View Full Version : Measuring Voltage with RCTIME

G McMurry
02-15-2010, 04:39 AM
There is some great material on www.emesystems.com/BS2rct.htm (http://www.emesystems.com/BS2rct.htm) that particularly interests me in my current Basic Stamp project. I would like to measure several voltages in a system.

12 Volt Automobile Battery voltage and charge current
30 to 120 VAC and Current (3 to 40 amps)
30 to 120 VDC and Current (3 to 40 amps)

In the above mentioned article, he talks about using RCTIME to measure voltage on a 12 volt battery...


Is it really safe to connect a 12 volt battery to the Stamp operating at 5 volts?

As for the higher voltages (up to 130 VAC) he suggests using the Analog Devices AD629 which has a high voltage differential input as isolation... This all seems too good to be true...

Have any of you ever done any of this? Any hints? Any shortcomings?

In general I am happy with the RCTIME manner of reading things on my system. I am already reading an oil pressure transducer and using LOOKUP to correct for irregularities and it works fine.



Mike Green
02-15-2010, 05:43 AM
You've got the resistor values wrong. The resistor on the left is 681,000 Ohms or 681K. This high resistor value protects the Stamp I/O pin from the 12V and limits the current through the I/O pin's protective diode to about 10uA.

Measuring higher voltages using an AD629 is possible because the AD629 has high value resistors in series with its input pins. I would stay away with any attempt to measure AC line voltage with anything other than an isolation transformer. If you're going to use a transformer, why not use a step-down transformer so you can measure voltages more in the 6VAC range. You can measure AC current the same way.

I assume the high voltage DC is isolated from the power source. The AD629 would be a reasonable choice for measuring this. You could also use the same circuit as used to measure 12VDC. The "fault" current to the Stamp I/O pin would be higher (like 100uA), but within range of the protective diodes' capability. Measuring DC current would require a series resistor in the ground lead of the 130VDC supply to transform a variable current to a ground-referenced variable voltage that can be measured by a similar circuit to the one from Emesystems.

G McMurry
02-15-2010, 06:10 AM

I missed that K when I was copying the schematic to post. I first tried to use the text method that the original article used but the forum dumped all the leading spaces. So I drew it quickly...

I have corrected the image now in case anyone else follows this thread.

I was planning on using a pcb mount transformer but that doesn't help me reading a shunt resistor for the DC current. I am looking at a Hall Effect Current Sensor from TAMURA the L08P050D15. It may be my best solution for both. You just pass a wire through it. It will read both DC and AC amps up to 50 amps and has an isolated output.

I still have some thinking to do with all this.

BTW -- In case I haven't always told you -- All of your answers in the past have always put me on the right path. I am moving along nicely with my project. THANKS