Motors not stopping anytime

Thread Starter

maliha tamkeen

Joined Jan 6, 2016
13
Hi
We are working on
http://www.sciencebuddies.org/science-f ... #procedure
When we tried first everything worked fine.We were able to turn off the motors in normal light at night.The motors turned on only with the torch light.We also set the potentiometer so that the motors are off on normal lights.
When we try now,the motors do not turn off ,even in dark.Please help.We have to get it resolve fast.
Please let us know.
Thanks
Maliha
 

TheButtonThief

Joined Feb 26, 2011
233
The back-emf from the motors may have killed the mosfets.
+1

Where coils and semi's are concerned, you should always place a flyback diode across the coil to prevent the back-EMF from causing damage to the rest of your circuit. Here, your MOSFET is removing voltage from the motor but it's still connected to the circuit and so current is still allowed to flow. When voltage over a coil is removed, the coil is left with an inductive charge (not unlike the charge in a capacitor), the coil wants to dump this charge very quickly and when it does, the current can spike enormously which will damage whatever components are in series with the coil if they're unable to handle such a volume. A diode over the coil in the reverse polarity to your conventional current flow will supress this current spike.
 

MaxHeadRoom

Joined Jul 18, 2013
19,442
Also I would have tended to control the gate digitally instead of in a linear fashion, use a comparitor LM311 for e.g. it would also allow for a sharper on/off setting.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
19,442
With the circuit shown, if using LDR's I would have thought the average latency of these type of photo detectors would result in a slow turn on/off of the Mosfet limiting the effect of opposing BEMF.
Max.
 
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