Op-Amp powered by voltage divider - Page 1
Forum section: electronics / beginners

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 19, 2015, 10:31:01 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2015, 10:35:43 pm
content: 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

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 19, 2015, 10:45:29 pm
content: Thanks for the help

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 19, 2015, 10:57:07 pm
content: 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.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 19, 2015, 11:04:03 pm
content: Is this ok?

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2015, 11:13:09 pm
content: 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.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 19, 2015, 11:20:03 pm
content: QuoteI don't understand what you mean. It has a gain of -2 to turn the 5V into -10V.

author: Stupid Beard
author_rank: Regular Contributor
timestamp: June 20, 2015, 12:23:35 am
content: Quote from: Aodhan145 on June 19, 2015, 11:20:03 pmI think you've got confused and are applying the math for a non-inverting amplifier to an inverting one. Seehttps://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifierFor a gain of -2, R48 would have to be 20K

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 20, 2015, 12:52:14 am
content: Thanks for the help.

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 20, 2015, 01:04:58 am
content: 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.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 20, 2015, 01:38:49 pm
content: 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?

author: flynwill
author_rank: Regular Contributor
timestamp: June 20, 2015, 02:18:34 pm
content: 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.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 20, 2015, 02:25:50 pm
content: Do you have any good reason for wanting to use zeners instead of linear regs?

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 20, 2015, 04:15:46 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: June 20, 2015, 04:28:12 pm
content: 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.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 20, 2015, 06:44:08 pm
content: 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 channelsPostive channel 0-32V (LT3080) at 8ANegative channel -1.25V to -32V (LM337) at -1.5AConstant Current limiting for both channels