PWM / shield

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bcwilson21
 
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PWM / shield

Post by bcwilson21 »

So I know that the arduino uno can do pulse width modulation but its maximum current output is only around 50mAh, are there any shields or other type of board that could increase this to an output that could pulse as low as 0.5A but also at least as high as 5A?

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adafruit_support_mike
 
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Re: PWM / shield

Post by adafruit_support_mike »

For that current level, you'd probably want to use power mosfets: https://www.adafruit.com/product/355

Driving them requires no DC current. There's a small AC current necessary to charge and discharge the mosfet's gate capacitance every time you turn it on or off, but that only becomes a noticable problem when you get into frequencies in the high megahertz range.

You do want to use a 5v signal.. mosfets begin to conduct current at around 2v, but the channel resistance is still a few ohms. Once the gate voltage gets up around 4.5v, the channel resistance drops to a few milliohms. You'll need that low resistance if you want to control 5A.

bcwilson21
 
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Re: PWM / shield

Post by bcwilson21 »

This will all be run off of a car battery so I will have no problem obtaining that voltage, so will this work if the pwm idled at say 14000hz and a .5A current with a peak around 5-9A?

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zener
 
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Re: PWM / shield

Post by zener »

That is a really high PWM frequency. The standard analog output frequency on the Arduino is around 400 Hz I think. What device are you driving?

bcwilson21
 
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Re: PWM / shield

Post by bcwilson21 »

Im going to be driving a HHO generator that will inturn act as the fuel for a motor, the generator produces hydrogen most efficiently at around 14,000hz and 43,500hz. So if im running a mosfet transistor I need to put in whatever voltage im wanting as my max current and just control the output with the PWM duty cycle, is that correct? I am also worried that by running through the transistor it would no longer hold the needed frequency.

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adafruit_support_mike
 
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Re: PWM / shield

Post by adafruit_support_mike »

14kHz shouldn't be any problem.

A mosfet is basically a resistor controlled by a capacitor, so the limiting factor on how fast you can turn it on and off depends on the size of the capacitor and the rate at which you can pump charge in and out of it.

Small mosfets, like the ones in a microcontroller, only have a couple picofarads of gate capacitance. It doesn't take much power to switch them on and off at gigahertz frequencies, which is good because the average microprocessor contains millions of them. Even a tiny amount of current per transistor racks up to some serious heat dissipation when you pack enough of them into a small space.

A power transistor's gate capacitance will probably be in the 1-2 nanofarad range.. a couple thousand times larger. You need to take the gate voltage to about 5v to get really low on-resistance, so you'll need to move about 10 nanocoulombs every time you turn the mosfet on or off. At 14kHz, you'll need 28,000 charge/discharge events per second, which comes to about 280 microamps.

That's the lowest possible current though, and at that level the gate voltage would probably rise and fall sinusoidally. To get crisp square waves, you want to move the charge faster. Assuming we want the mosfet to be fully on or off 99% of the time, we need to do all the charging and discharging in the remaining 1% of the time. That shifts the current consumption to the left a couple decimal places, putting us at about 30mA peak current on a 1% duty cycle.

That's still well within an Arduino's limits.

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