Constant power load using a constant current load - Page 1
Forum section: electronics / beginners

author: rochak.chadha
author_rank: Newbie
timestamp: October 24, 2012, 10:17:48 pm
content: I watched some of the videos by Dave on constant current load and since I was working on testing the capacity of batteries, where they come handy, I was naturally interested in them. I have an idea for a constant power source which are whooping expensive to otherwise buy. Suppose we automate the current load by putting a digital pot on the input and at the same time sense the current as well as the voltage across the source via a microcontroller, then using a simple feedback loop, wont it be possible to make a constant power load ?I am sure there is a catch here, so if any one is interested in the subject can you please explain it to me ?Thanks,Rochak

author: c4757p
author_rank: Super Contributor
timestamp: October 24, 2012, 10:47:50 pm
content: It should work. The catch is that it's not fast (there's no easy way tomultiplywith an op-amp, so as you clearly realize, you have to use an ADC and micro). It should still be fine for drawing constant power from a battery, as long as you get your feedback loop right. It just isn't good enough for a proper all-purpose constant power load.

author: Jonny
author_rank: Regular Contributor
timestamp: October 25, 2012, 09:04:07 am
content: Interesting idea. I to got inspired to make a constant current load so went and bought an Arduino with an LCD so I could get display of current it is set at. Pumped the PWM into an op-amp driving a FET and lots of RC filters cos I don't get the online calculator so threw numbers at it until it was mostly stable. I was using this to drain batteries like you, and also to charge them at a constant current.But constant power load? So if the source voltage goes up the current is reduced and vice versa? Might play with that idea but I don't know where it would be used cept in power supply testing perhaps? I will assume it would be much the same circuit with slight change in code.Jonny

author: BravoV
author_rank: Super Contributor
timestamp: October 25, 2012, 10:23:03 am
content: If you prefer "analog" stuff, the keyword is Linear Tech LT2940 , thanks to Rufus who introduced it.Made a thread about it while ago here ->Discrete-based-constant-'power'-dummy-load-(not-constant-current)Already got the chip, but still don't have time to start it.Edit : Linky to LT2940 ->http://www.linear.com/product/LT2940

author: Kevin.D
author_rank: Frequent Contributor
timestamp: October 25, 2012, 11:05:31 am
content: I was going to build one ages ago for stuff like testing batteries or supplies that will drive switching regulators since they are a constant power load ( constant current load always over estimates them slightly) . I suppose you could just use a switching reg dumping it's load into a resistor as one if that's all you had to hand.It's hard to do the multiplication required with discreet op amps ,but theres a few ready made ic's available for it specifically available . Somewhere around I got a max4210 power monitor ic I was going to use to build one .But then I decided that using a single micro and adc to do the math ,control loop ,display ,input controls etc is allot simpler.

author: rochak.chadha
author_rank: Newbie
timestamp: October 25, 2012, 02:18:18 pm
content: Thanks for your input guys. Yes, using a switched power supply is a great idea ( I never thought of that). But with a software controlled system, the possibilities are limitless. I am gonna go ahead and build one. Lets see how that goes.

author: Jeff1946
author_rank: Regular Contributor
timestamp: October 25, 2012, 02:38:31 pm
content: One way would be to use a buck or boost converter to supply a constant voltage to a load resistor. This is reasonible as it may be what you would be doing to supply for example, a high power led from a battery. Look up circuit examples for MC34063. They are dirt cheap and used in most automobile USB converters. They will work down to about 3 V.

author: c4757p
author_rank: Super Contributor
timestamp: October 25, 2012, 05:52:41 pm
content: Jonny, for a simple two-component RC low pass filter, the math is simple: frequencies at 1/(2?RC) are attenuated by a factor of 3dB (half power, 0.707 voltage). Choose C first because capacitors are commonly available in fewer values. Anything above is attenuated more, anything below is attenuated less. You just need to figure out your PWM frequency, then decide how much PWM noise you want to allow to pass through the filter. The more noise you cut (by setting the cutoff frequency lower), the slower the filter will respond to changes.