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Need to regulate wattage based on resistance of an exchangeable part. Answered

Ok, have a request. I need to make a compact device that will be DC powered (batteries) and produces a set wattage between 4.2watts and 12.7 ish. Can be a little lower a little higher on the scale.

I'll be putting different attachments ont he end of this device that will have different resistance between 3-4ohm down to around 1.0ohm. I need the device to read the resistance and adjust the voltage coming from the DC battery to match the wattage I set. Basically if I set it to 10watts I want it to always be 10watts no matter what resistance attachement is on it. I hope that makes sense. I'm sure there's a pre-existing board made by some one out there that does this or that could be salvaged from another device but I can't find one.

I know how to do variable voltage with a regulator but I need an auto sensing system where you set the output wattage. That goes a little beyond my expertise. Any help would be greatly appreciated, thanks in advance.

10 Replies

user
evolvapor (author)2012-03-22

Hmm, that circuit sounds vaguely familiar ;p

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steveastrouk (author)2011-08-19

You need a wattmeter. ie Measure voltage, measure current and multiply together. Use the current measurement to drive the voltage regulator.

Steve

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wallenc (author)steveastrouk2011-08-24

I know that part, what I'm not entirely sure of is how to do that in an internally contained device. do they make an existing module that will do that or will I hav eto program a microprocessor/controller?

I want the wattage to be user deffinable. working with a voltage range from 3.2-7.2

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steveastrouk (author)wallenc2011-08-24

The simplest way is to do it with a microcontroller, yes, you can get fancy chips to do it, but they are for big power meters, and not ones that work on DC.

Are you happy to have something dissipating the power not going into your load ?

Steve

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wallenc (author)steveastrouk2011-08-24

I'd rather it be as effecient as possible.

i've done a different set up that just had a step regulator for the voltage and I just did the math in my head based on the resistance of my accessory but I have a number of accessories at a number of resistances and I want a consistant wattage on them so constantly manually adjusting the voltage becomes annoying. I'd rather set the wattage and know i'll get the same watts no matter what accessory I have.

The power loss is something I'd like to save as much as possible to extend battery life but some loss is expected, I'd just like to mitigate it where I can. Size matters too, this will end up being a hand held device.

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steveastrouk (author)wallenc2011-08-24

So you need it all to be switch mode ?

Steve

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wallenc (author)steveastrouk2011-08-24

that would be preferable but I'm willing to sacrifice a bit of battery life for compactness. most of the switch regulators I've seen were obviously more bulky.

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steveastrouk (author)wallenc2011-08-24

Hmm, no, if you used a National Simple Switcher, it could be very small. The problem would be external control.

Steve

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wallenc (author)steveastrouk2011-08-24

external control of the voltage regulator? or to set the wattage?

I'm obviously a novice, I'v eused out of box parts that you basically have to solder together to make work. I know some of the internals but have never fiddled with MCUs or anything more complex than regulators, etc. The regulators I've used that were adjustable used a little pot to change em. I was hoping for something I could either use a digital display or even a pot where I could measure the various settings and mark them on the control knob.

The primary complication that I can't figure out how to do gracefully is read the resistance of the accessory, dothe math required, then automatically tell the regulator to adjust the voltage based on whatever setting I have selected. I appreciate your time so far by the way!

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steveastrouk (author)wallenc2011-08-24

Here's a circuit that uses a switching supply, and analogue techniques to do what you want. Its not easy.

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