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Electronics Help

A place to ask general spud cannon related questions.
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Unread postAuthor: Technician1002 » Tue Jan 18, 2011 5:52 pm

It is true that the Pull In Current is higher for a relay or solenoid than the Hold Current. Most AC devices take care of this automatically. As they pull in, the increase of metal inside the coil increases the inductive reactance raising the impedance and lowering the current.

High speed devices have either a dual coil or use a stepped current driver.

A good place to learn this is drivers for car fuel injection.
http://www.stealth316.com/2-fuelinjection.htm

http://www.national.com/mpf/LM/LM1949.html#Overview

This PDF application note shows the desired stepped current to the injector showing the initial current and the hold current. Note due to inductance, the initial voltage is much higher than the hold voltage.
http://www.national.com/ds/LM/LM1949.pdf
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Unread postAuthor: Davidvaini » Tue Jan 18, 2011 7:28 pm

yep, pwm is great for reducing power consumption, and being able to control the speed of a motor as well.
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Unread postAuthor: jhalek90 » Thu Jan 20, 2011 11:44 am

look for a cheap RC speed controller on ebay.

Speed controllers will take your PWN signal, and use it to control the voltage, and amps coming from another power source, to a motor.

This is how most high end RC cars work.
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Unread postAuthor: Technician1002 » Thu Jan 20, 2011 12:13 pm

Info on driving an RC servo with the Arduino is easy to find.

http://principialabs.com/arduino-serial-servo-control/
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Unread postAuthor: PVC Arsenal 17 » Thu Jan 20, 2011 3:11 pm

Technician1002 wrote:Info on driving an RC servo with the Arduino is easy to find.

http://principialabs.com/arduino-serial-servo-control/


Also quite easy to do as demonstrated here. Though I suspect the OP needs a motor for a different purpose.
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Unread postAuthor: Technician1002 » Thu Jan 20, 2011 3:36 pm

A servo and an RC speed controller use the same signal.
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Unread postAuthor: PVC Arsenal 17 » Thu Jan 20, 2011 3:42 pm

Ah, I didn't notice jhalek90's post and that you were adding to it. Along similar lines, Pololu makes good motor drivers but they get quite expensive. In any case it's still probably more difficult than the MOSFET route he's already talking about... Provided he can find one capable of PWM.
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Unread postAuthor: Technician1002 » Thu Jan 20, 2011 4:00 pm

MOSFET's do PWM just fine. When coupled into a SSR, speed is often limited. MOSFETS are common in inverters for car stereo amplifiers, power supplies, and RC motor control. Any MSOFET that can be used for high speed switching is fine for PWM.
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Unread postAuthor: PVC Arsenal 17 » Sun Jan 23, 2011 12:27 am

That's the catch though, it has to be suited for it. The nature of some MOSFETs is to stay on (in the case of N-channel) even after the signal on the gate it removed due to input impedance. I had one in my sights but the datasheet only states that it's been tested at 1Hz. If anyone can point me to a tried and true MOSFET good for PWM @ max. 10 amps I'd be much appreciative.
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Unread postAuthor: dewey-1 » Sun Jan 23, 2011 9:06 am

PVC Arsenal 17 wrote:If anyone can point me to a tried and true MOSFET good for PWM @ max. 10 amps I'd be much appreciative.


Check into IRFZ40 or IRFZ42. (these are not logic level gate drive)

I used them for my own designed RC Racing Speed Control. (Back in 1988).
They still work to this day. Used on 9T brushed motors.

I can give a partial schematic upon request via PM.

It had 6 MOSFETS in parallel and did not require a heat sink.

See following for data sheet as well:

http://search.digikey.com/scripts/DkSea ... FZ40PBF-ND
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Unread postAuthor: Technician1002 » Sun Jan 23, 2011 10:58 am

Almost any MOSFET used in the inverter power section of a high power car stereo amp will work fine. They are designed for low voltages (12 volt car power instead of 160V rectified house current) and work at high speeds with fast recovery times.

Other good sources is old RC speed controls, old computer UPS, DC brush less motors, 18 volt drill speed control, and other lower voltage high current devices.
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