PWM VS Potentiometer! When to use which technique?

I showed you how to build this modified van in a previous video and it's still useful and going strong but there has been a repetitive question about the speed control why the hell am i using PWM instead of just putting a potentiometer in serious to the van and yes you could just put about a geometry in serious this one here has a high value of resistance but it shows you that it is possible so why should you build this complicated circuit well in this video I am going to blow your mind with some basic electronics knowledge and give you the reason why I did this and I'm going to spoil myself here and say that the biggest reason is power lost we are working here with a battery so we do not want to waste energy on anything else than the span let me draw a simplified schematic of the PWM circuit the PWM represents a variable voltage source and our load is the van now we create an equivalent four-point-five voltage source with a duty cycle of 0.5 so we have a voltage drop of the complete 4.5 volts across our van no voltage is lost the current flows according to the characteristic line of the van practically we have no power loss in this circuit but in theory we have small voltage drops across the wire and the NPN transistor we could minimize the transistor lost with the n-channel MOSFETs like the pass 11 by thats not the point right now the only loss we have is the power the ne555 consumes to generate the square wave that is around 10 milliamps at 9 volts so a power loss of around 90 milli watts that is constant no matter how fast or how slow the van spins the loss does not change here practically time for the potentiometer to calculate this right we have to measure the characteristic line of the van so I hook it up to my bench power supply I started 9 volts and write down the amps then I go down by 0.5 volts and write down the amps again and so on remember kids the only difference between screwing around and science is writing it down in the end I have this nice chart which will help me now the simplified schematic for the potentiometer looks like this a fixed 9-volt source a resistor and a van let's do the math for the second line here about the eight point five voltage drop across my van Kirchhoff's voltage law tells us that the voltage sum in a closed netbook must be zero so we have zero point five voltage drop across the resistor that means the resistor wastes power of 0.5 volts multiplied by ninety three milliamps so around forty six point five million watts a second example would be 4 volts across the van so five folds across the resistor power lost or five volts multiplied by 48 million s equals 240 millivolts I did the calculation for the rest here now we can also calculate the resistor values resistance equals voltage divided by current so here we have zero point five volts divided by zero point zero nine three amps equals five point three seven ohms and the rest are just other values now you are thinking is this really tuned we can test it here is a 47 ohm resistor I put it in serious and BAM a voltage drop around three volts and a current flow of around sixty four milliamps just as we predicted now you look at your chart and realize well with PWM we have lost of 90 millivolts and here our highest is 230 millivolts but wait I want to feel the breeze so I only use the voltage levels until seven volts in this case it is pretty much the same so it is up to you whether you want to use PWM or a potentiometer for this van I would recommend a linear one up to 220 ohms and a quarter watt is enough as you can see but that is a pretty small load just imagine a bigger one like LED strip I did the same measurements and calculations and I got a power loss of around 1.6 watts I put 11 ohm resistor and Sirius just for fun and got unbearable hot I tried putting some drops of water on it to see it vaporize and it vaporized pretty good definitely use PWM for bigger loads because the efficiency is just better these were my main concerns with the subject of course there is plenty more to talk about but for now you know a little bit more don't forget to give me a like maybe you subscribe all up to you stay creative and I will see you next time

Channel: GreatScott!

Views: 407,066

Rating: 4.9362826 out of 5

Keywords: Pulse-width Modulation, Potentiometer, VS, greatscott!, tutorial, experiment, How-to, PWM, speed control, dimming, square wave, advantage, disadvantage, comparison

Id: 90g6RpvXBYY

Length: 4min 55sec (295 seconds)

Published: Thu Jul 31 2014

Reddit Comments

[u/Geekv2]

Why does he keep trying to PWM his van?

Seriously though, a great video for explaining the advantage of PWM for things such as LEDs or fans. I am a little curious as to why the PWM circuit draws 10mA. That seems a bit high.

[u/twerps]

PWM is what makes switch mode power supplies so efficient compared to the linear power supplies they replaced. 30 years ago, a laptop power brick would have 6 times the size and put off much more heat waste.

[u/pixel_juice]

For a second I thought this was /r/synthesizers!

[u/[deleted]]

This guys videos are wonderful. I've been watching for hours.