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10-15 AMPs w/ potentiometer. Answered

So, as a chemist I thought it would be fun to save some $ by making my own hot plate & magnetic stirrer.

I am using Thermoelectric cooling device (TEC) to heat my stirrer, and a small DC motor to stir.

Here's the problem - if you've ever taken highschool chemistry you will know that a hot plate with a stirrer has a potentiometer for Temp & Stirring speed - I'd like to do the same!

Problem:

the TEC takes anywhere from 5-20 amps in operation @ 12 volts DC. Adding a potetiometer to this in parallel is out of the question - I can't fine a potentiometer that can handle greater than 2 watts. ( i may just be looking in the wrong place)...

So.. all in all I'm looking for nearly 140-170 watts of power for my heating plate. (The motor will be using much less, almost negligble to the heating plate)

So, if I can't put a potentiometer in series for power regulation, how can I get my temperature to vary? If this is going to require a circuit board, I may need some help with a design :)

10 Replies

user
lemonie (author)2010-03-14

You want to be switching the heat on and off with a thermostatic control, or doing something like by controlling current by time rather than continuous flow.

L

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user
GreenD (author)lemonie2010-03-14

Oh, so I want a ... mmm timing device?

So... nooby knowledge is telling me: get sometype of timing device, have it connected to a power mosfet? 

So On...Off cycles?

Ugh! this is more work than I had hoped for :\

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user
Jayefuu (author)GreenD2010-03-15

Look up PWM - Pulse Width Modultation. It varies the on/off ratio of current to your device and is commonly used in temperature control of TECs.

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user
Jayefuu (author)Jayefuu2010-03-15

(I'll draw you a diagram later if you like, I'm off to play with my TEC for my project now,)

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user
GreenD (author)Jayefuu2010-04-08

You should get back to me on this!! I'm still interested.

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user
GreenD (author)Jayefuu2010-03-15

That would be awesome! thanks. I'll do some research

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user
lemonie (author)GreenD2010-03-14

How does your cooker / stove work? Mine clicks on and off except at full power, I believe it's a bimetallic device. Perhaps you could find one of those - they're current based.
See here - just your sort of area I think:
www.coleparmer.com/techinfo/techinfo.asp

L

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GreenD (author)lemonie2010-03-14

I haven't made it yet, its from scratch...

If you look online a magnetic stirrer with a hotplate with both speed of motor and temp° variability is well over 300$.

If you buy the parts and design it yourself, you can save atleast 100$.

I just am having trouble controlling the current that would theoretically be going through the device. Here is the instructable that gave me the idea.

https://www.instructables.com/id/Magnetic-Stirrer-w-Hotplate-for-30/

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user
lemonie (author)GreenD2010-03-15

Too weedy, you'd struggle to make a cup of tea with that. What are you planning to use for heat?

L

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gmoon (author)2010-03-14

Try searching for rheostat, rather than potentiometer.

Even so, your rheostat should be rated at ~240 watts (20 * 12), which won't be cheap.

An alternative way: use a power MOSFET.

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