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Topic: PWM for Peltiers.... (Read 2983 times)previous topic - next topic

Gibson486

Aug 24, 2012, 04:59 pmLast Edit: Aug 24, 2012, 05:19 pm by Gibson486 Reason: 1
I am starting to work on Peltiers....

I am kind of confused on how they work. So, we heat (or cool, depending on your perspective) by applying DC current. To get the opposite effect (to cool when heated), we apply negative DC current.

Here is where I am confused... Most peltier controllers use PWM, so that is not DC current. Correct me if I am wrong, but PWM chips (like the one found on MegaMoto) output PWM.

Are we relying on the avg current to dictate the temp of the cooler? In other words, the faster the PWM goes, the more avg current is going into the cooler and vice versa. So, effectively, we are turning the peltier off and on at some frequency to get the temp we want to achieve. Is that correct?

retrolefty

#1
Aug 24, 2012, 06:08 pmLast Edit: Aug 24, 2012, 06:10 pm by retrolefty Reason: 1

I am starting to work on Peltiers....

I am kind of confused on how they work. So, we heat (or cool, depending on your perspective) by applying DC current. To get the opposite effect (to cool when heated), we apply negative DC current.

Here is where I am confused... Most peltier controllers use PWM, so that is not DC current.
Well PWM is still DC current it just has a variable duty cycle switching from on to off, where the average on time determines the average DC current to the load, a peltier in this case.

Correct me if I am wrong, but PWM chips (like the one found on MegaMoto) output PWM.

Again PWM is just a form of controlling a variable DC. The only thing to keep in mind is that if you need reverse current PWM control, like for reversible speed control on a motor, or switchable heat and cooling for a peltier, the pwm controller must have a H-bridge output stage, that allows the switching of the polarity of the PWM current. If your load does not require reversing capablities then one can simple drive a switching transistor with the pwm control signal.

Are we relying on the avg current to dictate the temp of the cooler? In other words, the faster the PWM goes, the more avg current is going into the cooler and vice versa. So, effectively, we are turning the peltier off and on at some frequency to get the temp we want to achieve. Is that correct?

No, the switching speed (frequency) of the PWM signal does not effect the % of load current supplied to the load, but rather the duty cycle of the pwm signal determines the average current flow to the load. The ratio to on time to off time determines the average current, so 50% on and 50% off will supply half current capacity to the load, 25% on and 75% off will supply quarter current capacity, this is true regardless of the actual frequency of the pwm signal.
Lefty

Far-seeker

#2
Aug 24, 2012, 07:47 pm

I am starting to work on Peltiers....

I am kind of confused on how they work. So, we heat (or cool, depending on your perspective) by applying DC current. To get the opposite effect (to cool when heated), we apply negative DC current.

Since retrolefty did a good job covering the PWM aspect of your post, I'll dive into the theory a bit.

Peltier junctions are devices based around the thermoelectric effect, specifically the Peltier effect, which describes how heat can effect the flow of current between two different materials.  Like the Piezoelectric effect, the Peltier form of the Thermoelectric effect is a reversible process.  Therefore not only can the temperature differential of a junction effect the current flow, the current flow can cause a difference in temperature between the two sides of the junction.   With certain pairings of materials (e.g. dissimilarly doped semiconductors) this effect is strong enough to either cause a signficant temperature differential (i.e. heat one side of the junction, while cooling the other) if an electric current passes through it, or generate a small current at a specific voltage if a large enough temperature differential exisits accross the junction.

mauried

#3
Aug 25, 2012, 12:59 am
http://www.tellurex.com/technology/peltier-faq.php

You can control peltier devices with PWM , but there are some stringent conditions that apply
otherwise you will destroy the peltier device.

Gibson486

#4
Aug 27, 2012, 05:37 am
Thanks!

My biggest misunderstanding was thinking that the analog out on the controller was simply a standard DC output that was adjustable. Now I see that it is a pulsed output with the averaged DC voltage as the measured output.

So, if I wanted to control the temp, I could slow down the temp increase by lowering the duty cycle and speed it up by increasing the duty cycle. If I wanted to go from hot to cold (or vice versa), I would need to implement a H bridge. The code stays the same (for the most part), but i would need to switch polarity by closing the correct contacts on the H bridge. At least that is the idea, correct?

mauried

#5
Aug 27, 2012, 07:06 am
Yes, that will work.
Just make sure you have a decent heat sink and fan for the hot side, so you dont exceed the maximum temperature
the device can withstand.

MarkT

#6
Aug 27, 2012, 03:01 pm

Since retrolefty did a good job covering the PWM aspect of your post, I'll dive into the theory a bit.

Peltier junctions are devices based around the thermoelectric effect, specifically the Peltier effect, which describes how heat can effect the flow of current between two different materials.  Like the Piezoelectric effect, the Peltier form of the Thermoelectric effect is a reversible process.  Therefore not only can the temperature differential of a junction effect the current flow, the current flow can cause a difference in temperature between the two sides of the junction.   With certain pairings of materials (e.g. dissimilarly doped semiconductors) this effect is strong enough to either cause a signficant temperature differential (i.e. heat one side of the junction, while cooling the other) if an electric current passes through it, or generate a small current at a specific voltage if a large enough temperature differential exisits accross the junction.

Another way to think about it is this:  in a fridge the coolant is pumped round and forced to evaporate (change state) in one place (absorbing heat), and condenses somewhere else (dumping heat).  Think of the electrons as the cooling fluid, the change of state is when they are pushed from one material to another.  Its basically a heat pump using electrons (or holes depending on the semiconductor).  The change of state is more a change in entropy across a pn-junction I think.

Unlike mechanical heat-pumps the devices are low-efficiency (good mechanical heat-pumps / fridges push many times more heat energy around than are needed to power the pump - peltier devices use about as much power as the heat they move I think.  One reason the devices aren't very efficient is that the semiconductors that exhibit the effect conduct heat rather well (short-circuiting the heat flow).

As soon as you power down a peltier the temperatures will thus equalize rapidly.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Gibson486

#7
Aug 27, 2012, 03:44 pm
Thanks...one more question.

I am using the megamoto board. I will use it in the hbridge configuration.

However, I am confused. Looking over the schematic , I am not sure how you tell it to go in the reverse direction. There is an enable pin and a PWM pin, however, there is no select to control which 2 of 4 output fets to use... The data sheet does not really tell you either, but it shows that there are 4 fets, but i am still not sure how you select which fets to use in each chip.

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