On 18/12/2015 23:03, brass monkey wrote:
"Fredxxx" wrote in message
...
On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to have
learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current
does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

I dunno what the prob is, my bipolar PWM drive (circa 1980) worked fine for
years and may still be working
I never researched losses, inefficiencies etc etc, someone wanted something
cooling fairly accurately, thats what they got

No one here is doubting that a cheap and quick setup of PWM drive does
not work. It is a simple form of control ideal where efficiency is not
important.

What this thread has demonstrated is that someone who teaches students
doesn't seem to understand that PWM drive might be less efficient than
with a constant current.