Panel Current Meter Advice and Help

by tod_kuykendall on Sun Nov 25, 2012 2:58 pm

I ordered (and promptly received -thanks!) the Panel Current Meter because I'm building a portable GPS logger that will ultimately run off of LiPo and I want to know how much power I'm drawing and how to best minimize it. So my first test is an Arduino Dem powered from the USB bus and I have a PMB-648 GPS unit and an Adafruit MicroSD card logger both running off the 3.3V pin. (I'm going to be using a Mini Pro 3.3v for my final project.)

I powered the current meter from an external 9v battery (thin wires) to make the wiring easier and then tried to measure the usage at a couple of different spots in the circuit - red to red and black to black (thick wires). The first was to attach it to the 3.3v and GRND "rails" that I have set-up on 1/4 breadboard that I'm using to power my accessories. It seems like regardless of where I attach it the meter shows around .40 (with some minor variations) but the problem is that the light on the GPS unit that signals power shuts off and the output from the unit stops. Am I grounding the circuit somehow even when externally powering the meter?

This is probably a result of my inexperience with the current meter and I'm looking for any help or advice in correctly measuring the total current draw of my project so I can test different GPS settings and sleeping the Arduino configurations to find the minimum average power draw.
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Re: Panel Current Meter Advice and Help

by john444 on Sun Nov 25, 2012 7:01 pm

Hi Tod,

I am not part of the Adafruit team but I think I may be able to solve your problem.
I use several of these meters (and the V-meter) on my solar-power set-up.

First, keep in mind that this is a current-meter. It is used to measure Amperes.
Or, on this specific meter - it can display a min of .00-A to a max of 9.99-A
A Volt-meter would connect from the + supply to ground but, not an Amp-meter.

To properly connect the 'Panel Current Meter' (ID-574):
You correctly supplied power for the meter from a separate 9-v battery.
Nothing wrong doing that.
However, you seem to have mis-connected the current-leads.
It is necessary to break a connection in the power supply lead and insert the meter.
One way to do this is to cut the (-) lead from the AC-adapter (or battery pack).
The meter's black-wire connects to the cut-wire that attaches to AC-adapter's (-).
The meter's red-wire goes to the other end of the cut wire that goes to the Arduino.
That's it.

It is also possible to power the meter from the Arduino's AC-adapter (or battery pack).
To do this, connect the thin black-wire to the thick black-wire.
splice into the AC-adapter's (+) lead.
Now the AC-adapter provides power to the current meter as well as the Arduino.

If you are powering the Arduino from USB it is a little more involved to get to the
center power pins on the USB cable. But idea is the same:
Cut the USB cable and expose the USB power leads.
Insert the meter into the (-) power-lead.

Good Luck, John
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Re: Panel Current Meter Advice and Help

by tod_kuykendall on Sun Nov 25, 2012 7:37 pm

Hello john444, thanks for the reply.

I figured it was probably pilot error on my part and after reading your post and searching the web it seems pretty clear that what I need to do is "bug" or "tap" the negative lead by inserting the current meter on ground wire to measure the current. Do I have the right idea now?

In my current configuration running off USB makes this difficult unless I'm willing to switch to a battery source. Is it possible to insert the current meter later in the circuit and measure just the downstream current? That is, if I insert the current meter on the ground wire after it comes from the Arduino unit and goes to the GPS and SDcard reader and isolate that current? In this case it would allow me to check the usage of my GPS unit which is my primary concern.

Thanks again, and any further advice is welcomed!
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Re: Panel Current Meter Advice and Help

by adafruit_support_bill on Mon Nov 26, 2012 5:58 am

So you have an Arduino, GPS and SD card reader all running from USB power, and you want to measure the current draw of the GPS only? If you disconnect the ground wire from your GPS and insert you meter there, it will measure just the current used by the GPS.
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Re: Panel Current Meter Advice and Help

by tod_kuykendall on Mon Nov 26, 2012 12:03 pm

Thanks for the advice - I have it working now. It would be easier if the thicker wires fit into the breadboard because it's two more connections to deal with, but is certainly manageable. (I assume they're thicker because the panel measures up to 9 amps and it is required.)

I'll get to measuring and get an idea of how much LiPo I'm going to need to for what operation durations.

Thanks again.
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