Current limiter capacitor.

[On7tim7]

I am making a serial HHO generator and I need to know how I calculate a limiter capacitor to control the current that goes to the cell. The cell takes 12 amp at 125 VDC from a full wave rectifier that is fed from the ac 125 VAC with a serial ac capacitor and there is where I need to limit the current to the rectifier or it will be destroyed. Thank you in advance.

 

[dknguyen]

Just use a fuse if you want to limit the current...if you want to make it actively constant current then it's much more complicated.

 

[Willbe]

a limiter capacitor to control the current that goes to the cell.
12 amp at 125 VDC from a full wave rectifier that is fed from the ac 125 VAC with a serial ac capacitor
I need to limit the current to the rectifier or it will be destroyed.

Use a 400v cap. 200v units make a "bang" sound very quickly even though the peak value of 120v rms is only ~170v.

At 12 A you might need to take ripple current rating into account.

Z of 1 µF at 60 Hz = 2653 Ω, reactive.
So, if you want to drop X volts at 12A you need a Z of X/12. If X is 48v, then you need a Z of 4 Ω, so that's a cap of 2653/4 = 660 µF.
You'll need an electrolytic cap, and they are polarized, so two 400v, 1300 µF caps in series may do it.
Might be some bucks.

Maybe you should use an inductor; Z @ 60Hz = 377L, where L is in henries. It would then be 4/377 = 11mH.

Don't forget that reactive ohms are at right angles to resistive ohms, so if the load is resistive you're going to need to do some vector math.

 

[Hero999]

Why would he need to use 400V capacitors?

I've seen plenty of 200V capacitors used in US equipment and if you're connecting them in series then all the better. If you're worried about the inrush current then add a small resistor.

 

[Willbe]

It occurs to me that the current in the reactive element will be sine pulses, so an exact analysis might be messy. Laplace domain stuff is probably necessary.

Can you post a schematic starting from the 125v rms and going downstream?
The DC value of full wave rectified AC is not the same as the rms value of the DC voltage. Does the diode bridge feed a filter cap?

Yes, I guess you could put two 200v caps in series; you might need voltage-sharing resistors across them.

 

[Ubergeek63]

Why would he need to use 400V capacitors?

I've seen plenty of 200V capacitors used in US equipment and if you're connecting them in series then all the better. If you're worried about the inrush current then add a small resistor.

130V normal high line gives about 185V BEFORE you add in spikes and surges.

 

[Speakerguy]

I have taken apart commercially available electronic equipment that used 200V electrolytics on rectified line voltage. It's not a problem. Electrolytics will tolerate transient overvoltage OK for a brief spike if it's not too bad.

 

[On7tim7]

Very grateful.

I am very grateful to all of you. I have learned a lot with your answers. I’ll post the full schematic as Willbe asked and you will have a complete picture of my request. Thank you again.

 

[Oznog]

A series capacitor is a fairly poor choice for current limiting. In fact 110V as a whole is bordering on insane to try to moderate in an electrolytic cell. It's much higher than the voltage needed and there's a serious shock hazard by putting line voltage straight into an electrolytic cell.

What are you trying to accomplish? "HHO cells" being free energy or whatever is bogus. They make hydrogen and oxygen, the energy stored in H2 + O2 is less than the power draw off the outlet. People still put out plans and make claims, but no it's just a scam.

 

[Hero999]

130V normal high line gives about 185V BEFORE you add in spikes and surges.

A varistor normally gets rid of the extreme spikes and surges, the filter capacitor and input filtering choke handle the less severe overvoltages. A 50:mu:s 500V pulse (the varistor should higher energy surges) will only increase the chage on a 470µF by about 10V at the most when you consider the series resistance and inductance in the circuit.

All of the computer supplies I've seen use two 200V capacitors that are connected in series when the switch is in the 200V to 260V position and parallel when it's in the 100V to 130V position. I haven't had a single computer supply go bad.

Universal switched mode power supplies mostly use 400V capacitors some use 450V which offers more than enough head room, I think the highest I've seen is 500V but that was for military applications.

Using anything higher than a 250V capacitor on a supply rated for 130V at most seems like a silly idea, when 200V is alright for most applications. A 400V capacitor is four times larger than a 200V capacitor and is at least three times the price so it's both a waste of space and money.

Just take the correct surge protection precautions, use a 200V capacitor and all will be well.

Also 1300µF isn't a standard value and neither is 660µF the nearest values are 1500µF and 680µF respectively.

 

[On7tim7]

Hero999 I’ll take your opinion in consideration, thanks. Mr. Oznog, what I am trying to make is as I said, a HHO generator for welding and brazing is not a free energy gizmo. I have read that one of the best HHO (Brown gas) generator design is based on serial cell connection and it limited the necessary current by mean of a serial capacitor. I am preparing a drawing to show what is what I really need. Thanks.

 

[On7tim7]

Hi, here is the basic diagram without the sensors and associates safety control circuits, the cell capacity at rest is about 2,200 uf. This capacitance will varies with temperature, gas pressure etc.

 

[Ubergeek63]

Why would he need to use 400V capacitors?

I've seen plenty of 200V capacitors used in US equipment and if you're connecting them in series then all the better. If you're worried about the inrush current then add a small resistor.

I forgot there is also the capacitor current rating.

 

[Hero999]

That's true - something which is often ignored with capacitors.