| Products > Test Equipment |
| DIY Current load, keeping it as simple as possible, thoughts? |
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| SpottedDick:
OK, first off, I understand the issues with MOSFETs in parallel, so this discussion isn't about that. I want to try and build a 24V 20A capable current load that anyone can replicate using as few parts as possible. I'm planning on using it with the test controller software. My main goal was keeping component count and cost to a minimum, at the moment the only things I think are questionable are: 1. Using a 2 output DAC instead of a single output DAC, but the single output 4821 is suffering shortages at the minute. 2. The LCD display could be deemed unnecessary, as you'll be able to see everything on test controller, but I decided some sort of local feedback was needed, so kept it in the base design. Things like buttons, keypads, buzzers etc... can be added later. The point here is to get a stable base design and then the user can add "mods" like (for an example) a 3X4 keypad later. As a base design, does anyone see any major issues here? EDIT: Forgot to add, the Nano is getting its' supply via USB. (Note, I'm aware that there is a GND2 labeled on this image, it should just read GND) J2 is the Voltage sense for the load, J3 is the current input. 4 wire system for accurate voltage detection. The LM324 is used as a control loop for the base input to the MOSFETs, and also as an amplifier for the current shunt which outputs 75mV at 20A. |
| ledtester:
There have been a lot of electronic load projects discussed here. At the moment I'm only able to find this one: https://www.eevblog.com/forum/projects/smart-electronic-load/msg3602103/#msg3602103 Note the following: - each MOSFET is driven by its own op-amp - each MOSFET has its own shunt resistor - a compensation network for each op-amp (e.g. R19, C23) to prevent oscillations |
| SpottedDick:
--- Quote from: ledtester on June 25, 2022, 02:28:35 pm ---- a compensation network for each op-amp (e.g. R19, C23) to prevent oscillations --- End quote --- Thanks. I have to do a bit of reading on this and it looks like I will need to add this across U3A at least. |
| nctnico:
If you want to keep things simple: consider using switched power resistors. Resistors can dissipate much more power versus cost and have little frequency depedent behaviour. Also: don't use any analog parts from Microchip. If you look at the spec you'll see it basically is an 8 bit DAC. And the LM324 is a horrible opamp to use for almost any circuit. |
| mawyatt:
With a 3.75 mohm shunt, the offset voltage of the LM324 could easily create over an amp error!! With that low a shunt, you'll need a very low offset Op-Amp instead of the LM324. Edit: Also as mentioned, with the massive delay caused by the large number and size NMOS devices...and with the Miller effect, your circuit is almost guaranteed to be a oscillator. A well crafted frequency compensation network is a must. Best, |
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