Oscillating power transistor??

[Don A. Gilmore]

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

 

[John Popelish]

"Don A. Gilmore" wrote:

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.

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.


--
John Popelish

 

[Ban]

Don A. Gilmore wrote:

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?

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
diode across your inductive load to prevent the transistor from avalancing,
when it switches off. the anode to the collector, cathode to +
--
ciao Ban
Bordighera, Italy