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Topic: What is a part called, that rectifies powerful PWM (10 A)? (Read 4510 times) previous topic - next topic

Unknownsymbols

Dec 23, 2015, 10:37 pm Last Edit: Dec 24, 2015, 12:56 am by Unknownsymbols
Excuse me, people who knowledge the electronics and hardware, can you say a model of such device:
A thing which would allow to pass current from an electricity input (actually it's a computer 12 V unit, it allows anything 0-40A), and would be PWM-(Arduino-)controllable and let pass through itself 10 A. The current needs to be direct (and not waste as much heat as linear FET mode - a "switching" unit, that is). I know a FET in switching mode can be used to produce PWM-modulated voltage, and can be as powerful as 10 A, but the FET makes the voltage also PWM-shaped. I need direct voltage, but through a PWM-controllable device.

Do they exist? I watched about buck converters, rectifiers, operational amplifiers, PWM buck converters, voltage regulators, can't find such a class of devices. Switching power suppliers have a PWM, but that's inside them, to rectify voltage. I don't know how to attach it to the external PWM. In other regulators, there are voltage selecting resistors. But only resistors.
I also know that a "LRC circuit" could rectify a powerful PWM-electricity (FET-generated), but i can't assemble such a thing. I need a module.

For example, the nearest module i got to is: www.robotshop.com/en/10a-dc-motor-driver-arduino-shield.html
But i don't know yet about it. Does it produce straight voltage to motors?

To generalize a little, i need either voltage changing, or current changing, that's to control a thermoelectric plate. But still it has to be changable by 5 V PWM).

Paul__B

You don't "rectify" PWM (the term you are looking for is to "smooth" it) - you just use it to control your heater.

That's the whole idea - the heater doesn't care - being fed with full power for half the time is exactly the same as being fed with half power (which would incidentally, correspond to 1/√2 times the voltage).

DVDdoug

I'm not sure if you can buy a module like that.   What you're looking for is a "PWM controlled switching voltage regulator".

As Paul says, you don't need a linear voltage for a heater.   You don't need PWM either.   You turn a heater on 'till it hits the target temperature, then you turn it off.  Then on again when i falls below the target.

dwightthinker

#3
Dec 23, 2015, 10:56 pm Last Edit: Dec 23, 2015, 10:57 pm by dwightthinker
Why do you require a smooth DC current. If you're
just using it to heat a heat plate, a FET power switch
would be fine.
If it is a noise problem issue, you'd need a heavy inductor
in series. The size and current handling would depend on the
parameters you are using. It would average the PWM voltage
for you.
Dwight

DrAzzy

If you need 10A at a variable voltage, your best hope is probably a digital potentiometer replacing the voltage setting resistors in a buck or boost converter. What voltage do you need? If you need more than 5v, your options for suitable digipots is limited.

I did something like this a while ago, using a digipot to control the output voltage of a buck/boost converter. I don't know if I have the notes anywhere. It wasn't terribly sophisticated.
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MorganS

I think a MOSFET is the correct device for this. A Peltier will work just fine with a PWM waveform. It won't work well with a simple on-off thermostat. That causes thermal stresses in the Peltier.
"The problem is in the code you didn't post."

Unknownsymbols

It's not a heater, yes, it's a Peltier-unit (thermoelectric cooler). I know that it can be controlled through just PWM. And i do have such an assembly now - a MOSFET controlled by PWM. But i've read that TEC-s don't like PWM very much - not only slow cycling, but also PWM. That they operate with better efficiency on direct current (and wear out slowlier).
I need to maximize the efficiency. The assembly i have approaches limits otherwise. Hm, it's strange that such a module isn't widely known.
I need either 10 A at a variable voltage, or 12 V at variable current.

Making a digitally controlled resistor - maybe i'll manage to do it. Will look in it. (So the reaction of regulators will be acceptable, they don't expect that resistance to be set only once, at least i know now.) It needs to be 0 to 12 V. But i don't need many steps. 6 or 8 steps would be enough.

Slow cycling (without PWM - turn on, turn off for seconds) - hm, if in some opinions it causes bad effects to the cooler, better not to do it. It seemed bad to me. Maybe because i will have big temperature on the expel side - once off, the temperature will crawl back very quickly, hardly this will be efficient.

PWM controlled switching voltage regulator
will search yet, quickly it now returns the same regulators with PWM inside.

