Need PWM with current regulation 8-18 volts DC max 30 amps

[ki7ay]

I would like help coming up with a PWM that I can use to provide regulation to a hydrogen generator system. It would need the following:

1. Current readout inside the car (LCD??)
2. Ability to set the current inside the car.
3. Capacity of no more than about 30 amps.

I would like to be able to set the current desired and have it stay right on that value with no operator intervention. Although linear circuits would work, I would like to stay with a PWM device for efficiency purposes.

Anyone got any schematics already, or ideas where to find them?

Thanks, Jim Darrough

[MrAl]

Hi Jim,

So you are saying that your input voltage is 12v or is it higher like
24vdc ?
The reason i ask is because the circuit topology depends greatly on
this information.

[ki7ay]

Hi Al.

The input will be from an automotive alternator, which as you know means a voltage swing from about 12 to 14.6 or so.

Thanks for looking.

Regards, Jim

[MrAl]

Hi again,


Ok, with a 12v nominal input and 8v output you would need a buck,
and with 12v input and 18v output you would need a boost, so the
topology you need is a buck/boost.

Im not sure if you are aware of this but with a 12v input and 18v
output at 30 amps the input current would be around 50 amps,
possibly a bit higher.
Is this an acceptable input current?

[ki7ay]

No, 50 amps is way too high. All I am looking for is a way to provide regulated current under varying load and varying supply voltages. I have seen PWM motor controller circuits online that are pretty inexpensive to purchase and relatively simple to build, but as I mentioned, I want to set the current (actually set a maximum threshold) as in a current limit that will work under the aforementioned varying load and supply voltage conditions. What will happen without a current limit is as the device warms up, it conducts better, and if left unchecked can go up as high as 20 - 25 amps.

Thanks again.


Regards, Jim Darrough

[philba]

I would take a cue from how stepper motor controllers limit current. They basically use a sense resistor and a comparator. The sense resistor is used to generate a voltage based on the current (ohms law) and that is input to the comparator. The ref input to the comparator is used to set the voltage level at which the comparator output changes and shuts off the current flow. Use the comparator output to control a mosfet to turn the current off/on. After some period of time, the current flow is restarted and the process repeats. It is sometimes called chopper based control. For higher currents like yours, look at shunts instead of resistors.

Allegro has a number of chips that do this. I don't know if you can use them in your application but at the least, you can study what they do and learn a lot. the 3977 is one example - http://www.allegromicro.com/en/Products ... /index.asp

phil

[ki7ay]

Hi Phil.

That is a very interesting suggestion. I was leaning towards a PWM motor controller and a 0.1 ohm 1% 25 watt precision resistor inline to sense current. A comparator circuit sounds like a good idea for this application.

Thanks for your input.

Jim Darrough

[philba]

You will also need a way to time turning the mosfet back on. Maybe a simple timer would do it. This will define your duty cycle. well, sort of anyway...

[Engineer1138]

There's an appnote on Atmel's site (www.atmel.com of course) on using one of the tinyAvrs in a constant-current supply.

If you go big I have an IEEE journal somewhere with a paper on building a high-precision 900A constant-current supply

[MrAl]

Hi Phil.

That is a very interesting suggestion. I was leaning towards a PWM motor controller and a 0.1 ohm 1% 25 watt precision resistor inline to sense current. A comparator circuit sounds like a good idea for this application.

Thanks for your input.

Jim Darrough

Hi again,


Ok, well since you said you need 18v output you need either a boost
circuit or a transformer to step up the level of the voltage. How did
you intend to get this with a stepper motor controller? Do you really
need 18v output or not?