Maintain current

[sp1r0]

Sounds good, keep that FET cool!

[ridelong]

The pwm works great.

Measuring actual current will be interesting. The frequency is low, so the current measuring has to be integrated.

The voltage must be resistor divided to be proportional to current. I'm thinking 200 mv= 200 amps so it works with the cheap digital readout modules.

Will work on this over the weekend, unless I go diving.

[Atechguy]

The pwm works great.

Measuring actual current will be interesting. The frequency is low, so the current measuring has to be integrated.

The voltage must be resistor divided to be proportional to current. I'm thinking 200 mv= 200 amps so it works with the cheap digital readout modules.

Will work on this over the weekend, unless I go diving.

Glad to hear , i need a stable pwm , just when i think i have it under control , it seems to get hot enough to blow a 30 amp fuse , i will be changing from 8 gauge to 10. to put a liitle resistance , i believe the my cell only needs a couple of grains of NOah or just distilled water ,has 50 small plates .

[ridelong]

Atechguy,

I don't think the current feedback pwm I'm building will help you. I made it very slow to minimize the amount of switching, to minimize fet heating.

For example, if it is set to 5 amps, it might draw 10 amps for 1/2 second, then be off for 1/2 second, thereby maintaining 5 amp average.

In pwm, the fet is full on or full off, that's why they are so efficient.

My circuit will regulate average current very well, but by being a pwm cannot regulate current at any given time.

[Atechguy]

Atechguy,

I don't think the current feedback pwm I'm building will help you. I made it very slow to minimize the amount of switching, to minimize fet heating.

For example, if it is set to 5 amps, it might draw 10 amps for 1/2 second, then be off for 1/2 second, thereby maintaining 5 amp average.

In pwm, the fet is full on or full off, that's why they are so efficient.

My circuit will regulate average current very well, but by being a pwm cannot regulate current at any given time.

Will this not help to control current rising out control especially after the cell gets warm from under hood heat and current flow, when i start cool it draws about 6 amps but after hour or less it can be 22 amps. ??I know the series design can be a problem but i didn't want to add another cell and i can't easily open up this cell and reconfig. plates, do you recommend a more fully controlable PWM?? Thanks

[ridelong]

Atechguy,

I played with my current feedback pwm today. I can speed it up (x1000) and still run a cool fet. If you use a slow blow fuse, I think this circuit might work for you. Make sure you don't have something intermittently shorting out.

Do you know how to post a link or a picture? I'll post schematic, but I still need to work out how to adjust current and read it.

The prototype is strung all over my workbench. I am in the process of making a neat one in a box, but still don't have the current sense resistor, so i'm stuck until it arrives.

[Atechguy]

Atechguy,

I played with my current feedback pwm today. I can speed it up (x1000) and still run a cool fet. If you use a slow blow fuse, I think this circuit might work for you. Make sure you don't have something intermittently shorting out.

Do you know how to post a link or a picture? I'll post schematic, but I still need to work out how to adjust current and read it.

The prototype is strung all over my workbench. I am in the process of making a neat one in a box, but still don't have the current sense resistor, so i'm stuck until it arrives.

updates i now suspect i may have a short in a cell ,now when i start cold it is about 21 amps when previosly it was 6 amps. I sorry i didn't make my own .
No i haven't posted pictures before, i have reduced my wiring down to 10 gauge hope it helps abit, what does that mean speed adjusted to x1000 ??pusles?? I can get a slow blow fuse, i pretty sure i don't have short. Thanks

[ridelong]

Atechguy,

What is 1000x? A pulse width modulator works by changing the on and off times. The circuit parameters determine the total on-off cycle times. I first made my pwm very slow, about a second for 1 on-off cycle. I did it slow so the fet would stay cool. After reading your post, and realizing that this won't help you, I tried speeding up the circuit. It worked fine and the fet is cool. Now, 1 on-off cycle ocurrs in 0.004 seconds. BTW it is only 250x original.

If your circuit is somehow drawing high current the short on time of the pwm should limit it enough using a slow blow fuse to not blow. If it is a dead short, it will blow with or without a pwm.

[Atechguy]

Atechguy,

What is 1000x? A pulse width modulator works by changing the on and off times. The circuit parameters determine the total on-off cycle times. I first made my pwm very slow, about a second for 1 on-off cycle. I did it slow so the fet would stay cool. After reading your post, and realizing that this won't help you, I tried speeding up the circuit. It worked fine and the fet is cool. Now, 1 on-off cycle ocurrs in 0.004 seconds. BTW it is only 250x original.

If your circuit is somehow drawing high current the short on time of the pwm should limit it enough using a slow blow fuse to not blow. If it is a dead short, it will blow with or without a pwm.

I just cleaned out the cell to and put straight distilled water in , iwas amazed at the production i was getting at 6.25 amps, i also emailed the company i bought the cell off of and there willing to help me out, i'm not sure if there is a short or not but was suspecting that miniature metal flakes could build up and cause a short, i am still interested in your pwm . To be able to have some control with your pwm for changing conditions like winter time , to add more amps for more production when it is cold out.Thanks

[Jaxom]

Any progress on this? I had two thoughts:

First, use an inductive-capacitive filter on the input side to quell the spikes in current draw when the FET switches. This should stabilize the current through the fuse so that normal (as compared to slow-blow) fuses can be used.

Second, can this circuit be modified to change the output current based on an external input? I.e. use a 0-5v input to proportionally vary the output between 0 and 40A? I would be VERY interested in such a design, but I'm too rusty on my electronics to design it myself.

Good work so far! Keep it up!!