Lm358 voltage controller

[be80be]

How does one get this to go from 0 to 10 volts don't really care if it's less on the 10 volt side but I need the 0
I'm thinking I need to lower the ground by the 1.59 volts some how.

 

[AnalogKid]

The input common mode voltage range of a 358 extends below the opamp's negative rail, which means it includes GND, which is half the problem. But the other half is that the opamp output must go high enough (with respect to the emitter) to turn off the transistor, and it cannot. Look at the 358's output voltage range for your operating voltage. One chart shows the max output voltage at around 13 to 13.5 V with a 15 V rail. that is 1.5 V of headroom, best case, and that will keep the transistor turned on at least partly even when the opamp output is saturated high.

One solution is an opamp with a true rail-to-rail output. another is a couple of small signal diodes in series with the 3906 base, and a 10K resistor from the base to the emitter. This will turn off the transistor when the opamp output is high enough to stop conduction through the diodes.

ak

 

[alec_t]

Try this :-

 

[AnalogKid]

As above, recommend a resistor from the Q1 base to emitter to guarantee correct operation at very low output voltages.

Rethinking my first response, it is possible that adding the resistor (and adjusting the value of the 358 output resistor) is all that is needed.

ak

 

[be80be]

Thanks im try it all out Been trying to learn LTspice

 

[alec_t]

As above, recommend a resistor from the Q1 base to emitter to guarantee correct operation at very low output voltages.

Good idea for the real world, but not needed in my sim because the LM358 output can get very close to ground and shut Q2 off firmly.

 

[alec_t]

Been trying to learn LTspice

Keep at it! Well worth the effort.

 

[AnalogKid]

Good idea for the real world, but not needed in my sim because the LM358 output can get very close to ground and shut Q2 off firmly.

True, but shutting off Q2 equates to turning off Q1 *only* in the world of sim. Sim programs consider a transistor with a floating base to be off. The universe has a different opinion.

ak

 

[dr pepper]

LTspice is pretty good, I wouldnt use it a first, now its a handy tool.
If you cant find some components like 555's, uc3842, Tl492 etc do a google search you can add them to its lib.

 

[schmitt trigger]

A way I have seen it is to replace your 2N3906 with a darlington transistor.
The larger Vbe drop of a darlington, will compensate for the LM358 output's inability to swing all the way to the supply rail. Which is necessary to fully turn off the PNP pass transistor.

 

[dr pepper]

Bc516 is a pnp darlington with silly high gain, I think you can still get them.

 

[schmitt trigger]

Now that you mention it Dr, the OP's schematic may have originally intended to employ a darlington PNP.
Just look at the base resistor's value!

 

[dr pepper]

Yep its a bigun.

 

[gophert]

How does one get this to go from 0 to 10 volts don't really care if it's less on the 10 volt side but I need the 0
I'm thinking I need to lower the ground by the 1.59 volts some how.

View attachment 123450

The gain of this schematic is the same as a non-inverting op amp circuit = 1 + RF/R2 = 2

your output voltage will be 2x the input from the pot.
To get 0-10v, you'll have to cut the input voltage in half.
The ways to do that are
- put a 10k resistor in series above the pot
OR
- remove RF and R2, connect (+) input of op amp to collector of PNP.
-