will my 120v rectified be more than 150v?

I'm expecting to run no more than 6A through this. Will I get a reasonably filtered power source? Or a big boom?

Thanks!

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,395
1,607
Sure will. Danger, you will put your eye out.

I'd give more advice but with all the TLA's and such of your post I don't know what you are talking about.

3. ### ScottWang Moderator

Aug 23, 2012
4,858
767
Not quite sure what you want.

But if you tring to using rectifier and capacitor to transfer 120Vac to Vdc, then the Vmax rating of 150V/35000uf is not enough, you better choosing as 250V/35000uf or higher than 250V.

AC120V x 1.414 = 170Vdc.

If you are not good at ee then you will put yourself in dangerous.

4. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
I guess a HHO for a torch is OK. Your cap will about 120*1.4 volt after rectification. Somehow I have a feeling that using a cap or not will not have much to say in this setting. Also your setting will create HUGH inrush current then turned on. It will for sure zap your fuse(s) and rectifier. Before you do more, I would suggest looking here.
http://en.wikipedia.org/wiki/Electrolysis_of_water See what you can find in the reference section. But also be aware that it is a lot of nonsense on the net regarding this topic.

5. ### atferrari AAC Fanatic!

Jan 6, 2004
2,652
768
What HHO are you talking about here?

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,003
745
Cue music to Mission Impossible.........................

7. ### Sue_AF6LJ Member

Mar 16, 2013
45
27
Y'all know that is a lot of energy in a little can.
I wouldn't want to accidentally short that cap out.
There are formulas for determining the amount of capacitance needed for a given load current. To a point more is better, but you run into complications when you are dealing with large caps like that one.

For one thing You can start blowing out things like diodes, on/off switches because of the hard start condition that occurs when a cap that large is charged from a non current limited source. Furthermore when there is a failure somewhere on the way to or at the load, large amounts of current are sourced from that cap and that explosion you are concerned about can happen elsewhere.

This can be like those videos where they guy wants to start a camp fire and all he has is gasoline.... So instead of using a teaspoon of gas, he uses five ounces and wakes up in the ER...

So be very careful...

8. ### richard.cs Member

Mar 3, 2012
162
31
With the capacitor directly after the rectifier you can expect 170V dc and the current will be drawn from the mains in huge spikes. That capacitor will probably die from overvoltage and a 250V rating would be better - you don't generally want to run them too close to their maximum rating, I wouldn't use a 200V one here. But the capacitor you use will have to handle huge amounts of ripple current and may overheat from that anyway if it's not designed for it.

What voltage do you actually want, or have you not chosen your number of plates yet? One way around the high peak currents is to make the supply choke-input where you have a large inductor between the rectifier and the capacitor. You get lower output voltage (typically dc out is then nearly the same as your rms ac in). The advantage is you don't get the huge inrush and ripple currents that you get with a capacitor input. You can also use a smaller capacitor and yet get smoother dc.

You could use a microwave oven transformer with the secondary removed and an air gap added to the core as a choke.

9. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,328
I think someone's on the road to earn a Darwin Award....

Tell me Uber_Goober, are you familiar with Wile E. Coyote?

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10. ### Sue_AF6LJ Member

Mar 16, 2013
45
27
Long time ago, whey I was young dumb and really stupid.....

I had this 410V ferroresonant power supply that was used to run old tube type computing equipment. I didn't know much about that kind of power supply back then and I was trying to see if it would develop just a little more voltage. Well first off this thing was not rated in milliamps it was rated at 5.0A... (you can see this coming right...)
So what I did was to disconnect the large bleeder resistors.
I figured I would get the 450V I was looking for.
Oh did I mention this thing had 12 150MFD 500V caps in parallel, well it did...
So with the bleeder resistors disconnected I powered this thing up...
CRAP only 425V.....
So I shut it off, and the voltage just stayed there at 425V....
Oh I can fix that. (Here hold my beer)
So with a screwdriver with a quarter inch shank in one hand I shorted the output..... BANG
I had spots in front of my eyes for five minutes and my ears didn't stop ring for two hours, the screwdriver became useless as a screwdriver as the end of it was blown off.
Lucky it didn't wipe out my remaining eyesight......

DOn't ever understatement the danger of capacitors.

THE_RB, shortbus, #12 and 1 other person like this.
11. ### Uber_Goober Thread Starter Member

Jan 19, 2013
45
4
Hi Richard - Its a 61 plate configuration with all plates in series so its set up for 120v. The place I bought it from recommended this vs. going with 12v cells and high current. But I almost think that would have been better now. Yeah I had a feeling I was getting into dangerous territory so I thought it would be a good idea to ask here. Even though its a dumb question, it would be dumber not to ask lol. I had a much smaller capacitor blow up once and I was just lucky the case it was in was facing the other way - it directed the blast away from me and capacitor bits were everywhere. The place I bought it from said to hook it up with just the rectifier, but I didn't think that was a good idea either (for efficiency, not so much safety).

12. ### Uber_Goober Thread Starter Member

Jan 19, 2013
45
4
I will check that out, thanks. Yeah most people working with this think they are solving cold fusion or something. This isn't going anywhere near my car though. I actually just have a use for an oxygen/hydrogen torch and they run about \$6000 retail. I may have been better off getting a 12v cell - even at the higher current it looks like it would have been easier to deal with. But I've already got this one.

13. ### Uber_Goober Thread Starter Member

Jan 19, 2013
45
4
HHO is just water electrolyzed into hydrogen and oxygen gas. A lot of people are doing these to hook into their cars thinking they are inventing something magic. I'm not out to break the laws of thermodynamics, but the gas generated actually makes a very useful torch somewhere between MAPP and oxy-aceteline in temperature - and the input fuel is just water and an electrolyte. I'm doing some acrylic work and people like HHO torches for acrylic polishing because the gas is cleaner.

14. ### shortbus AAC Fanatic!

Sep 30, 2009
4,019
1,542
Unless the accumulator for the hydrogen is positioned over the correct polarity of plates, you are going to be making a very explosive mixture being sent to your torch.

Is your life and work space worth more than the cost of a small tank/cylinder of hydrogen?? Only you can make this call. They are available in a 20 cu ft size; http://www.airgas.com/browse/productDetail.aspx?Category=195&product=HY 20

15. ### Uber_Goober Thread Starter Member

Jan 19, 2013
45
4
Hey I wasn't aware that was available. If the price isn't too high I will likely go that route. Thanks for the pointer.

Regarding the safety of the HHO system though- these are relatively safe. The HHO cell doesn't separate the hydrogen and oxygen, and only a small amount is in the cell at any given time before it is forced out and into a bubbler. In the bubbler it is filtered through water in a plastic tube. If the HHO was ignited after the bubbler, it would stop at the water barrier. I'm actually constructing my system with two bubblers- and the last one has a pressure valve in case the small amount of HHO accumulated does ignite. Then the gas is flowing through two separate flashback arrestors before going to the torch head. There are a few points in the system where gas accumulates, but only a small amount. Now if that small amount in a small space ignites it is still likely to damage the equipment and could potentially do harm if the plastic shattered. But barring a leak, there won't be enough gas anywhere for a major explosion.

But I'm doing this because I want an HHO torch, not because I want to do electrolysis. If I can buy it at an OK price, I'll do that.