That's what I'm thinking too. He needs to breadboard something, or simulate it and see what the actual problems are, rather than try to over-kill it in the ruminating stage of design...
The arduino outputs 5 volts, or 3.3V, and if he only has that to work with, it may not be enough to fully turn on a so-called logic level mosfet. But it is so easy to use the Arduino to run a flea power boost converter so that shouldn't be a problem to just step up voltage with an inductor and switching transistor.
And then too, the Arduino is over-kill for most projects.
a small 12VDC (about an amp or so) pump motor with PWM, using an IRLZ34N as the power mosfet.
more quickly? Or is a logic-level mosfet already designed to efficiently t urn on and off with the weak logic level signal?
I already breadboarded it. I put a 270 ohm resistor (calculated from V=I R, with I set to the max safe current the Arduino could put out... I forget exactly what it was, and V set to 5V) between an output PWM of the Arduino to the gate of an IRLZ34NPbF mosfet, and put one of these pumps between +1
8V and the drain:
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ic+pump
I rather doubt that 80 mA figure; I'll have to measure it one of these days .
My Arduino program displays the current power setting (0-100%) and if I, fo r example, send a "6" followed by Enter to the serial port, the Arduino set s the PWM at 60% power. The C code is somewhat laughable but I can post it if you really want.
It works fairly well, but I noticed I had to put a heatsink on the MOSFET, which had me wondering why.
Oh! I put an SB140 schottky in parallel with the motor.
I'd doubt the 80 mils too. They don't give the conditions, like speed and back-pressure. Peristaltic pumps aren't known for their free-spinning characteristics, they are better suited for intermittent use (especially when directly driven). If the tubing isn't silicon tubing I'd recommend a change because the energy used to pinch the tubing is waste. (and I don't ever remember seeing any peristaltic with a direct drive, they always used gearboxes, even the ones driven with stepper motors)
Motor current would be worth checking.
I worked in a pharmaceutical chemistry lab doing electronic and electromechanical design.
I take it the Arduino is basically just chopping the power to the pump motor? No inductors or filter caps involved? What frequency are you chopping power at? Have you looked at the waveforms with a scope?
As for current limiting to the gate drive, that may not be necessary or desirable. The gate is essentially a capacitor. (of 880 pico farads in this case) The current is just necessary for a brief time to charge and discharge the gate. Do you have any idea what the peak current the Arduino chip can sink and source? If you have 18 Volts to play with it may be better to just drive the gate with a higher voltage (but absolute max for that part is 16 volts...)
A scope might tell you what is going on to cause it to heat...
I designed a variable speed (large) fan for my last tower computer. The specs on the brushless fan said it was 500 MA at 12 V and the mosfet was toggling at 15 KHZ and ran cool, with no heat sink. But then I wasn't using straight pwm and driving a brushed motor either, I was using it as a switching mode power supply and sending pure DC to the fan motor, except on the highest setting, then it was just turning on the mosfet with no chopping.
My drive circuit was a single ended npn transistor, but only because I started out using pwm and a BJT transistor. (it turned out that the fan didn't like that; pulses from the drive would alias with the pulses to the fan coils) When I changed the programming for smooth DC (added an inductor diode and filter cap) and switched to a mosfet I just left the transistor in the circuit. (too lazy to rewire the perf-board I was using, and 12V meant there was no need for a logic level device)
=IR, with I set to the max safe current the Arduino could put out... I fo rget exactly what it was, and V set to 5V) between an output PWM of the Ard uino to the gate of an IRLZ34NPbF mosfet, and put one of these pumps betwee n +18V and the drain:
altic+pump
ays.
for example, send a "6" followed by Enter to the serial port, the Arduino sets the PWM at 60% power. The C code is somewhat laughable but I can post it if you really want.
T, which had me wondering why.
I think it's either around 400 or 900 Hz, and nope I have no scope.
Here's the schematic for it. I was practicing my CAD drawing skills too...
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tocad/pwm.png
Brushless fan... there's a controller board in there too. Seems kind of str ange when you think about it, PWMing the power to a brushless fan which wil l also have its controller board, Hall sensors, etc. buried under the fan b lade hub. Ceramic iron magnets too... and how do I know this? I somehow m anaged to corrode one, and decided to take it apart :p
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