Electronic Load CV Mode - Page 1
Forum section: electronics / projects

author: Ditiris
author_rank: Contributor
timestamp: July 13, 2020, 08:56:27 pm
content: I'm trying to design an electronic load and determine what features to include and what specs to target. How does an electronic load CV mode work? I've looked at a few designs and it's obvious how you could sense the voltage of the load, but I don't understand how you would set the voltage. Is it some kind of feedback loop to the current sense?

author: duak
author_rank: Super Contributor
timestamp: July 14, 2020, 03:58:31 am
content: A constant voltage load is basically a shunt regulator. Instead of comparing the reference level to the voltage across the current sense resistor, the op-amp compares it to the applied voltage. If I were to design an electronic load with a CV mode, I would design it such that the CC feedback could override the CV feedback to prevent the pass element from being damaged. This is important if the power source can deliver more current than the load can handle, eg. a battery. You could use a current limiting fuse, but it's nice to not have to replace a fuse if the load is not set correctly.An elegant way to protect the load is to servo the current by actually working out the power (V x I) dissipated. Some loads have constant power mode, but I expect they need a multiplier in the feedback loop.I needed to simulate a forklift battery recently to test a 24 V 50 A battery charger. It would have been nice to have a CV load for this test. I had some 50 A transistors and some 0R5 power resistors handy so I rigged them up with some zener diodes to turn them on sequentially to simulate a battery.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2020, 11:47:57 am
content: Consider the traditional CCS circuit: opamp, FET with shunt resistor, load.Move the amp's feedback connection to a voltage divider on the load terminal. Boom, you have a shunt regulator!More generally: if you prepare both of these situations, and connect the amp's feedback to the wiper of a potentiometer going between the two extremes, you can also have avariable resistanceload! (The pot can be digitally programmed, or it can be synthesized with some clever, albeit dated, analog circuitry.)(Note that you can'tliterallystring a pot between a resistor divider and a shunt resistor; the pot will pull down on the divider, and the shunt will impose some of its voltage even when the pot's all the way to one side. Easy solution: buffer the divider with an op-amp follower. Ideally, the same would be done on the shunt signal, but its value may be low enough not to worry about this. If you're making this for very small current ranges, this would be a good idea.)Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 14, 2020, 12:24:45 pm
content: Quote from: Ditiris on July 13, 2020, 08:56:27 pmCV is constant voltage, so it is like Zener - doesn't connected with load current but with load voltage. A quick example may be a comparator which is controlled by set voltage and load voltage. If load voltage is larger then set voltage - then it works to put a current through regulating element.A sophisticated CV-mode electronic load will control load current too - just only to not overload a regulation element.