Op-Amp powered by voltage divider

[Aodhan145]

Can you power an op-amp like a lm358 with a voltage divider? (I only need a few milli-amps on the output.)  I am taking 32V down to 12V.

[Ian.M]

Not advisable. 

However you could use a dropper resistor and a 12V Zener.  Choose the dropper resister so that when the OPAMP is providing its max output current, there are still a few mA flowing through the Zener

[Aodhan145]

Thanks for the help

[AG6QR]

The classic model of a voltage divider composed of two resistors says that it divides the voltage according to the ratio of the resistances.

That model ONLY works if no current flows into our out of the voltage divider.  Remember, the classic model is derived from Ohm's law, combined with the fact that the currents in the two resistors are equal.  But if any current flows into our out of the divider, the currents flowing in the resistors will no longer be equal.

And that explains why you can't reliably use a voltage divider for anything that's going to source or sink significant current.

[Aodhan145]

Is this ok?

[Ian.M]

D7 is the wrong way round.

The Zeners will need to be 500mW and the dropper resistors 1W.

The gain doesn't match the signal voltage ranges you have given as the circuit after the 0 to 5V node is a simple unit gain inverting amplifier.

[Aodhan145]

The gain doesn't match the signal voltage ranges you have given as the circuit after the 0 to 5V node is a simple unit gain inverting amplifier.

I don't understand what you mean. It has a gain of -2 to turn the 5V into -10V.

[Stupid Beard]

The gain doesn't match the signal voltage ranges you have given as the circuit after the 0 to 5V node is a simple unit gain inverting amplifier.

I don't understand what you mean. It has a gain of -2 to turn the 5V into -10V.

I think you've got confused and are applying the math for a non-inverting amplifier to an inverting one. See https://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifier

For a gain of -2, R48 would have to be 20K

[Aodhan145]

Thanks for the help.

[Phoenix]

Are you applying the 0V-5V at ISET? If you are, you need to consider all the input resistors R44 R43 R47 for the gain of the circuit. In this case it would be:
OUTPUT/ISET = (10k)/(10k+10k+10k) = 0.33 an attenuator!

If you're expecting the labelled node to be 0V-5V, the input at ISET would actually need to be 0V-15V.

[Aodhan145]

ISET is 5V PWM with varing duty cycle. Then it turns into an analog voltage. Does it need buffered so it doesn't affect the op-amps gain?

[flynwill]

You just need to increase the value of the feedback resistor, 62k will give a bit more than the gain of 2 you are seeking.

[nitro2k01]

Do you have any good reason for wanting to use zeners instead of linear regs?

[Aodhan145]

Zeners are easier. All I need it for is for at most 1 or 2 milliamps from an op-amp. It would be a waste to use linear regs I would also have to have capacitors to stabilize the voltage and it would be a waste of space on the pcb as well.

[Ian.M]

Also, common low current linear regulators typically have a maximum input voltage rating of 35V.  That doesn't leave much safety margin when fed from a 32V supply.

Regulators with higher input voltage ratings are available, and may be worth using if this design is going to production, but if its a one-off a KISS design philosophy using in-stock or at least common parts gets the job done well enough today.

[Aodhan145]

It is just a diy bench power supply for me. It wont be in production. It uses a lot from Daves µSupply except it is not battery powered has different specs.

• Positive and Negative voltage channels
• Postive channel 0-32V (LT3080) at 8A
• Negative channel -1.25V to -32V (LM337) at -1.5A
• Constant Current limiting for both channels