Choosing the right transistor?

[ormo]

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:

**broken link removed**

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]

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]

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]

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]

A typical 555 will only work down to about 4.5V.

 

[ormo]

Sorry, 5.5V, my bad

 

[smanches]

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

 

[ormo]

I'm still rather confused by all of this, does this look suitable?

**broken link removed**

 

[smanches]

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

You need something like this. **broken link removed**

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