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DC Motor Timing project

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mesamune80

New Member
Hi all,

i had encountered a very weird problem form my motor timing circuit,ok now i'll explain my problem:

1) when SW is push and release (trigger oin 2 of 555 Timer),the IC start its count (set it to be 5 minutes time)...but after about 3 minutes when the pedal is press then my timer got stopped.But if you let the time elapse without pressing the pedal from the start then the time will count to 5 minutes.

2)The motor seem to loss power (slow down) if compare to direct power up form 6V cell.
is there any method to boost my output current to my motor from the 555 timer ?

i am uisng the timer to control my time for my motor,i want it to be after 5 minutes my motor power will be cut off.
Here i attached my circuit,please take a look. thanks .
 

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Mr RB

Well-Known Member
Hi, your 1N4006 diode shoudl be across the motor, not the transistor.

Your TIP120 has heaps of gain but even at saturationt he TIP120 typically is about 1v lost. So you are getting 5v to your motor, not 6v which as you noticed is a serious power difference.

Since it only needs slow switching, what about using a relay?
Or you could replace the TIP120 with a N-Fet that has a low Rds "on" resistance.
 

Hero999

Banned
Seconded, use a MOSFET, IRL540 will do.

The battery voltage probably falls pretty quickly.

What sort of battery are you using?
 

mesamune80

New Member
Thanks for all the replies,i am uisng Rechargeable Valve Regulated Lead Acid Battery initial current is less than 1.35A my battery model is LONGWAY 3FM4.5 (6V 4.5Ah/20HR)
i'll try to switch diode across the transisitor and add a 100nF capacitor parallel with my motor does this helps?
 

Sceadwian

Banned
No it won't help but the diode is still in the wrong place. The cap will just help reduce noise, again good but it won't replace the fact that you shouldn't be using that transistor as Mr RB said. Go with Hero's suggestion, us a MOSFET instead, it will replace the diode drop of transistor with an extremely low on state resistance of the FET, giving the motor the maximum voltage it can pull of the battery.
 

Sceadwian

Banned
They're not useless ubergeek just not ideal. I've used 4148 as freewheeling diodes on stepper motors, if the frequency he's using is low enough the type of diode he uses is irrelevent.
 

Ubergeek63

Well-Known Member
They're not useless ubergeek just not ideal. I've used 4148 as freewheeling diodes on stepper motors, if the frequency he's using is low enough the type of diode he uses is irrelevent.
1N4148s are switching diodes, the ones he is using are LF rectifiers. They are both slow and have high forward drops.

At the 6V rail he is working with he is throwing away over 10% of his power between the darlington and the diode. Changing to a FET and schottky can easilly cut the losses in half.

Dan
 

mesamune80

New Member
Thanks for the replies,ok so what i should now is change my TIP120 to FET (or should i use MOSFET) which is better and cost efficient ?and diode which suppose to place across the motor should change to schottky.What kind of FET i shold use and schottky diode i should change to? How to choose both of them? Thanks.
By the way my motor is bidirectional dc motor.
 
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