Voltage Regulator Resistance

parsko

Hi all,

Though I don't currently have the part number on-hand regarding the specific Vreg I am questioning, the question is pretty general.

Should my 3.3V Voltage Regulator have a measurable resistance between Vout (3.3V) and Ground (0V)?

I have a Vreg (whose flag is at 3.3V, not ground (grumble! grumble!)) that, when measured, shows a value of ~570 Ohm between Vout and Ground. When I measure the 5V Vreg I use on my homebrew board, it shows infinite resistance (and it's flag is connected to ground!!!). I have a spare 3.3 Vreg, and it measures ~570 ohm also. I get a good 3.3V out of it, but I think the resistance is messing with my circuit.

I think it is fine, and this is common, but I'd love if if someone would confirm, for sanity's sake. Any other thoughts are welcome as well.

Thanks,

-Parsko

Mike Green

The resistance seen at the output of a voltage regulator depends on how it's measured and on the specific design of the voltage regulator. Resistance is usually measured by using a battery of a known voltage to force current through the resistance and measuring the amount of current that flows. Depending on your meter, the polarity of the voltage used could be either way and most regulators have some kind of diode structure on the output that would prevent current from flowing into the regulator from the output. That's the infinite resistance you sometimes see. If you connect the meter the "other way" some current will flow depending on the design of the regulator, the voltage used in the Ohmmeter, and other resistances in the Ohmmeter circuit.

Bottom line: 1) Don't use an Ohmmeter to measure "resistance" at the output of a regulator. The value you measure is not very useful.
2) The apparent "resistance" at the output of a regulator is not a useful measure. What you're interested in is the maximum current available from the regulator (that will affect what you can draw from it), the voltage drop from input to output (that will affect what the input voltage has to be and the amount of power converted to heat), the thermal resistance of the packaging (that will affect how well the regulator can get rid of excess heat), and the transient response of the regulator to sudden changes in load (that will affect how much filtering will be needed to supply power transiently).

Post Edited (Mike Green) : 3/5/2008 9:34:11 PM GMT

Paul Baker

The output resistance of a power source doesn't have any real meaningful effect on a circuit (notice I said power source and not signal source). The only real impact is that the load's resistance isn't so low that more current is drawn than the regulator can handle. If that happens you'll need to use a regulator which can handle the current required.

Now the output resistance of a signal source does matter, especially at higher frequency signals and when there is·more than a few inches between the source and the load (transmission line effects).

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.