# Constant Power to a Varying Load

1. Jul 10, 2010

### fobos3

Is there a way to deliver constant power to a varying load. Any ideas will be appreciated.

2. Jul 10, 2010

### Staff: Mentor

Could you elaborate please - the question doesn't seem to make much sense.

3. Jul 10, 2010

### fobos3

Say you have a resistor with resistance R. What I want is to deliver say 1W to the resistor no matter it's resistance.

4. Jul 10, 2010

### skeptic2

Monitor the voltage across the resistor and the current through it. In a microcontroller multiply the voltage and current and compare to desired power setting. Adjust the voltage until calculated power matches the desired power.

5. Jul 10, 2010

### fobos3

I've attached my idea.

U2 is a differential amplifier with gain 1. U1 is there to make sure the voltage at + and - is the same.

#### Attached Files:

• ###### Constant_Power.pdf
File size:
15.4 KB
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126
Last edited: Jul 10, 2010
6. Jul 12, 2010

### rbj

the idea is good. i think R1 should be lower resistance and U2 gain be larger.

what do you plan to use for your analog multiplyer? you might wanna look into something from Analog Devices, say the AD633 or AD734 or something like that. you can get some of them pretty cheap ($8) at Digi-Key. you also want to have some pot in there to set and control the power. 7. Jul 12, 2010 ### fobos3 I was thinking with two log amplifiers then a summing amplifier and exponential amplifier. $$e^{\ln a+\ln b}=ab$$ With something like LM324(quad) which costs 10p(£0.10). I'll post a circuit tomorrow because I'm too tired now. P.S. I just found that it costs$1.69 on Radio Shack. You've got to love the uni labs.

Last edited: Jul 12, 2010
8. Jul 12, 2010

### rbj

you still need a logarithmic element (actually 3 of them). run-of-the-mill diodes don't follow the curve good enough. what do you plan to use for that?

9. Jul 12, 2010

### fobos3

Transistors. Here's my final schematics. I haven't calculated the values of the resistors so all are 10k.

Edit
Still need to figure out the gain for each op amp taking into account the offset voltage. In particular the log amplifiers.

#### Attached Files:

• ###### Constant_Power.pdf
File size:
15.6 KB
Views:
120
Last edited: Jul 12, 2010
10. Jul 13, 2010

### rbj

rots 'o ruk finding transistors true enough to the Ebers-Moll equation to be accurate loggers. this is how Analog Devices makes their money, by somehow designing accurate transistors for their logging functions.

also, make sure you're getting your polarities right. it looks like you are (with the inverting summing amp), but i'm a little concerned about the output of the exp amp.