# Help

#### micro

Joined Feb 10, 2006
26
Recently I have read a Simple DC motor PWN speed control circuit from
http://www.electronics-lab.com/projects/mo.../038/index.html. Inside the article and below the word of important, there have mention that "the drain (or collector) current must be equal to maximum motor current (at power supply voltage, when it is blocked)". May I know what is mean by maximum motor current? Can I check it from the data sheet (Which part of a data sheet, can you please guide me?) or measure from the 12V DC motor. Thanks for your kind help.

#### Papabravo

Joined Feb 24, 2006
20,385
Originally posted by micro@Apr 30 2006, 06:14 AM
Recently I have read a Simple DC motor PWN speed control circuit from
http://www.electronics-lab.com/projects/mo.../038/index.html. Inside the article and below the word of important, there have mention that "the drain (or collector) current must be equal to maximum motor current (at power supply voltage, when it is blocked)". May I know what is mean by maximum motor current? Can I check it from the data sheet (Which part of a data sheet, can you please guide me?) or measure from the 12V DC motor.  Thanks for your kind help.
[post=16701]Quoted post[/post]​
It just means that if the motor wants to draw 3 Amps at 12 Volts that you should not expect a 2N7000 in a TO-92 package to do the job. In this case I would be looking for a part that could handle 6A of drain current so as to not push the part to its design limit. The other parameter of interest is how much voltage can appear across the source-drain terminals without causing a breakdown, when the device is off. In this case I would look for a 24 Volt part for the same reason. It is good thing to have some margin between your operational point and the part's design limits, or Absolute Maximum Ratings. That said, you might decide to cut your margin down from a factor of two to a factor of 1.5 How and when to do this comes with experience.

The datasheet for the part will give you the maximum drain current and the maximum drain-source voltage. These numbers will be in a paragraph called "Absolute Maximum Ratings"

Datasheets for motors are less consistent than those for semiconductors, but you should still be able to find numbers or graphs that relate torque and speed to current and voltage.