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PWM
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M98Ranger Offline
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Post: #1
PWM
Okay, I have been trying to figure out what a pulse wave modulator is going to do for an HHO setup and I haven't come to a conclussion yet. If you truly understand why it is used, would someone please explain it to me? I have been reading everything I can find and still don't understand how it can help us.

I mean, I understand the workings of it, that it affectively creates a dirac delta sort of output that an analog monitor could display very well. For instance, I understand that it is used on a lot of vehicles now a days to "modulate" the voltage controlled pressure applied in anti-lock brakes to aid in stopping force. I also understand that by starting the voltage at zero (assuming current is in phase with voltage), current too is at zero. From that point they both increase in phase with each other until the pwm cuts open the circuit contact. Why would you do that!???

If a pwm is utilized for cooling it seems to me that you could do the very same thing just by modification of your design parameters. Plus, you could probably do it cheaper.

I must be totally missing the boat. Please correct me. And please if you truly don't understand it, don't act like you do. Just ignore me if you don't understand it. But if you do understand it and you were obliged to explain it to me I would be very very thankful.
12-29-2008 07:20 PM
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Gary Offline
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Post: #2
RE: PWM
Think of the PWM as a light switch, flipping on and off very fast.
Think of the electrolyser as an inefficient capacitor.
Because of the capacitance, the current can stop intermittently without stopping production.
This is how I understand it, in simplest terms.
12-30-2008 01:23 AM
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benny Offline
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Post: #3
RE: PWM
(12-30-2008 01:23 AM)Gary Wrote:  Think of the PWM as a light switch, flipping on and off very fast.
Think of the electrolyser as an inefficient capacitor.
Because of the capacitance, the current can stop intermittently without stopping production.
This is how I understand it, in simplest terms.

Think of each single cell more as a leaky capacitor, rather than an inefficient one. The electrolyte between the plates acts in the same manner as a resistor connected between the plates would do.
Visualize a standard electrolytic capacitor with a resistor connected across the positive and negative terminals of the capacitor.

Apply a voltage across the capacitor terminals and the capacitor will charge at an inverse exponential rate, fast at first, then slowing as the capacitor charge approaches maximum.
If we then disconnect the applied voltage, were there no resistor present, the capacitor would remain in a charged state for a long period of time. However, we do have to consider the resistor. This would cause the capacitor to discharge at a rate depending on the resistor value, and charge state of the capacitor. Again, the capacitor charge would discharge rapidly at first, then more slowly as the charge state reduced.

What a PWM is used for is to periodically boost the charge state of your capacitor by, as Gary says, switching the applied voltage on/off, and thereby maintain a reasonably constant voltage level across your capacitor. This is dependent on several factors.
Applied voltage, capacitor resistance, and size, in uF, of your capacitor, and the switching rate and mark space ratio of your applied voltage from the PWM output

What you have is a method of applying voltage for less than 100% of the time, but, with self discharge of the capacitor/resistor, maintaining internal current flow through your system as the capacitor discharges when the applied voltage is discontinued..

Think of this as being a bit like the way in which a switch mode PSU works, but with less components (inductors, feedback loop monitoring, etc), where your resistor represents load across the SMPS.

Not an exact analogy, but close enough, without getting too complex.
(This post was last modified: 12-30-2008 03:09 AM by benny.)
12-30-2008 03:05 AM
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M98Ranger Offline
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Post: #4
RE: PWM
Thank you Gary and Benny. Benny that was a great explanation. Now I understand how it could be used as claimed to keep current flow high and heat relatively a bit lower. Makes sense!
12-30-2008 09:49 AM
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