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Topic Title: FET Motor Control
Topic Summary: Start/Stop Control Of A Small DC Motor Not Working: Advice Requested
Created On: 01 April 2010 04:38 PM
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 01 April 2010 04:38 PM
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mikewagstaffe

Posts: 10
Joined: 10 June 2002

A colleague of mine, is attempting to start and stop a very small 5VDC electric motor with from a 5V low current output on an interface card.

He has produced a circuit using a small signal BS107 MOSFET, which I have uploaded a picture of, it can be seen here.

As far as my understanding of MOSFETS as switches is concerned I cannot see any reason why this would not work, and it does indeed work on another application to drive a sounder.

The Transistor appears to work perfectly with no load connected, however, when the motor is connected, the result is with the transistor gate pulled to 0V the motor runs slowly and when pulled up to 5V it runs quickly, again indicating that the circuit is switching.

Any advice on why this is not simply starting and stopping the motor when applying 5V/0V respectively to the gate would be appreciated.

Thanks in advance for any advice on this.

Michael Wagstaffe BEng Hons EngTech MIET
 01 April 2010 07:13 PM
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DavidParr

Posts: 238
Joined: 19 April 2002

Since the motor is running fast or slow, you must not be fully switching the device, and effectively operating it in its linear region. In other words it is not fully switching off when the Gate is brought low (so the motor goes slow rather than stopping). The resistance of the motor coil may well be similar to that of the on resistance for a 5V power supply.

Have look at the data sheet, bearing the above in mind for a VDS that is effectively very small.

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David Parr BSc.CEng MIET
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 02 April 2010 10:41 AM
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dbelly

Posts: 1
Joined: 07 January 2005

Check that you have got Source and Drain wired in the correct way around. If they are wrong the body diode of the FET will be forward conducting all of the time and this would explain why the motor runs slowly when the FET is off and there is a diode voltage drop across the FET. The FET will still operate (switch on and off) wired in backwards (Source and Drain wrong way around).

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David Bell CEng MIET
 02 April 2010 12:14 PM
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DavidParr

Posts: 238
Joined: 19 April 2002

I totally agree with David Ignore my earlier babblings

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David Parr BSc.CEng MIET
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