What is a PWM?

PWM stands for pulse width modulator. It looks a little scientific and complicated at first glance, but I assure you you don't need advanced math or physics to understand what it is and what it does. For instance right now you might not know what "pulse width" is, and you might not know the definition of "modulator", but we will define both of these in this article.

We'll use an HHO cell to help illustrate a PWM and what it does. They are actually used in many other places, but we'll stick to an HHO setting. Lets compare it to a cell that is hooked up directly to 12 volts and ground. In this circumstance, the cell is getting 12 volts 100% of the time. There is no interruption of the voltage supply. By comparison, a pulse width modulator will give the cell pulses of 12 volts, separated by 0 volts, instead of a steady supply. If we were to slow down time, we would see 12 volts, then 0 volts, then 12 volts, then 0 volts... Later we'll get into the ramifications of what this does to cell production, but for now we just want to describe what it is and what it is doing.

Frequency

The frequency of the pulses in different PWM designs is sometimes fixed and sometimes variable, but for this examples we're going to say that the frequency of the pulses is once every 2 seconds. They actually usually run much faster than this but for our example, we're going to say that the output of our PWM is on for 1 second, and off for one second. In other words our cell will get 12 volts for 1 second, and will get 0 volts for 1 second. So we'll define our first term here. The "Pulse Width" here is 1 second out of every 2 seconds. That's what's meant. How long is the pulse on compared to the time it is off.

Our frequency in this case is one cycle every 2 seconds. From this you can see that a "cycle" includes the entire period of time between the start of a pulse, until the start of the next pulse. In this case it's once every 2 seconds. Now, lets cut the time in 1/2. The pulse is on for 1/2 second and off for 1/2 second. Now the cycle is once per second. Frequency is measured in Hertz (sounds like "hurts", but is not nearly as painful, honest). Hertz = cycles per second. Our example is 1 cycle per second or 1 Hz (Hz is the abbreviation for Hertz).

Lets speed up our example to the pulse being on for 1/4 of a second and off for 1/4 of a second. Now the cycle is 1/2 of a second, or 2 times per second. The frequency is said to be 2 Hz.

Some PWMs have a fixed frequency, and some have a way to adjust the frequency within some fixed range. A couple of common fixed frequency PWMs you'll run into are 100 Hz and 500 Hz. These equate to 100 pulses, or 500 pulses per second. There are also variable frequency PWMs that will have a range of frequencies that they will adjust to.

Duty Cycle

Now we get into why we're doing this in the first place. This is a related concept that you will see quite often when reading about PWMs. It's called "Duty Cycle" or "Duty Percent". This is a term that is borrowed from the physics text books, but is still a very simple concept. Duty cycle is the percentage of time that the pulse is "on" compared to the time it is "off". In our examples above, the duty cycle has always been 50%. The voltage is on for the same amount of time that it is off, or exactly 50% of the time. Notice that each time we doubled the frequency, our duty cycle remained 50%. In our examples, the pulse would be on for the same amount of time that it was off. It doesn't matter what the frequency is, the duty cycle just tells us the percentage of time that the pulse is on compared to the time the pulse is off.

So duty cycle describes the width of the pulse over time, or the "pulse width" as a percentage. Lets look at another example: The pulse is on for .75 seconds and off for .25 seconds. If we repeat these ons and offs, always making the voltage be on for .75 seconds and off for .25 seconds, we have a duty cycle of 75%.

The graph below shows 4 PWM signals, by graphing voltage over time. For a car the voltage at the high point will be 12 volts, and the low point will be 0 volts. Graphs a and b show 50% duty cycles. Graph c shows about 10%, and graph d shows about 80% duty cycle. Notice that as the duty cycle goes up, the amount of time that the signal is high goes up. This means current will flow in the cell 80% of the time, using graph d, but only 10% of the time with graph c.

Duty cycle can even be 100%. But you don't need a PWM to provide a cell with a 100% duty cycle. You just hook it straight to 12 volts. It can also be 0%. Again, you don't need a PWM for that, you just disconnect the cell from all power. That's the equivalent of a 0% duty cycle. But for any duty cycle above 0% and below 100% you need a pulse width modulator.

Modulator

Now we're ready to define modulator. Modulator loosely translates into "changer". With a PWM you can change the pulse width. There are a couple of reasons for doing this, and we're only going to touch on them in this article. But as the duty cycle goes up, and the voltage is on for a higher percentage of the time, current will flow for longer times. So my "modulating" the pulse width, we have an immediate way of changing the current flow through our HHO cell.

When folks use a PWM, they usually add a little more electrolyte than if they were providing a straight 12 volts. This is because the voltage isn't on all the time. By adding 15% more electrolyte, you can then run your PWM at 85% duty cycle, and you should get the roughly the same current draw and the same amount of HHO production. But now you can control it. But turning down the duty cycle, you can reduce the overall current flow. By increasing the duty cycle you can raise the overall current flow. PWMs usually have a knob that will control the duty cycle directly, so you will have instant control over the current.

Summary

That's the basics of what a PWM is. Its a device a device that breaks up a DC voltage (like the 12 volts from our car's electrical system) into pulses which can be changed to our needs. When we change the width of the pulses, we are modulating them. Hence it should now be clear what "Pulse Width Modulator" means.

Constant Current PWM

One last concept, I should cover before ending off on this discussion. So far we have discussed a PWM that the user can control to vary the pulse width. However, there is a particular type of PWM on the market that controls itself. It's called a "Constant Current PWM", sometimes abbreviated CCPWM. This device has a knob where the user can set the highest amount of current that he wants the PWM to allow. The PWM in this case will sense the current, and if the current tries to go higher than this setting, the PWM itself will reduce the duty cycle. Most PWMs that are sold for HHO systems do not have this capability, yet this is probably one of the most important functions a PWM should do.

Usually a cell will run at a certain amp level when its cold. But as the cell runs, it warms up over time, and as it warms up, it will draw more amperage. Because its drawing more amperage, it will heat up some more, and this causes it to draw even more amperage. And so on. This is called "Current Runaway". If you don't have a constant current PWM, you have to measure out your electrolyte, so that the PWM runs well below it's capacity when cold, so that when it gets warm it won't go into runaway. Even a PWM that doesn't have constant current capability has the same problem. Although if a user had an amp meter in his car, and a knob to control the duty cycle, he could manually control the amount of the amps.

Let's say you have a single cell that you want to run at 15 amps. With a Constant Current PWM, the first thing you will do is set the maximum current level to 15 amps. Then, you can set your electrolyte level so that when the car starts cold, it's running at full capacity, or 15 amps. The duty cycle would be high at this point, lets say 90-100%. As the cell warms up and the current creeps up to 16 amps, the CCPWM will sense this, and will dial back the duty cycle to 85% (for instance), so that the current again drops back down to 15 amps. No matter how much the cell warms up, and tries to draw more current, the CCPWM will sense the change and adjust the duty % accordingly.

This is the type of PWM I recommend if you build or buy one for your vehicle.