Dewheater and Peltiers

[MarkS]

A diode would effectively short the pelt out and you'll get a much less efficient cooler.

Surely the diode should go in series so it allows current to go to the TEC but prevents it coming back out.  So I would need a 3amp(ish) diode.

[Mac]

if the transistors need protection from the back EMF

Correct me if im wrong.
but the diode used to protect things from back emf was mainly for relay coils
mainly because the emf that is generated by the collapsing magnetic field is huge,
which destroys the trasistors, ect.

The peltier is a resistive network, not an electromagnetic coil.
Therefore no back EMF produced.

Yes there is a voltage generated because of the Hot / Cold junction. but this should be the same as the voltage needed to run it. 12v.

This low voltage should not be enough to damage the transistor.

As for the diode protecting against back EMF,
the diode is placed across the coil reversed biased, so that the collapsing field has a current path to follow.


Mac.

[Ian]

Mac, yes. The EMF produced by a TEC in that manner is less than impressive to be honest. I tried to find a datasheet for some numbers, but failed miserably. I'm sure I've got one in my library somewhere...

Mark, the diode is placed in parallel across the device you wish to protect. The plan being it provides a low impedence path for the reverse current, so carries much more current than the relatively high impedance part you're trying to protect.

Putting the diode in series with the TEC is less desirable as it would need to carry the full on current to the TEC and dissipate a proportion of the power. A Si junction diode (which is the only type you'd use in this case) drops 0.6V across itself, meaning you'd not get the full 12v to the TEC, and the diode itself would dissipate 7.2W for no good reason.

[MarkS]

The EMF produced by a TEC in that manner is less than impressive to be honest.

Ian,

I've just measured it by getting the camera down to near thermal equilibrium (25C across the TEC) disconnected it from the dew controller and connected it to a meter:  either 1.45V back EMF or 450mA current - but clearly not both at once! 

The final stage of the dew controller circuit should easily cope with a back EMF of 1.5V.
We clearly don't want to short it out with a diode.

Mark

[Ian]

the datasheet would be able to tell you the maximum review collector-emitter voltage, but generally they can tolerate up to about 5V. The TEC is well clear of this.

[JohnP]

Mark - agree with Mac on this one. I doubt Diode would be needed & certainly shouldn't be placed in series - you normally do this to prevent against accidently connecting battery around the wrong way or when you need to drop volts by .6V for some reason or 1/2 wave rectification in AC ckts..... Like Ian says no point in dissipating .6V X 3A - 1.8W for no reason...

[Ian]

only Ian didn't say 1.8W  I put all the wrong numbers into the right equation. Can't trust my own head

[MarkS]

In the meantime,

2 Peltier-Canons wired in series running off one channel of a dew controller.  0.8 amps total consumption (including fans) keeps their temperatures to 14C below ambient - more than sufficient for the freezing temperatures expected this weekend.



Mark

[Ian]

just add tomato sauce and a healthy number of meatballs

Would you like some cable ties?

[Mac]

JUST
ADD
WATER.