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High(ish) power passive dummy load

Used as a dummy load to test a Keithley 228 Voltage Current source

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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(*steve*) submitted a new Showcase Item:

High(ish) power passive dummy load

I am in need of a dummy load to test my Keithley 228 Voltage Current source. (And yes, it does sound like a vacuum cleaner)

I need a 100 ohm load capable of 100W, a 1 ohm load capable of 100W, and a 0.01 and a 0.1 ohm load capable of 10A.

So my requirements are:
  • 0.01R 1W
  • 0.1R 10W
  • 1R 100W
  • 100R 100W

But
  1. I prefer to be conservative
  2. It's really begging for some intermediate vales.
So the specs I'll shoot for are:
  • 0.01R 50W (0.7V 70A)
  • 0.1R 50W (2.2V 22A)
  • 1R 200W (14V 14A)
  • 10R 50W (22V 2.2A)
  • 100R 200W (141V 1.4A)
  • 1K 50W (220V 0.22A)
  • 10k 50W (700V 0.7A)
These will all have kelvin connections as I will normally want to measure the voltage across the resistor without worrying about the lead resistance. Whilst this is really only a factor with the lowest ranges, it's easier to make all of these resistors the same.

I am also toying with a method to trim the value of these resistors to closer to their marked value. And also (perhaps) to do something about absorbing any transient voltages generated due to these resistors being inductive (the instructions on the Keithley 228 are quite specific about this being a requirement for inductive loads).

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After some administrative issues, I'll show you what I managed to get done today...


Here is the heatsink drilled and with all but 2 of the resistors mounted. I haven't used heatsink compound yet because I might rearrange the resistors.

Below the heatsink is the CAD drawing of the layout which was taped to the heatsink to allow me to centrepunch the positions for the holes.

At the bottom of the image is a 50W resistor that doesn't need a heatsink. From the relative size of this 50W resistor and the ones mounted on the heatsink it should be clear why the smaller ones need a heatsink.

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I finally received my last two resistors (Thanks RS for the free shipping :))

Obviously I need to carefully chose the wire to use to connect the resistors to my front panel. The wire has to both carry the current and not contribute to the resistance.

In practice it's always going to contribute to the resistance, but in the case of the 0.01R resistor, the lead resistance is so significant that I need to do a kelvin connection to the resistor. The...

Read more about this showcase item here...
 

davenn

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Sep 5, 2009
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hey Steve
nice job , well done
You riveted the resistors to the heatsink, they sort look like rivets ??

Dave
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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No, they're small screws. But they're black, so I understand why you can't see the detail :)
 
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