Sinking constant current with multisim - Page 1
Forum section: electronics / beginners

author: crispytato
author_rank: Regular Contributor
timestamp: May 21, 2014, 05:06:24 am
content: Hello all.I'm working on an assignment for uni, it involves building power supplies and characterizing them using NI's Multisim. I have a few designs, but one problem, I have no idea how to sink a constant current in order to test the supplies at their specification! I've talked to a couple of my classmates about the assignment and they've used resistors for the job, but I'd like to go above and beyond the accuracy attainable with this method.I've done this many times in my own projects using LTspice, but we're only allowed to use multisim for this project so I'm stuck.Could anyone with experience in this program point me in he right direction to find a constant current sink device among the (many, many) Multisim libraries?Thanks for taking the time to read this post!-chris

author: Kremmen
author_rank: Super Contributor
timestamp: May 21, 2014, 05:49:31 am
content: There is a current source component in the standard library, in "Sources" IIRC.Also, there are controlled current sources that you can use to create all kinds of current profiles just by adding a signal generator to the control input. And controlled switches if you need to switch the "load" on and off.

author: crispytato
author_rank: Regular Contributor
timestamp: May 21, 2014, 08:14:24 am
content: I found it, that's great!I'm really enjoying MultiSim now that I've gotten comfortable with it's UI, it has a lot of features that are much more accessible than SPICE tools that I've used, and has over all it has been a great tool for learning with. I just wish they'd fix some of the graphical glitches that it seems to exhibit on intel graphics hardware, it can be a real pain on my Surface pro.Thanks very much for helping me out, Kremmen

author: zapta
author_rank: Super Contributor
timestamp: May 21, 2014, 08:50:52 am
content: Quote from: crispytato on May 21, 2014, 08:14:24 amGood.Otherwise you could use a very large negative voltage source (e.g. -1000V) with a serial resistor (e.g. 1K for 1A) to the output of your PSU. Changing the output voltage of your PSU will not change much the voltage and current of the resistor. Never tried it but may work in the simulator (resistor power does not matter).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2014, 11:43:53 pm
content: ^ Which BTW might give you some idea of potential startup conditions.If you're talking linear supplies, probably not a problem; you can tell the simulator to start from steady-state (DC operating point) and it'll already be working from t=0, assuming it works at all.For switching supplies, you have to set up the conditions yourself (a switching supply is an intrinsically unsteady condition), and you may have to worry about startup conditions, like, the CC load dragging the output infinitely below ground before the supply starts up -- or something like that.This is the downside to a CC source/load: when it says CC, it *means* CC, whether the voltage is where you thought it should be, or not.A more physically useful CC load would be one in parallel with a pre-biased diode, so the CC starts drawing through the diode at low voltages. The diode could also have a breakdown voltage specified (BV in the .MODEL dialog) so the voltage can't go infinite either.Regarding dynamics, CC loads aren't usually a good choice, because many loads are not constant current sinks! They do occur (e.g., most linear audio amplifiers), but resistive -- and negative resistive! -- loads are also very common. A resistive load will usually act to dampen the output's step response, while a negative resistance (typical of a switching supply, which tries to draw higher current at lower voltage!) will tend to accentuate it; in the worst case, it can begin oscillating, or even get latched in startup (current draw at a low voltage is greater than the supply's capability and it gets stuck -- a bistable hysteresis condition).Perhaps you can add a PULSE source (a small amplitude VPULSE in series with a load resistor, or a small amplitude IPULSE in parallel with the resistor, or for the CC condition, just the IPULSE alone at whatever two current levels) to observe the transient step response, do some hand-waving, and get extra credit for it?Tim