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Thursday, 17 October 2013

Sometimes I just don't get it


In order to try and confirm or otherwise my (in) sanity, I have been doing some simple experiments with a L200 voltage regulator.

On the bench right now I have a breadboarded circuit something like this:

As you can see I have stolen this image from elsewhere on the net.

Now, in my build these are the values:

  • R1 is 1K
  • R2 is 100R
  • Vin is 15V
I have three meters hooked up to this circuit, in the image below there is no load connected to the output and the meter on the left is measuring the voltage accross the Rsc or R2 resistor, the meter in the middle is measuring the output voltage at Vout in the image above, and finally the meter on the right is measuring the current drawn from the circuit.

So, I have an output voltage set at 13.8 (using the pot in the schematic) and an output current of 0 (as there is no load) - the voltage drop across Rsc or R2 is 145mV.

Now, given that my Rsc is a rather daft value, I expect the current limiting of the regulator to kick in at:

I limit = 0.45/R = 0.45/100 = 4.5mA

So to pull some current without starting the internal current limiting I have calculated the load needed to draw 2mA from the regulator.

R = V/I = 13.8/0.002 = 6900 ohms

So I now stick a 6.8K resistor across the output terminals:

So now my current draw is as calculated at 2mA ish (it's 1.97mA) but my output voltage has dropped from 13.8 to 13.34 - that's a whopping drop of 0.46 volts.

I thought the whole idea of a voltage regulator was to stop that happening as the load varies?

Confusing, egh?

** UPDATE **

A quick post on the RSGBTech Yahoo! group has allowed others to point out my Muppetry!

Firstly there is an internal current pull from the voltage setting components, so my very low current limit of 4.5mA needs to consider that and didn't.

Secondly the input voltage is probably too close to the desired output voltage to allow the regulator to regulate!

I have increased the input voltage to 18V and decreased Rsc to 10R and the circuit is working as expected.

Good, egh?

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