using TIP transistors to control speed of a motor

[Korawy]

Hi all,,,

I have TIP142 can i use this transistor to control the speed of dc motor (instead of mosfet) through PWM from pic18f452?

 

[kam1787]

depends on the motor and the current/voltage it needs, especially the inrush current

 

[Korawy]

the Motor current at most is less than 2A, and its voltage is 18 volt...
The freq of PWM is 20KHz..

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the Motor current at most is less than 2A, and its voltage is 18 volt...
The freq of PWM is 20KHz..

 

[kam1787]

TP142
ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value Unit
VCBO Collector-Base Voltage (IE = 0) 60 80 100 V
VCEO Collector-Emitter Voltage (IB = 0) 60 80 100 V
VEBO Emitter-Base Voltage (IC = 0) 5 V
IC Collector Current 10 A
ICM Collector Peak Current 20 A
IB Base Current 0.5 A
Ptot Total Dissipation at Tcase ≤ 25 o

It is not only the running current but also the inrush or cranking or initial current which you need to address, which can be up to 10x the running current. So this MIGHT be 20A in your case.

Which is at the peak current for this TIP. Depending on the motor performance, they may work for a while but could fail in a period shorter than desireable.

Further, you are using PWM which could make matters worse in terms of the duration of start-up.

 

[Korawy]

I measure the current the motor draw under normal condition i found it less than 1A.
I think the inrush current u determine is the max current a motor can draw, and i measure that max current through forcing the motor to stop and i found it less than 2A.

PWM frequency may be a problem as i see in the datasheet that TIP is for low frequency applications, but i know that there are applications that works on a lot of MHz's.

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[kam1787]

What equipment did you use to measure the inrush current?

You are using 20KHZ in your PWM which ok for the TIP.

 

[Korawy]

I used a multimeter to measure the current..!

 

[kam1787]

I used a multimeter to measure the current..!

The inrush current peak has a time intervals of MILLISECONDS. Much much too fast for your VOM or DVMM to capture. You need a storage scope to measure. It is too fast for the multi-meter to see but certainly fast enough to burn-out a transistor!

If the running current is <1A, then - usually - the inrush could be as high as 10A, in which case the TIP will be fine. :grin:

 

[Korawy]

when i measure the current, i used a power supply which supplies less than 3A, and the motor works fine with no problem, even when i force to stop with my hand.
Later i was using batteries which current rating are 4Ah....!

 

[kam1787]

batteries, depending on the type, can supply needed current inrush

likewise for a power supply - no idea as to your power supply so can not comment further

It seems you doubt the magnitude of the inrush current or refute advice/suggestions, so you may try researching it or build your circuit