Discrete based constant "power" dummy load (NOT constant current) - Page 1
Forum section: electronics / projects

author: BravoV
author_rank: Super Contributor
timestamp: November 03, 2011, 03:40:55 am
content: Again, its not constant current dummy load, but constant power/wattage dummy load and adjustable too.Has anyone ever built, designed or encountered such circuit design ?Preferred discrete components based, rather than using micro controller, and the sole reason is only for improving skill and understanding in analog realm.I've been thinking for quite sometime to build it if I can find any schematic, and it will be a nice addition at the bench measurement tools families. Not overly ambitious, maybe under 100 watts, DC based and at few amps with max voltage something around 12 or 24 volt would be nice.Not sure, probably my "google-fu" is not good enough, still can not find it at the net.Really appreciate if you ever designed one or even better made one and of course if you don't mind share it here please.

author: IanB
author_rank: Super Contributor
timestamp: November 03, 2011, 04:43:29 am
content: An LED buck or boost driver will sort of do this. Such drivers are designed to supply a constant current into a load with a constant voltage drop. That makes their power consumption on the input side almost constant, allowing for varying converter efficiencies.

author: Rufus
author_rank: Super Contributor
timestamp: November 03, 2011, 05:08:59 am
content: You need to multiply amps and volts in your load to sense the power you are trying to control.Analog multipliers are a bit finicky and old-school, however, there are some modern battery power monitoring chips which include one.Look up the Linear Tech LT2940. The data sheet has a couple of constant power load application circuits.Maxim do a similar chip, there may be others.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 03, 2011, 02:53:37 pm
content: IMO, it's a pretty good idea. I know adjustable electronic loads (Google that term in quotes) have been around for a long while, but I don't know if they offer a constant power feature. I wouldn't be surprised if they did. (OK, opened another tab and foundhttp://www.hotektech.com/EPSFK.htm).Best,CH

author: fmaimon
author_rank: Unknown
timestamp: November 03, 2011, 03:16:54 pm
content: If I'm not mistaken, Elektor did an article that is exactly what are you looking for (search for high power adjustable load). For constant power, instead of using a constant voltage for the reference, it used a voltage derived from the load input to control the current.

author: BravoV
author_rank: Super Contributor
timestamp: November 03, 2011, 04:26:13 pm
content: Thanks guys for the suggestions and feedbacks, I will look further into those.@Conrad,Thanks for pointing that out, yeah, its an industrial grade constant current/resistance and power capable and I'm aware of that kind of tools, its just I'm really into learning and believe if we can build it diy style even with lower accuracy and capabilities, and probably less features, I will be happy enough if I have this unique tool at my bench.Actually I've been looking at this bunch of dedicated multipliers/dividers chips from Analog Devices.Here ->http://www.analog.com/en/special-linear-functions/analog-multipliersdividers/products/index.htmlThese might be suitable as the multiplier for the analog readings in calculating the power for dummy load, and some are actually have quite impressive performance and not that expensive too.Please share your thought about these realtime analog math ICs.PS : @fmaimon , Welcome to eevblog forum, noticed that it is your 1st post here.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 03, 2011, 06:05:44 pm
content: Quote from: BravoV on November 03, 2011, 04:26:13 pmfor your purpose the AD633 is just about right, and reasonably priced for a single chip solution at 4.23/1kTo do the same thing discretely, you would need a quad op-amp, a multitude of external gain setting resistors, a couple of transistors,and wire it all up into a sum of 2 logs configuration, i.e. the transfer function would be W = ln(X) + ln(Y) =>> W=X*Y, where X and Y would be your voltage and current inputs.The parts, labor, and time investment in this to get a similar accuracy as the AD633 would exceed $4.23/1k, not to mention more parts means more points of failure.The picture below shows a single logarithmic amplifier. Vout = -ln(Vin). You'd have to build 2 of these, 1 for each input, sum them, and invert the final sum to get ln(x)+ln(y). Thus the parts list I mentioned above.--Cheers

author: Rufus
author_rank: Super Contributor
timestamp: November 03, 2011, 07:01:02 pm
content: Quote from: codeboy2k on November 03, 2011, 06:05:44 pmExcept the LT2940 I already mentioned is half that price, is specified for operation on 6v to 80v single rail supply instead of fixed +/-15v, and has one multiplier input with a 200mV full scale suitable for direct connection to a high side current shunt. It even has two constant power load example circuits in the data sheet.I don't know why I bother posting sometimes.

