Electronics Gurus - Constant Current Sink

Frozennova · 4 Nov 2015 at 22:07

Hopefully an OCUK electronics guru can help me on this one.

I'm looking for a schematic/bill of parts required to build a constant current sink circuit measuring battery capacity

So far I've found the LM317 and LM338 which can be use as a constant current sink however they will only allow me to sink upto 1.5A for the 317 and 5A for the 338.

For a real world representation of capacity I need to be able to sink 15-20A.

My first though was to use 8x LM338 in parallel with each sinking 2.5A with a 0.5ohm 5w resistor, however I can't find any information as to whether this actually work in practice or there is a better way

Frozennova · 5 Nov 2015 at 13:57

The only other thing I can find is using an N-Channel MOSFET and and OP-AMP but I can't seem to find any information on how the circuit works, and what each component is doing

Frozennova · 8 Nov 2015 at 21:40

Initial voltage of 4.2v with the intention to stop discharging @ 3v

Frozennova · 8 Nov 2015 at 23:15

GSWAudio said:
Then with a very capable heat sink and a few big FETs you could do it linearly.

I've just setup the circuit in that diagram you've posted in a Simulator and mostly understand how it works now.

Am I correct in thinking Vin needs to be a lower voltage than the minimum load voltage, so I want to stop draining the cells at 3v use 2.5v for Vin?

Rsense and Vin determine the load on the cell. So with the vin @ 2.5v and Rsense @ 0.25ohm I will see a 10A load.

What affect does the value of R1 have on the circuit, from simulating it changing its value from 1ohm upto 1k ohm and removing it all together make no difference at all.

Where and how would I add a FET or multple FETs to sink the current

Frozennova · 15 Nov 2015 at 15:36

Thought I'd revisit this today using the Op-Amp + NPN Transistor circuit posted above.

I've simulated it using 4 of those combinations in Parallel with 4x0.47ohm Rsense Resistors which would allow me a variable load upto my 20A target.

First of all would this be a viable circuit to actual produce?

Falstad Simulator

Would a 741 Op-Amp and TIP141 NPN Transistor be suitable for this application. Each Transistor would be seeing 5A max with the TIP141 being rated for 10A, and finally I would swap the 4xparallel 0.47ohm resistors for 8xparrallel 1ohm resistors as these would rated and 10w and see a maximum of 6w load