# Oscillating power transistor??

Discussion in 'Electronic Basics' started by Don A. Gilmore, Sep 18, 2004.

1. ### Don A. GilmoreGuest

Hi guys:

I'm working with a circuit where I take a square wave signal from an op amp
comparator and use it to drive an inductive load. Obviously, the op amp
can't handle much current, so I connect this output to the base of a power
transistor and drive the inductive load with it.

The problem is that, for some reason, I get a spontaneous oscillation from
the circuit when I'm not even applying any signal. It's a strong signal and
shows up clearly on the scope. I would filter it out except that it is
relatively close in frequency to the signal I plan to apply to the inductor
(on the order of hundreds of hertz).

What is causing this and what should I do to eliminate/prevent it? Thanks
for all replies.

Don
Kansas City

2. ### John PopelishGuest

If your driver applied a voltage across the inductor, and holds this
voltage till a current develops through the inductor, and then turns
off, the inductor must ring at its self resonant frequency. to
produce a controlled voltage across the inductor, you must apply
voltage to it from a source that has a low impedance at all parts of
the waveform. Without seeing a schematic of your circuit, I have no
way to know if it does this.

3. ### BanGuest

It is probably that your opamp is not able to swing close enough to the
ground rail and the power transistor cannot switch off.
In the first place you need a 1k resistor between the opamp output and the
base, and another 1k resistor from base to ground. Then you will need a
when it switches off. the anode to the collector, cathode to +

4. ### CFoley1064Guest

Subject: Oscillating power transistor??
Hi, Don. Try this Kansas City Standard method (view in fixed font or M\$

VCC VCC
+ +
| |
| |
- C|
^ C|
| C|
| |
VCC | |
+ '---o
| |
|\| 2.2K |
-|-\ ___ |/
| >-->|--|___|-o -|
-|+/ | |>
|/| .-. |
| 2.2K| | |
=== | | |
GND '-' |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The diode in parallel with the load will help absorb the inductive kick. The
diode in series with the op amp output will help ensure complete turnoff if
your op amp doesn't go down to the negative rail. The resistors are necessary
because transistors are current-driven devices.

Good luck
Chris