Power Mosfet capability

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gil
 
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Power Mosfet capability

Post by gil »

Can I use the N channel power MOSFET (Vds 30V max, max current 62A, Vgs 2.35V max, product ID 355) to power my actuator motors?
Since Arduino pins supply -5V; will I have to use an npn transistor to turn on +5V to the gate?
Motor 1: 24V, 3.5A at full load
Motor 2: 12V, 10.3A at full load

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adafruit_support_mike
 
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Re: Power Mosfet capability

Post by adafruit_support_mike »

The mosfet can handle both of those motors easily.

You won't need any additional parts between the mosfet and your Arduino. Mosfets are voltage-controlled devices, so you don't have to worry about current consumption. As long as you take the gate voltage to 5v, it should be happy.

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gil
 
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Re: Power Mosfet capability

Post by gil »

Now let me complicate matters. I am currently using 2 transistors and 2 relays for each motor. One transistor is paired with a DPDT relay to choose motor polarity. The second transistor is paired with a SPST relay as an on/off switch. I had to use TIP120 s for the higher power motor to drive relays with 12V coils.
The power mosfet sounded good in that I could dispense with the electro-mechanical relays. Then I started doodling on paper to create the circuit diagram and came up blank. I thought I could use 2 mosfets for each motor - fire the one for the polarity I wanted. Now I'm afraid I can't use them. Am I missing something?
Thanks.

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Franklin97355
 
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Re: Power Mosfet capability

Post by Franklin97355 »

What you are looking for sounds like an H bridge circuit. Google should give you good results

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gil
 
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Re: Power Mosfet capability

Post by gil »

It appears that I have already designed an H-bridge without knowing what it is. In any case, many of the H-bridge designs appear to require more than 2 signals from the microcontroller. Since I only have 3 unused pins and may need them for additional functions, I will continue pouring over various designs until I find one that works for me. I may ultimately need to change out my microcontroller for one with more I/O. Thanks.

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adafruit_support_mike
 
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Re: Power Mosfet capability

Post by adafruit_support_mike »

A basic two-input H-bridge looks like this:
bridge.jpg
bridge.jpg (23.46 KiB) Viewed 372 times
P1 and P2 are P-mosfets, N1 and N2 are N-mosfets. B1 and B2 are buffers with an 'output enable' pin, and I1 is an inverter.

Ignoring the buffers for a moment, the mosfets on either side share an input, which means they work in opposition. If signal A is HIGH, P1 will be closed and N1 will be open. If signal A is LOW, P1 will be open and N1 will be closed. Inverter I1 guarantees that signal B is always the opposite of signal A, so if A is HIGH (P1 closed, N1 open), B will be LOW (P2 open, N2 closed).

The FWD signal that goes into I1 decides whether current will flow from P1 to N2, or from P2 to N1.

The buffers are controlled by the RUN signal, which decides whether A and B get to the P-mosfets at all. If the RUN signal disables the buffers, P1 and P2 will be shut off by their pull-up resistors. If P1 and P2 are closed, it doesn't matter whether N1 and N2 are open or not. No power can reach either one of them.

That design isn't the best possible H-bridge.. there's room for improvement on things like shoot-through current when P1 is closing but still partially open and N1 is opening too. You can also do useful things by controlling the mosfets individually. This makes the basics visible though.

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gil
 
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Re: Power Mosfet capability

Post by gil »

Thanks. I like this concept. I had to research buffer and inverted buffer. Simple and useful. I may play with this. I do worry about the shoot-through current. In my present design; to mitigate against differences in relay actuation time; I put a 20ms delay between actuating the DPDT and the SPST on/off. With this design I see that the RUN signal is necessary before signal A has any effect. In any case, this has possibilities.
I have since torn apart my breadboard prototype and built my controller. Motor 2 is a linear actuator that of late has been mounted in the structure; i.e. out in the weather. Rainwater got into the housing and seems to have damaged the reed switch. This has happened even though the actuator is rated IP66. I no longer get a signal from the reed switch. If not one thing, it’s another.

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adafruit_support_mike
 
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Re: Power Mosfet capability

Post by adafruit_support_mike »

Microcontrollers do make running H-bridges easier.. you can assign an output pin to each signal of interest, then program your break-before-make timing explicitly.

There are also chips that do the whole job for you. H-bridges are workhorse ciruits, so it was only a matter of time before someone put a well-behaved circuit into a single package. The L293D (http://www.adafruit.com/products/807) uses Darlington transistors rather than mosfets, but contains four half-bridges (one vertical P/N pair in the circuit above) along with enable pins and proper switching. Each half-bridge is limited to about 600mA, but you can stack them or use them as mosfet drivers.

IC H-bridges are limited by the amount of power you can run through the package before the slicon die explodes or the metal connections melt. In hand-wavey terms, that's usually about 1A. Discrete mosfets can handle a lot more power, but the bigger they get, the harder they are to run. A mosfet is basically a resistor controlled by a capacitor, and the more current that can flow through it, the bigger the capacitor tends to be. To change the voltage in a large capacitor quickly, you need to pump a lot of current in or out for a short time.

Conveniently, the point where IC half-bridges reach their upper limit as direct motor drivers is roughly the same point where they become good power mosfet drivers. Instead of driving the motor directly, you use the chip to drive the mosfets that drive the motor.

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