author: johnmx
author_rank: Frequent Contributor
timestamp: November 03, 2011, 09:55:09 pm
content: I made it using discrete components, inc. opamps, in my dummy load project. It works in constant power mode if you set the output current to the maximum. The power can be adjusted through R18 (see schematic):https://www.eevblog.com/forum/index.php?topic=2487.msg33809#msg33809

author: codeboy2k
author_rank: Super Contributor
timestamp: November 03, 2011, 10:09:08 pm
content: Quote from: Rufus on November 03, 2011, 07:01:02 pmYes, you're absolutely right. I didn't look up the LT2940 you mentioned, I just went to the OP's links. I assumed the OP would look that up and make a decision.Now I've been to Linear's website, and studied the LT2940 datasheet. It looks good, I like it too.Please don't be discouraged from posting, without your post I wouldn't have known about the LT2940 or your thoughts on it,and now I will consider it for one of my future projects too.Cheers!

author: EEVblog
author_rank: Unknown
timestamp: November 03, 2011, 11:10:49 pm
content: Quote from: Rufus on November 03, 2011, 07:01:02 pmPlease keep posting, your post was valuable.There is usually no "right" or "best" answer for something like this, so the more alternative's people post, the better for the OP and everyone involved.I for one went and looked up the device the LT posted, so I learned something, and I'm sure others did too.As for the OP's question, I've only down a constant power load with a PIC micro, so can't help there.Dave.

author: amspire
author_rank: Super Contributor
timestamp: November 03, 2011, 11:45:46 pm
content: Another way you can achieve a true multiplication is to convert the sensed current into a variable pulse width. so 1% current = 1% duty cycle. 99% current = 99% duty cycle.You pass the sensed voltage through an analog switch controlled by the current PWM signal and integrate the result. The output of the integrator (ie resistor+capacitor) is volts x current.You just have to make sure that the frequency of the PWM is much higher then the frequency response of the load regulator circuit.Richard

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 04, 2011, 12:18:04 am
content: If you want to go old-school, have it regulate the temperature of a load resistor.And if you want to go really really old-school, they used to make power meters which were like moving coil meters, but instead of having a permanent magnet, they had an electromagnet on the stator, so the needle deflection would show the product of the voltage in one coil and current through the other.Put something like this in a control loop and away you go...!

author: BravoV
author_rank: Super Contributor
timestamp: November 04, 2011, 06:23:29 am
content: Quote from: Rufus on November 03, 2011, 07:01:02 pmC'mon Rufus, its just a misunderstanding, cheer up.Anyway, took a look at it's datasheet, its a very nice ic, and you're right, its all there and single rail supply will be handy too, thanks for pointing that out, I will take a deeper look at it.Quote from: johnmx on November 03, 2011, 09:55:09 pmThanks, that is one neat circuit you got there, definitely bookmarked.Quote from: mikeselectricstuff on November 04, 2011, 12:18:04 amThat's too old school for me Mike.

author: sub
author_rank: Regular Contributor
timestamp: November 04, 2011, 01:21:14 pm
content: If you're happy to use a PWM solution that needs calibration and varies with temperature, you could use a FET-based solution that doesn't require a multiplier.The idea would be that you bias your FET's gate about its threshold voltage, and use its quadratic V/I curve to find the V^2/R power dissipated by your load resistor. Use this to generate an error voltage and provide negative feedback to the PWM controller.Or you could just use a thermistor, your call.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 05, 2011, 02:58:36 am
content: Over what voltage and current ranges are you working at? How good does the transient response have to be?A relatively easy way with only analog parts is to use a switching regulator supplying a load resistor.