[NSRCA-discussion] Foam cutter power supply

[Ed Alt]

Paul:
The way a light dimmer works is to chop a variable percentage of the AC voltage waveform off, so the brighter the light is, the more full and complete the 60 hz sine wave is that represents the voltage applied to the load (the bulb). As you reduce the output, it chops off more and more of the output.  It has to work this way, otherwise the dimmer would be the size of a toaster oven to have enough mass and surface area to dissipate the heat that would build up in it if all it did was to drop a portion of the voltage across it.  In other words, it is either all the way off for a period of time (no power dissipated in it at that time), or all the way on like it was a piece of copper wire (no appreciable power disspated at that time).  Anyway, what you are apparently reading is the what the meter still "thinks" is the RMS (root mean square) voltage of the sine wave.  It probably is reducing the meter reading somewhat, but not nearly as much as you might expect.


The reason this works for heating the nichrome wire at the right temperature is for the same reason the light bulb would get dimmer.  You are changing the average power delivered to the resistive load (the nichrome wire), so even though there are still some high voltage peaks, the average power is managed effectively.  I hope this helps.

Ed
  ----- Original Message ----- 
  From: Paul LaChance 
  To: NSRCA Mailing List 
  Sent: Thursday, May 21, 2009 4:25 AM
  Subject: [NSRCA-discussion] Foam cutter power supply


  Hi all,

  I was told about a way to make a power supply for a foam cutter using 2 dimmer switches.  I was also told to measure the final voltage to get an approximate starting point of 15 volts.  I am not getting a voltage change with the dimmers.  I have tested to see that there is a difference by using a drill to see that it is reducing the speed.  Does anyone know how I can measure to get a starting point or is it going to be trial and error?  

  Thanks in advance,

  Paul


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