Clamping the flyback energy of an inductive load to prevent damage to switching transistors.

This circuit is just to demonstrate the high voltage generated by the collapse of the magnetic field in an inductive device such as a relays, selonoid valves, or vurchuly any other electromagnetic device at the moment the switching transistor turns off. This high potential is in opposit polarity to the power supply and therfore adds it's potential in series with the power supply presents itself across the collector-emitor Junction of the switching transistor. This potential is often several hundred volts and will quite often severely exceed the breakdown voltage of the transistors Junction thereby destroying the transistor. In this example I used LED'S as the clamping diode so that you can see the potential being clamped to the forward bias voltage of the LED and Bled off slowly. Noted by the slow decay in the intensity of the LED. Normally you would use a much higher current diode such as a 1N4007 which is good for 1 amp continuous and will do several times that in this service. Opening one of the switches in series with the LED'S will allow you to view the high potential with the scope. Also just wanted to mention that the resistors in series with the inductors is there because every circuit assumes a zero resistance for the inductor and is not an adjustable parameter when you open the tools (click on the little wrench). Without the resistors the current is so high that it gives you a short circuit fault as soon as you close the switch. Well it did before I added the transistors. They actually have that adjustable parameter in them and any series resistance no matter how small will stop the short circuit error it just can't be zero otherwise the current is infinity which has no solution.

published 8 years ago