LM723 Precision Logarithmic Scale Current Limitation - Page 1
Forum section: electronics / projects

author: iMo
author_rank: Super Contributor
timestamp: January 23, 2019, 04:06:08 pm
content: As a big fan of the LM723 vintage regulator here is a simulation which works, but has not been tested in hw yet. Feel free to testIt is a "precision current limit" setting with logarithmic scale.It means the potentiometer's dial shows "decades" of max current limits (like 1mA-10mA-100mA-1A-10A on the dial).In below picture you may see the Load currents from 1mA to 10A measured by the log amplifier range from 220mV to 590mV at the comparator's input in pretty linear way.You may see below the simulation. I took my junkbox opamps, you may use better ones, of course.For simplicity the opamps are powered from the "bridge voltage", you may lower the voltage for the real opamps you want to use.Also mind the diode in the log amp has some tempco of -2mV/degC.The wiring requires a "Low-Side shunt" (not a problem I think).The Q2 and R7 simulate the internal LM723 current limiting transistor. The R2 should be connected to the LM723 CL pin (the base of the internal CL transistor).Still, in this version of simulation the current you limit includes the operating current of the LM723 as well (I've been using the internal LM723 Vref as the reference for the current limit comparator).

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 09:34:22 am
content: Hi,two comments:- You could try to use a temp-compensated log amp circuit- where is your loop compensation when in current mode ? This could be tricky because the loop is now very nonlinearHave funWolfgang

author: BravoV
author_rank: Super Contributor
timestamp: January 25, 2019, 09:49:10 am
content: Subscribed, and thanks.

author: Rerouter
author_rank: Super Contributor
timestamp: January 25, 2019, 10:39:37 am
content: Just redrawn to make it easier to read the schematic.

author: iMo
author_rank: Super Contributor
timestamp: January 25, 2019, 01:00:24 pm
content: Quote from: Rerouter on January 25, 2019, 10:39:37 amMind your "PSU" is the bridge/filter voltage (input to 723 reg), and your GND is the output terminal of the PSU.The 723' GND is wired to the GND.@Wolfgang: the dynamic parameters of the circuit have not been tested nor simulated. It waits on anadventurous enough reader to try it out

author: iMo
author_rank: Super Contributor
timestamp: January 25, 2019, 02:36:12 pm
content: Here is a "more complete simulation" with a simple 723 wiring.I added the C5 and R16 - that RC suppresses an oscillation of the loop when the current limiter triggers.It is an simulation only, and the actual behavior depends on many factors.Use at your own risk

author: iMo
author_rank: Super Contributor
timestamp: January 25, 2019, 07:07:35 pm
content: Another iteration V4:1. a switch used instead of the current source at the Load side2. changes in resistor's values around the opamps3. more realistic bridge impedance model4. the C5 has been removed - a faster response now5. some minor improvements.

author: BravoV
author_rank: Super Contributor
timestamp: January 26, 2019, 03:52:16 am
content: Please attach the .ASC file too, thanks.

author: iMo
author_rank: Super Contributor
timestamp: January 26, 2019, 07:52:36 am
content: The source. You need the latest Bordodynov library, however.