# Peltier modules - Different voltages in parallel?

• posted

I'm a computer scientist and know very little about electronics. After a while of google searching, I haven't been able to find a solution to my problem.

I'm building a simple DC device that uses 1 to n Peltier modules to generate electricity to power a motor. I heat one side of the module and cool the other to get ~.3-.6v and 60-120mA output. I've found that using 1 peltier module is more efficient than multiple ones in parallel (or series). From my research, I think that the dynamic nature of the output due to the uneven heating/cooling of the modules makes it a different voltage in parallel problem (or different current in series). I think that the highest voltage module is leaking power into lower voltage modules and therefore reducing the output to the motor. I understand that I could add a diode in series after each peltier module in order to stop that backflow, but that would effectively be eliminating the usefulness of all but the highest output module. So, what can I do to utilize all of the power being generated from these n modules?

Using the water analogy -- I'd like pour power into the top of a reservior after each module to allow the pressure to build up enough so that there would be enough force to overcome the pressure of the backflow through a pipe at the bottom of the reservoir. Would I use a capacitor for that? How could I use one without a real ground?

Thanks!

-Rob

• posted

Commercial Peltier modules are built up of lots of elements, apparently connected in series.

which suggests that you should be able to connect several modules in series.

If this generates more volts than you want at a lower current than you want, you could always use a DC-to-DC converter to transform the high voltage to a lower, or vice versa.

The Farnell catalogue lists a couple of pages of such devices - admittedly, mostly aimed at converting 5V, 12V, 24V and 48V DC into something in the same ballpark.

All of them use some kind of switch - these days it is usually a power MOSFET or two - to chop the DC input to produce AC, which is then put through a transformer to get a higher or lower AC voltage, which can then be rectified to give the DC output you want.

This can be quite efficient - 90% or better.

There are ways of performing similar tricks with capacitors, but transformers can shift a lot more power per unit volume.

With a bit more ingenuity you can build a circuit which can handle a moderately wide range of input voltages - modern "universal" switch-mode power supplies accept mains voltages from 85V rms to 265V rms.

Your voltages sound more like the output from a solar cell, and you might find that you could use something intended for that application.

--------------- Bill Sloman, Nijmegen

• posted

interesting... sounds like a similar problem with solar cells, perhaps you can look for some info on how that is done...

in order to get the most power out of each module and combine them if each has different V and I, you would need a small switching power supply for each module then coordinate and combine the output of the switchers, not simple.

Mark

• posted

Mount all of them on the same pair of heatsinks (one on each side). That will tend to equalize the voltage/current outputs and allow for either series or parallel operation.

```--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."```
• posted

Mark,

What sort of switching power supply do you mean? I don't have much knowledge in the area...

-Rob

• posted

Bill,

The area I'm working with is a few sq inches, so each module doesn't produce the same V or mA output, due to differences in temperature. Series and parallel circuits work worse than 1 alone. Futhermore, the heat/cold aren't constant, so I can't even guarantee that the individual modules will have a different, yet constant output. Basically, all I know is that I have n modules that will put out a range of voltages and currents, at any given time. I *wish* my problem was that I have too much voltage!

-Rob

• posted

Luhan,

I thought about that, but the area is too big. I have about a 5"x2". I already have thick sheet metal on either side that seems to be evening things out more than direct heating/cooling. I doubt I'd be able to get it much more even. Even if the temps were similar, wouldn't even minor differences cause a loss in useful power?

-Rob

• posted

The temperature difference across each Peltier junction sets up a potential difference across that junction. If you stack up the junctions in series, the potential differences just add up.

If the junctions are in series, you have to have the same current running through all of them. Because each junction has an associated resistance you lose some of that potential difference as a resistive drop in each junction. This also just adds up over all the junctions, but it doesn't have to distributed in the same way as the thermally generated potential differences.

This won't be as efficient as drawing the optimum current from each junction, but if the junctions aren't seeing vastly different thermal environments, such that you'd be better off by-passing some of them, my guess is that a single series string of juctions driving a single pulse-width modulated DC-to-DC converter is going to be the most practical way of handling your job.

Putting modules in parallel would be much more difficult - the high voltage strings would just drive curren through the low voltage strings as well a your load, as you've already said.

--------------- Bill Sloman, Nijmegen

• posted

switching power supplies chop the DC input and can convert the voltage and current by changing the duty cycle of the switching. They are also called DC to DC converters. Also called flyback, boost or buck converters.

I don't know if there is something off the shelf for what you need.

I'm thinking a bunch small boost type supplies whose outputs are similar to current sources so the outputs can simply be put in parallel. You need a power supply designer to help you with this.

Mark

• posted

I read in sci.electronics.design that snipped-for-privacy@ieee.org wrote (in ) about 'Peltier modules - Different voltages in parallel?', on Fri, 1 Apr 2005:

I agree entirely, and I was tempted to post an article advising this, but I wasn't sure that there was some subtlety involved that affected the advice.

```--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except```
• posted

In the usual application, the thermoelectric generators are arranged into identical series strings that are then paralleled so as to achieve an equivalent resistance equal to the load for maximum power transfer. Anything else is either less than optimal in power delivery or a waste of thermoelectric modules. Note that deviation from less than identical parallel string voltages is not so important when they are all driving a node that is at 1/2 their open circuit potential. The OP is quoting ridiculously low output voltages and unconcerned with telling us whether this is open circuit or loaded, so either he has a gross resistance mismatch or a pathetically unworkable temperature differential. Either way- as usual- the thread is a boring waste of time.

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