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$10 Motor Driver Possible?

 
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liam.zbasic



Joined: 25 Mar 2008
Posts: 163
Location: Southern California (Blue)
Posted: 02 January 2011, 6:24 AM    Post subject: $10 Motor Driver Possible? Reply with quote
Looking for a simple and reliable MOSFET motor-driver circuit . The input will be a 5V PWM signal from my ZX-40 device and the output will be a 6-15V signal to the motor (20A peak). I have a Pololu VNH2SP30 driver in use and I need another. However, they're priced at $60. I'd rather use that for a microcontroller and build my own driver if the total parts cost less than $10. Is this possible? Thank you.
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mikep



Joined: 24 Sep 2005
Posts: 771
Location: Austin, TX
Posted: 02 January 2011, 16:15 PM    Post subject: Reply with quote
Nothing is simple (or cheap) when you are talking about 20A peak (stall) current. The chip used by the Pololu VNH2SP30 is over $11 from Digikey (although it is not stocked).

In terms of a MOSFET H-bridge circuit, you really should have some prior experience before trying one with this type of current. Did you try using Google to find a circuit or append/read one of the many Robot forums?

I did find this circuit after a few seconds searching with Google: http://letsmakerobots.com/node/18633 . I have no knowledge about whether this circuit will work or not.

Note that just the FETs alone cost in the order of $2-$3 each. Then you need a heatsink plus all of the other components. So doing it under $10 for a one-off is probably not possible whichever circuit you choose.
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twesthoff



Joined: 17 Mar 2006
Posts: 199
Location: Fredericksburg, VA
Posted: 02 January 2011, 16:30 PM    Post subject: < $10 motor controller Reply with quote
Any specifications? Do you need to reverse the motor or just control it's speed?
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GTBecker



Joined: 18 Jan 2006
Posts: 472
Location: Cape Coral
Posted: 02 January 2011, 18:27 PM    Post subject: Reply with quote
mikep wrote:
... you really should have some prior experience before trying one with this type of current...

Ah, Mike, you're a spoilsport! How can you deny Liam such a formative experience? There's nothing like forming a dead short across a nice stiff supply with just a timing error! Let him learn!

Liam, he's right, of course. If you are intent on building your own, it might be wise to do it first with small parts and a small motor on the bench - and a current-limited supply. The nature of an H-bridge makes it easy to blow something up if you simultaneously turn on both the high- and low-side drivers on either side of the motor. Incidentally, a very likely time for that to happen is when you reset the processor - when all pins float.

Most H-bridge ICs and modules include logic that makes that impossible, so they are a good place to start - even if it costs more.
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liam.zbasic



Joined: 25 Mar 2008
Posts: 163
Location: Southern California (Blue)
Posted: 02 January 2011, 21:36 PM    Post subject: Reply with quote
mikep - yes, I found plenty of MOSFET-based circuits but what I found lacked some kind of guarantee of reliable operability. I figured one of the Zbasic users might have one already. Anyway, the let letsmakerobots circuit looks promising. I have most of the components except the MOSFETs. The idea of my own motor driver came after coming across this:

http://www.sparkfun.com/products/10256

twesthoff - yes, I need forward & reverse control with speed.

GTBecker - When you say "... simultaneously turn on both the high- and low-side drivers...", are you referring to all 3 driver inputs (PWM signal, "A" and "B")?

Overall, looks like the VNH2SP30 bare chip or equivalent is the best approach for cost, reliability & fire prevention!

Thanks all.
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GTBecker



Joined: 18 Jan 2006
Posts: 472
Location: Cape Coral
Posted: 02 January 2011, 21:52 PM    Post subject: Reply with quote
liam.zbasic wrote:
... 3 driver inputs (PWM signal, "A" and "B")...


On what device? Those signals are already a level above the MOSFETs.
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mikep



Joined: 24 Sep 2005
Posts: 771
Location: Austin, TX
Posted: 03 January 2011, 0:16 AM    Post subject: Reply with quote
liam.zbasic wrote:
The idea of my own motor driver came after coming across this:

http://www.sparkfun.com/products/10256
This circuit is misleading. If you read the comments on the page you will find that you need some kind of higher voltage driver to be sure to properly turn on and turn off the MOSFETs. I'm sure you already know this.
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spamiam



Joined: 13 Nov 2005
Posts: 689
Posted: 03 January 2011, 15:04 PM    Post subject: Reply with quote
mikep wrote:
you need some kind of higher voltage driver to be sure to properly turn on and turn off the MOSFETs. I'm sure you already know this.


Most MOSFETS require 10v at the gate (Compared to the source voltage, which is ground for a low-side MOSFET) to fully enhance. There are some logic-level MOSFETS that can enhance pretty well with 5v at the gate.

You need an H-bridge circuit for the MOSFETS, and probably could use a high-side/low-side MOSFET driver. These may also have logic designed for H-bridges too!

Total price could easily be $5 for the driver, and $5 each for the N-Channel MOSFETS, plus heatsink(s). A couple extra dollars for some glue logic (to prevent simultaneous turn-on) if required.

-Tony
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