Current measurements on LDO regulators
A customer asked about trying something with the Island Keyer II kit:
http://wb9kzy.com/ik2.htm
He had purchased the regulator and had soldered it but wanted to power the kit with 3 x AA cells, about 4.5 volts instead of using a 9 volt battery or 12 volt supply with the regulator.
Here is the schematic of that regulator circuit (and SSR):
His idea is to simply connect the +4.5 volts to the output of the regulator (LM2936). I told him I'd try it first and get back to him. I connected an LM2936 circuit on a small breadboard with the regulator input open circuit and the output connected to 4.5 volts from a supply. I measured this current going into the LM2936 output:
The reading was dancing between 4 and 5 uA so I'm assuming that 4.5 uA is a fair approximation of the reading. Also, no smoke or damage happened. For reference, the LM2936 uses 7 uA when powered by 9 volts on the input and with the output connected to a sleeping Island Keyer II PIC.
Another assumption, that the 4.5 uA is drawn by the voltage divider circuit used on the LM2936 as shown here in the datasheet:
These voltage dividers are present on all linear regulators such as the Seiko (ABLIC) S812C50:
With the LM2936, Ohm's Law tells us that the value of those two resistors in series is 4.5 volts / 4.5 uA = 1,000,000 ohms or 1 megohm
Just for the heck of it, I also tried measuring the current of the Seiko S81250 regulator:
It is zero microamps ! But notice that there is no minus sign, here is a picture of the current with no voltage on the Seiko output:
There is a minus sign although the current still reads zero. Using a little interpolation (or imagination ?) that's telling me that there is some current being measured when 4.5 volts is applied but that it is less than 1/2 microamp so the meter displays a zero without a minus sign. Now using that same Ohm's Law 4.5 volts / 0.45 uA (assumed) = 10,000,000 ohms or 10 megohms !
So to really know I'd have to get a better DVM than my Metex which has 3 3/4 digits of precision. I do have an HP voltmeter with 4 1/2 digits but it is squirrely (possibly the switches are bad).
Why not measure the resistance directly ? I tried and got infinite ohms for both the Seiko and the LM2936 (I had already broken down the camera tripod so no pictures).
Sometimes it is fun to get out the meter and use the basics like Ohm's Law !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
