What si that series resistor with Diode in and Inductor Switch circuit?

[Shahriar]

Hi
When an inductor is to be switched by a transistor, Usually a Diode is placed
in parrarell with inductor to Protect Transitor from High voltage created with Inductor
when Switch turns off. (I have forgotten the English name of this diode)

But sth. a Resistor is placed in series with this diode? What is that resistor for and how to calculate it?

ThanX in advance
Shahriar

[audioguru]

When current is suddenly stopped to an inductor, the stored energy will try to keep the current flowing. Without a diode then there is nowhere for the current to go so the voltage goes very high. The diode conducts when the voltage tries to go high then the current keeps flowing until the inductor discharges its energy.
If the inductor is a relay then its release will be delayed. A resistor in series with the diode will allow the voltage to rise by the amount of current that was in the inductor, then the discharge is quicker and the relay releases quicker.
The calculation is the time for a certain amount of current in an inductor of a certain value to discharge, and the inductor's current in the resistor creating a rise of voltage due to Ohm's Law.

[Shahriar]

Tell me if I got it. ;)

The resistor will Dissipate power to remove the stored energy in Inductor. without Resistor it takes longer.
I think for relays, because The resistance is high enough, they don't put this resistor but for an inductor which its Resistance is low, they put such Resistor.

BTW, what was the name of this diode in English? I have forgot it.

ThanX very much
Shahriar

[audioguru]

Catch diode, protection diode, flyback diode, freewheeling diode. Lots of names for the same thing.

[indulis]

A resistor in series with the diode will allow the voltage to rise by the amount of current that was in the inductor, then the discharge is quicker and the relay releases quicker.

You sure you want to stick with this...  (hint: a resistor LIMITS current)

[audioguru]

[You sure you want to stick with this...

[indulis]

Think about it... what makes a "better short", a diode, or a diode in series with a resistor when it comes to discharging an inductor?

[audioguru]

A relay without a flyback diode releases immediately when its current is stopped.
I have read that when a flyback diode is connected without a series resistor then the current circulates and the release is delayed.
Adding a resistor in series with the diode is half-way between the two connections above, so the release should also be half-way in time.

[indulis]

The diode is there on relay coils to protect the switching device. If someone put a R in series with a diode across a relay coil, it was to limit the current thru the diode. To control rise and fall time's, most common are RC and RDC snubbers.