I need help in choosing a flyback diode across a DC motor. How can I choose a one with proper current ratings? What is the equation for current passing through it?

 

At the very first moment, the same current as DC motor draw from PSU want to flow through the flyback diode. But this current is not continuous so, the 1A diode will work in most of the cases.

 

You need to select a diode with a peak current greater than the maximum current drawn by the coil in regular operation. For example, a typical 1N400x is rated to withstand 30A for 8.3ms according to this datasheet, which is why it is used at large with small solenoids, relays and motors.

Wikipedia has some equations about the process:
https://en.wikipedia.org/wiki/Flyback_diode

 

You need to select a diode with a peak current greater than the maximum current drawn by the coil in regular operation. For example, a typical 1N400x is rated to withstand 30A for 8.3ms according to this datasheet, which is why it is used at large with small solenoids, relays and motors.
Wikipedia has some equations about the process:
https://en.wikipedia.org/wiki/Flyback_diode

I require about 250 mA peak for my motor. It stays on for about 2 to 3 seconds.So is there any diode which you can suggest for this?

 

The 1N400x series that rsjsouza mentioned should work fine.

 

To calculate the current though it, put a small value resistor like .25 or .5 ohm in series with the diode, then put an oscilloscope across the resistor and measure the voltage. The voltage is equal to the current times the resistor value. The current will be a tad higher without the resistor, but this will get you really close.

 

The 1N400x series that rsjsouza mentioned should work fine.

Thanks. But I want to know why

 

Thanks. But I want to know why

Why?
Because it has more than sufficient current capability (1A) to carry the transient peak motor current of 250mA.

 

The 1N4000 series of diodes has been extremely popular so they are usually easy to find and inexpensive. I was very surprised recently when I saw a datasheet from Diodes Incorporated saying "not for new design" - through-hole parts are disappearing as surface mount is used more and more.

There are lots of other diodes with adequate voltage and current rating that would work. For 250 mA I would use something like a 1N4148, which is also very popular, if I needed a part that was physically a bit smaller. It is in a DO-35 package whereas the 1N400x series are in DO-41. The 1N4148 has very short "forward recovery" and "reverse recovery" times which can be important in some applications but would make no difference for your motor circuit.

With small DC motors, a capacitor or a resistor and a capacitor in series are often connected across the motor to suppress radio frequency interference produced by the motor brushes. Sometimes this is also adequate to limit the inductive voltage spike, but it must be confirmed carefully. If there is any doubt, it is best to include the diode.

A single diode across the motor cannot be used if the motor is driven bidirectionally. In that case other methods such as "back to back" zener diodes (two zeners, each with a voltage rating higher than the maximum that would be applied to the motor connected in series, either anode to anode or cathode to cathode) or a bidirectional transient voltage suppressor (functionally similar to the zeners but higher transient current handling) might be used. With "H-bridge" drive with MOSFETs, the "body diodes" of the FETs will provide a path to limit the spike voltage. With H bridges made with bipolar transistors, it is common to add a discrete diode across each transistor.