Turning off the LM395.

Aug 19, 2010 6 Replies
G

The following simple single supply op-amp with transistor power booster won=92t go down to zero. With the input to the opamp at zero the output from the opamp is at 20mV and the emitter of the LM395 is at 0.6 volts.



(V+)---------------+ | | |\ C Vin----+ \ |/ (LM395) | >--5k--| +-- / |\ | |/ E +---10k-------+-------+ | | R=3D2.5k R load =3D 50 ohm heater. | | (V-)-+---------------------+


The LM395 is not a =91real=92 transistor. It=92s got overload and thermal shutdown circuitry, which I like. It=92s possible for the user to short out the load and with the LM395 I don=92t worry so much about something failing. When I replaced the LM395 with a real NPN. (TIP31C) the circuit was fine. (Base resistor reduced to 500 ohms, V out of opamp



0.43 volts, V emitter =3D 0.2mV.) The LM395 is really more like a Darlington which might be the root of the problem. Anyone know of a simple way to get this to turn off with the LM395? Oh the opamp is a LT1014.

Thanks George H.


Vote
Twitter Facebook LinkedIn
L

current for the protection circuit etc. ?, datasheet says upto 10mA @ vbe =3D 0

0.6V/50R =3D 12mA

-Lasse

Vote
Twitter Facebook LinkedIn
J

The input of the LM395 sources current. So the 5K resistor is part of the problem.

A real r-r opamp would be better, something that gets down to 50 millivolts maybe. If you stick with the LT1014, add a resistor from its output to ground.

Is there a negative supply available?

Are you seeing this error with the 50 ohm load connected?

John

Vote
Twitter Facebook LinkedIn
G

mal

t

Opps, sorry my mistake. A better schematic looks like this.

| GND | | | +--------------+

I've got a Hobbs filter* on the output to reduce the noise. I had it ground referenced and the cap is polarized (100uF Al-electro). And the rest of the circuit ran from +/- 15 volt rails. Kind of amazing that it worked at all. I originally had the Hobbs filter on the power rails before it entered the voltage control circuitry. That worked fine to filter the power supply crap, but I forgot about the noise in the voltage control.

Why is it that the simple circuit that you spend the least amount of time on, is the one that bites you just as you are about to ship the product? (Murphys law #137.)

Thanks for listening, sometimes that's all that is needed.

George H.

*The rest of the world calls this a capacitor multiplier.
Vote
Twitter Facebook LinkedIn
G

rmal

rt

Thanks guys, (Lasse and John) I replaced the polarized cap with a

10uF film and everything looks nice. Now I've got to go and remeasure the noise.

George H.

Vote
Twitter Facebook LinkedIn
J

Yup. We think real hard about the complex parts and get sloppy on the "easy" stuff.

Bizantine mixed-signal stuff works first time, but the voltage regulators don't.

John

Vote
Twitter Facebook LinkedIn
S

^^^^^^^^^

I like that.

Bizantine: (adj) A highly complex or intricate and occasionally devious corporate structure or arrangement of related companies.

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required