How about using a Relay as a PWM ?

[Bob]

 I realise a Relay will probably burn up fairly fast making it chatter all the time, but on the other hand I have seen old ford relays chatter for Hours at a time and never get warm !
....( thats when you discover the key switch in the old truck stuck on the start position....)
but... at a slow rate probably 30 or 60 clicks a minute.... if the on time was equil to the off time the end results would be the same as a normal PWM.... and... sense some relays can handle 300 amps and be on continiously too..... I'ed think all it would take is a hefty capasitor and a relay
to make a high amp PMW.....
the capasitor to charge up slowly and then throw the relay....and a Pot to adjust the on -off time....?
  not sure how you'ed accomplish that, but perhaps some of you guys can help me with the idea... might be a realy cheep reliable (albeit... NOISY) PWM... but a 300amp PWM is hard to come by
....
whatcha think guys ?

...
Bob.......

[Manta]

Won't work.

Where are you going to find a relay with the current carrying capacity ?

Manta

[Bob]

I bought one that handles 300 amp max  and can be on continuously....bought it from a generator/starter motor repair place for $30 bucks if memory serves me correctly...
.... yah I know it sounds silly... but I think it'ed work if you can get a relay that won't fry the coil inside it.... and one that can be on continuously should fit the bill.
...
even if ya can't figure out the timing capacitor and pot, you could always use a small DC motor and close the relay with a cam hooked to the shaft,.... as long as the motor is a slow turning motor....like 100rpm to 500rpm or so... eh ?
and then control the speed of the motor to control the relay closing...
but that would give you a 300amp PWM and a PWM that big is hard to come by.... hard to say how long it might last though.... probably be lucky to last a month in operation....
but that is a total unknown....
  I bet it would work....
the results would be the same as a Electronic PWM....(just allot slower) its just a Mechanical PWM...
....
Manta: Assuming... I could find a Relay that wouldn't burn up.... could I make it work then ?
....
Bob....

[biggy boy]

Best type of relay to use for constantly turning on and off is a solid state relay.
This is what they use for electric heating elements in industry. The elements are turned on and off at different rates of speed to control the temperature ramp up and ramp down and to keep the temp steady.
you can get them with Dc voltage controls to work with 12 volts.
The reason for not using a electro mechanical relay is the contacts will get pitted and eventually weld together form the constant arcing. As the contacts open an arc is produced and tries to jump across the gap.

I have used and wired in many many relays and contactors over the years. See some nasty damage to contactors/relays.
I've been an electrician for more then 25 years.

[Bob]

yah ! as I said I dunno how long it would last...probly not very long at all !... but it is a thought.... no doubt an electronic one is far better ! but its hard as heck to get up to 300 amps too !.
.... who knows some one might use the idea.... don't think I will HAHAHAHAHHAHAHA!
....
Bob........

[biggy boy]

Seeing as you have it you could give it a try.
Are you planning on running anywhere near 300 amps through it?

Ya for the cost of a solid State relay you might as well get a PWM. big bucks for one to handle 300 amps 

[Bob]

Actually No... I don't plan to use a PWM at all....
 the reason being, is I don't have one, and doubt my eyesight will allow me to make one up, even a simple one, and  even if I did, its just another part to break.... so I wan planning on useing a "Current Limiter" instead....
it will have to be a hefty one because I plan on running close to 200 amps....
but if I understand the principal correctly, you simply put a resistance inline with the cell to ground...  I am not sure of the formula to figure the "Current limiter's" Resistance....
perhaps Manta can Clue me in on that .... but some guys use light bulbs to make their Current Limiters, and that seams like a smart move to me to use THAT Approach, instead of a gadget that will be prone to frying.
...
  I think its fairly simple to figure, if I want to limit my current draw to 30 amps all I have to do is put something in-line with the cell that will only draw 30amps itself... this throttles
the electrons down and they cannot go through any faster than that.... and figuring how many amps something draws is fairly easy by ohms law ( E=I*R) or (P=I*E)......
...
the problem with this Idea is I don't know if it will "Draw" say 30amps all the time or start out way low and only reach 30 amps after its been on for some time.(which I suspect!!!)
....
However, that way I can run a MAX concentration of KOH in my cell and get the benifits of LOW resistance in the electrolyte (less heat) and not have a problem controling the amps either.
...sense I will more than likely use a Electric heater element as my "current Limiter" chances of it braking are very minimal....unlike a light bulb. <GRIN>
....
at least thats the plan.... we'll see how it works out in reality when I can find the time to get back at the building of Hydroxy stuff....
... I figure all the cells togather will probably draw 200amps.... so I will use eather 4 seperate current limiters (1 per cell) or hook them all togather and use just one... dunno yet.
so if I can get 5LPM from each cell at 50 amps.... I'll have it ! ... but we shall see !
....
Bob....