Electronic Load for free - just a resistor away ! - Page 1
Forum section: electronics / testgear

author: EE-digger
author_rank: Frequent Contributor
timestamp: March 03, 2018, 01:09:01 am
content: After a long day, working from my home lab, I stared at the orange LEDs on an inexpensive, small (Circuit Specialists CSI3005T) bench supply in front of me. It was charging a single NiMh cell that I was plotting a charge curve for on Benchlink. I thought about how the LEDs looks so neon-like, sort of like old Sperry 7 segment displays or nixies. Yeah, long day.A thought occurred to me that has occurred every few years. Why can't I use the current limit function as a load? Each time I hooked a supply, set to current limit, backwards across a source, the numbers are all wrong, the supply starts clicking, etc. etc.This time I wondered for a moment ... why can't you do it? I have numerous electronic loads at work but spend my own money elsewhere. Having that convenience in the home lab, possibly to ANY load current level as you will see, would be nice ... sometimes.I put a series 20ohm power resistor in series with the supply, set it for 4V, 100mA limit to start, and hooked it backwards against the cell I was testing.This setup produced a perfect 100mA ! At this voltage and with this resistance, I was able to cleanly dial from 50mA to 400mA.So what happened? Your results may vary but for starters, I think you need that resistor to develop enough voltage to satisfy your supply so a short detection does not occur, and possible some other low output voltage protection.Interestingly, this cheap, single output supply delivered precise current to almost perfect 3 digits on my 34401A DMM. Possibly due to alignment of the stars, moon and the angle of the supply to the bench. But it was cool to see how well it worked.Also, ** important warning **, this "load box" is active ! While it's probably fine with any voltage source, be sure that if your source output drops too low, it is not damaged by having a voltage applied to it.I apologize if this idea has appeared before. I did some searching and did not find anything similar.

author: Brumby
author_rank: Unknown
timestamp: March 03, 2018, 04:06:00 am
content: This, perhaps?http://powersupply.blogs.keysight.com/2012/10/what-is-bipolar-four-quadrant-power.html

author: EE-digger
author_rank: Frequent Contributor
timestamp: March 03, 2018, 01:25:37 pm
content: Very good stuff ! Thank you.We have various Rigol supplies at work, and at home I have a number of Agilent bench supplies, this one Circuit Specialists and a few big honking Lambda and Kepco units.Of all the small bench supplies, I don't think any of them can sink, although I've only looked for this capability on occasion.To get around this, I simulate Li-ion batteries for products using any suitable supply and a 2 ohm, 50 W resistor across it. At a given simulated cell voltage, like 4.05V, I just dial up enough current to develop this across the 2 ohm, then set the current limit higher by what the product needs and some added margin. Then I can dial the simulated Li-ion up and down all day while observing what the charge system is doing. Could have thrown a bit of resistance in the output, but most of the products have been fairly low power and it wouldn't matter much.

author: EE-digger
author_rank: Frequent Contributor
timestamp: March 03, 2018, 02:56:05 pm
content: I did some more experimenting this morning and this method of creating a load is somewhat fidgety.When the source you are applying this electronic load to is somewhat higher in voltage than my single NiMh cell (1.4V), say 3 or 4 volts, there is more residual current drawn in addition to the CC limit you've set.Updated - a few basic constraints are in order. The supply used as the electronic load will probably need to keep its output above zero volts by some small amount so its CC limit can function. The voltage of this supply will need to be higher if the source you are loading is higher. The resistor will need to be large if you want to regulate small currents and vice versa.Summary - you probably won't be dialing in zero to 1000mA but when setup for your situation, you can get a good load current adjustment range. Have fun

author: macboy
author_rank: Super Contributor
timestamp: March 05, 2018, 02:30:31 pm
content: Think about it this way:What actually you did was connect the power supply in series with the cell, and put a load resistor across the new series-connected power supply. With a current limit set, the power supply will reduce its own output voltage enough to push only the required current through the resistor (and therefore the cell).

author: macboy
author_rank: Super Contributor
timestamp: March 05, 2018, 02:43:17 pm
content: I've used this technique to safely discharge battery packs: a load resistor is chosen so that at full battery voltage, the current is just a little lower than desired. Then you add the series connected power supply, which provides enough extra voltage to provide the desired current (obviously that current must be within the supply's rating). Importantly: a second power supply, usually the second channel of the same power supply, is connected in parallel directly across the battery. You set this one to the battery cut-off voltage (say 10.5 V for a "12" V Pb battery, or 0.9 V per cell for NiMH) and to a higher current than the load current. When the battery voltage falls to this discharge voltage, the second supply takes over the supply of current. You are still dumping the same amount of heat in the load resistor, but no power is coming from the battery any longer. This ismuchless efficient than a real electronic load but provides a true constant current discharge and safe cut-off.