DC Current or Voltage Doubling Circuit?

[NealG]

Hello,
I am trying to build a temperature control circuit. The output should be DC (current or voltage) and is fed to a heater blanket. The system should be able to supply ~15W with a 24V supply at max output. Originally, I was planning to use a 20 or 24 bit DAC then amplify; essentially, a DAC controlled DC current source.

To help with the linearity of the system identification that I will have to do, I would like the output of my controller to be a proxy for power. Either V^2 or I^2 is fine. Here is the part that I would like some advice about: what is the best way to realize this?
- I could add a layer of abstraction in my approach to restrict the output states. Instead of 0-2^24-1 states, I could restrict it to 0:2^2^12-1 states where each state is actually S^2 but this would reduce my resolution to 12 bits (using a 24 bit DAC)
- I could use an IC like AD633 analog multiplier but somehow this seems like overkill
- I have searched for DACs with nonlinear outputs but didn't find much
- Could an DC to RMS conversion circuit be used?

Is there a simpler realization that I'm not thinking of?
Thanks for your help!
N

 

[KlausST]

Hi,

First things first: give an almost realistic specification.

If you do this - with a minimal effort about error estimation - you will see that a 24bit DAC is absolute nonsense.

***
Heater: "15W" says nothing. Give the resistance or rated voltage and current.

Either V^2 or I^2 is fine. Here is the part that I would like some advice about: what is the best way to realize this?

If the heater has (large) NTC characteristic, then use "current regulation" otherwise use voltage regulation. I recommend PWM.

I would like the output of my controller to be a proxy for power. Either V^2 or I^2 is fine.

If you want to linearize for power then your output value need to be square-rooted first, not squared.

but somehow this seems like overkill

Definitely yes. See my first sentence.

- Could an DC to RMS conversion circuit be used?

DC_value = RMS_value. Thus a "DC to RMS" conversion is useless.
(the only difference between DC value and RMS value is that RMS value is always positive. Thus the correct formula is: RMS_value = |DC_value| )

Is there a simpler realization that I'm not thinking of?

As said: PWM (as one example). In detail it depends on your specifications.

Klaus

 

[NealG]

Hello Klaus,
Thanks for your reply.
- 15W with 24V says everything (including resistance) R = 38.4ohm.
- PWM is not a suitable option as my application is very sensitive to noise.
- Temperature is linear with power. If my circuit outputs a voltage then P is proportional to V^2, if it outputs a current then I is proportional to I^2. Therefore, T is proportional to I^2 or V^2 (not sqrt of either).
Hopefully this clarifies things.
N

 

[KlausST]

Hi,

- 15W with 24V says everything (including resistance) R = 38.4ohm.

but you said:

with a 24V supply

I have designed temperature controller supplied with 400V AC able to drive 8kW heating power into loads with resistance range 128mOhms ... 20 Ohms (400V max, 250A max)

--> What I want to say: I was confused by the "24V supply" --> in my eyes you don´t want 24V as supply voltage, but as ouptut voltage (= heater voltage). I understand now.

Temperature is linear with power. If my circuit outputs a voltage then P is proportional to V^2, if it outputs a current then I is proportional to I^2. Therefore, T is proportional to I^2 or V^2 (not sqrt of either).

This all is correct. Thus you need to compensate for the "square" that happens "outside" your controller. You need the square-root inside your controller.

this would reduce my resolution to 12 bits

True, but 12 bits is well enough for a temperautre controller. It´s a 1:4096 range (in steady state). If I understood correctly, then you have a feedbacked system.

Example: In my home there we have a 1 bit controller (ON/OFF bimetall thermostat with internal heater) and it maintains room temperautre fairly constant, even when outside temperature varies widely.
Don´t forget: with a regulation loop you have an additional parameter "time".

Some questions: Do you really think that your power supply output voltage is stable within 0.025%? And do you think that your heater resistance is stable within 0.025% over temperature and time?
Mind: copper changes it´s resistance about 0.4% per °C. This is 16 times the error you need - and this with a variation of a single °C!

--> Start to really develop your system ... with specifications and error estimation.

Klaus

added:
Onother - maybe interesting - calculation: 15W / 4096 is about 3.7mW. This is less than the sun transmits on a square with about 2mm of length.

 

[FvM]

The interesting point is required power control resolution. You don't need fine resolution for system identification, only reproducible power steps. Power control resolution is relevant for dynamic temperature error, the requirements can be calculated if you have a rough estimation of "plant" transfer function. 24 bit current resolution sounds like overkill, but if you don't need the DC accuracy (I believe you don't) it's just a cheap audio DAC. Depending on the intended controller sample rate, a filtered pwm output may serve the purpose as well.