opamp driving power transistor driving coil

[Jon]

I want to switch a coil on and off completely, from a power transistor
at the output of an opamp or comparator. Right now I am trying the
opamp, and I can't get it to turn off completely, since the opamp can
only go to -14 V or so, and the transistor emitter is at -15. What am I
missing? This seems pretty simple. Also wouldn't the comparator have
the same problem, since the open collector output is at a transistor
drop above the negative rail?

Aye. I should know this stuff...

 

[Tim Wescott]

I want to switch a coil on and off completely, from a power transistor
at the output of an opamp or comparator. Right now I am trying the
opamp, and I can't get it to turn off completely, since the opamp can
only go to -14 V or so, and the transistor emitter is at -15. What am I
missing? This seems pretty simple. Also wouldn't the comparator have
the same problem, since the open collector output is at a transistor
drop above the negative rail?

Aye. I should know this stuff...

A comparator should pull you down to 0.2V or so (Vcesat), your
transistor needs 0.6 (check the comparator datasheet).

A rail-rail op-amp or something like an LM358 should pull all the way to
ground as well.

Got the right snubber on the coil?

 

[Ban]

I want to switch a coil on and off completely, from a power transistor
at the output of an opamp or comparator. Right now I am trying the
opamp, and I can't get it to turn off completely, since the opamp can
only go to -14 V or so, and the transistor emitter is at -15. What
am I missing? This seems pretty simple. Also wouldn't the
comparator have the same problem, since the open collector output is
at a transistor drop above the negative rail?

Aye. I should know this stuff...

You can pull down the base voltage by applying a resistive divider to the
transistor base. In this case we can divide the current into 10mA base drive
and 4mA which gets eaten up by the additional resistor. The voltage when low
is now below 100mV, which will shut off the transistor.

+-----+------o
| | +15V
C| |
C| -
C| ^
| |
+-----+
|
|\ |
-|-\ ___ |/
| >--|___|-+----|
-|+/ 2k2 | |>
|/ .-. |
| | |
180| | |
'-' |
| |
+------+-----------o
-15V
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[Jon]

You can pull down the base voltage by applying a resistive divider to the
transistor base. In this case we can divide the current into 10mA base drive
and 4mA which gets eaten up by the additional resistor. The voltage when low
is now below 100mV, which will shut off the transistor.

+-----+------o
| | +15V
C| |
C| -
C| ^
| |
+-----+
|
|\ |
-|-\ ___ |/
| >--|___|-+----|
-|+/ 2k2 | |>
|/ .-. |
| | |
180| | |
'-' |
| |
+------+-----------o
-15V
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

aha! thank you! i knew there was a simple solution...

it works.

 

[Norm Dresner]

I want to switch a coil on and off completely, from a power transistor
at the output of an opamp or comparator. Right now I am trying the
opamp, and I can't get it to turn off completely, since the opamp can
only go to -14 V or so, and the transistor emitter is at -15. What am I
missing? This seems pretty simple. Also wouldn't the comparator have
the same problem, since the open collector output is at a transistor
drop above the negative rail?

Aye. I should know this stuff...

You could also try to insert a diode in series with the Op Amp output.

Norm

 

[Jon]

You could also try to insert a diode in series with the Op Amp output.

Norm

ah. to create a voltage drop. is there a benefit to one method over
the other? switching times or something?

 

[Ban]

ah. to create a voltage drop. is there a benefit to one method over
the other? switching times or something?

Well, the diode should be in the emitter leg of the transistor, I suppose.
The disadvantage is that the output voltage to the coil is reduced by the
diode drop, the current might be quite high too, depending on the solenoid.
The additional resistor method increases the current consumation by 4mA when
"on"(dependent on size of resistor). It is the cheapest method.
Or use a comparator IC like LM339. The pullup resistor would be 4.7k with
+/-15V rails. no resistor to gnd needed, because the O/P goes down to 100mV
or so. But this constellation draws also current (6mA) when the solenoid is
off (because of the pull-up).
Lowest power consumption with a N-FET as switch and CMOS opamp with rail to
rail outputs. The gate voltage should be divided by 2 with a resistor
divider 15k + 15k, better to have a lower supply voltage then the connection
can be direct and switching is faster.