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Choosing the right transistor?

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ormo

New Member
I am having trouble finding the right transistors to drive motors. I have 3 identical PWM circuits based on the 555 timer. Each one will drive one of these motors:

3v DC Motor 1.5 to 4.5 volt operation 13000 rpm on eBay, also Motors Servos, Components Supplies, Electrical Test Equipment, Business, Office Industrial (end time 11-Jun-09 23:37:01 BST)

My problem is finding the right transistor to drive the motors.

If it helps, the weight on the motor is just a small plastic mirror - nothing heavy.

Thanks.
 

Wp100

Well-Known Member
The spec sheet states over 1amp full load so you need a transistor capable of say 3 to 5 amp to handle start up surges etc.

Should find something in Maplin thats suitable BD243c looks big enough and cheap - don't forget a little heatsink if you intend running heavy for a long time.
 

crutschow

Well-Known Member
Most Helpful Member
A power MOSFET would probably be easiest to drive. Use one rated for 3A or greater. They typically take 8-10V or so for best turn-on, so operate the 555 at a 12V supply or greater (15V max.).

What is you PWM frequency?

A search at Maplin came up with an IRFZ24N, IRF740, and IRF540, somewhat overkill for you requirements, but any one should work.

Make sure you place a suppression diode across the motor (cathode to plus motor connection) to suppress inductive spike voltages that can blow the transistor.
 

ormo

New Member
Down to the design of the rest of the project the 555 will be running on 3.3V - will the BD243c still work well here? The PWM frequency is about 145Hz
 

crutschow

Well-Known Member
Most Helpful Member
A typical 555 will only work down to about 4.5V.
 

smanches

New Member
Use a logic level mosfet then. Most are "full-on" at around 4.5V.
 

smanches

New Member
The one you listed is not a logic level fet and must be driven by 10V or more.

You need something like this. IRLIB4343PBF

The only factor that matters is the Rds_on parameter. Make sure the parameters say 4.5V or 5V, not 10V.
 
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