And that motor driver - doesn't it produce smoothened current? (guess no because it doesn't have big coils, and they usually have)

MarkT

Peltiers are more efficient with constant voltage/current drive than PWM: http://www.meerstetter.ch/compendium/pwm-vs-direct-current

The device the OP was flailing for is a _controllable_ DC-DC converter (which could be an LC filter
after a PWM power stage, like class-D amp)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Paul__B

You are of course correct that TECs do not like shock stress, so that "simmerstat" switching is indeed undesirable and PWM is much better.  The frequency (several hundred Hertz) is such that thermal shock is not a concern.

So what is the problem with PWM?

Well, Peltier devices are essentially semiconductors like LEDs - they have a threshold voltage and are essentially current driven as they tend to maintain that threshold voltage.  In effect, they behave as a voltage "sink" which performs their "work" in series with an internal resistance which is simply a power wastage and of course, interferes with their cooling effect (but of course, benefits somewhat, their heating effect).  This means that feeding more voltage disproportionately increases this energy loss, so driving more current for less of the time, is indeed, less efficient.

What you are looking for is a constant-current driver.  A simple inductor in series with your PWM driving FET, and a flyback diode across the combination, will suffice.  It is not a module as such unless you obtain a complete constant-current driver at your required specification.  Someone else may care to suggest appropriate parts for the inductor and diode. :smiley-lol:

Peltiers are more efficient with constant voltage/current drive than PWM: http://www.meerstetter.ch/compendium/pwm-vs-direct-current

The device the OP was flailing for is a _controllable_ DC-DC converter (which could be an LC filter
after a PWM power stage, like class-D amp)
Oh, so I needn't have bothered writing all this! :smiley-eek:

dwightthinker

I'd thought he was using it to heat.
He is right that they work more efficiently cooling with a constant
current.
It is because If you turn the voltage off, The difference in temperature
makes the peltier a generator, and a leaky one at that, as some of the
current it generates flows through the device, making it warmer.
It is something like paralleling solar panels without blocking diodes
and having one panel in the dark. The one in the light will waste
power in the one in the dark.
He is, most likely, more worried about ultimate temperature delta
than power source efficiency.
You can add a large inductor in series with your PWM power source to
smooth it out. You might try some secondary winding on some scrap
HiFi equipment.
The primaries may have too much resistance and inductance but
at about 500Hz something like a 24V winding might do a good job
of averaging a PWM signal into a smooth DC. A small amount of ripple
shouldn't be too much of an issue. Anyway, give it a try and see
if it is close.
If your going to make hundreds of these, you can calculate the inductance
and a reasonable IR drop for optimal performance.
The other option is to go with a D/A and a linear power source ( possible
modified old power supply ).
Dwight

Paul__B

Do remember that a (power) transformer will not work as a DC choke!

dwightthinker


MorganS

I think because if you leave the other winding on the transformer it will act more like a transformer than an inductor.

Losses will be higher because the transformer core won't be efficient at the higher PWM frequency. It is also possible to reach saturation with a high DC current through the transformer. Then the core has no additional benefit.
"The problem is in the code you didn't post."

Paul__B

Just saturation.

DC chokes require an air gap.

(No, the other winding is irrelevant.  If both windings are similar, they can be put in series.)

Unknownsymbols

#14
Dec 26, 2015, 03:53 am Last Edit: Dec 27, 2015, 04:47 am by Unknownsymbols
Quote
so I needn't have bothered writing all this
No, it's good to read, because what i've now found from "controllable dc-dc converter"s, like IR3475M or Semtech SC190, is when the chip has some easier scheme than external resistors for voltage setting, they all still require a ton of components as drawn in typical applications. Coils, capacitances, resistances. It's weird why they don't make circuits with all introduced. I think i won't be able to gear such a chip up. Or like in the solution like from this article: arduino-digitally-controlled-step-down-buck-dc-converter on eevbldg it's the same too.
That way the digital resistor way also becomes too complicated.

And special Peltier-controllers (i think i also searched for them before, but this page is the most clear), these are too expensive. Like 235 E. So maybe i'll unevitably have to wire up the inductor and a diode. If it's just as much, maybe i won't mess it down. But i'll have to calculate them.


Also i can't fully get this idea: you both, dwightthinker and Paul_B, write that constant current is better. Is it just "than PWM", or "among the constant current and constant voltage" still constant current is better? I know that an inductor will do the constant current way, but if i ever can implement any of'em fully, just what to choose.


Quote
The other option is to go with a D/A and a linear power source ( possible modified old power supply )
Is it possible to use a DAC here? (i don't understand why linear source though) From the parameter searcher i got the feeling they are small power devices.

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