Contextual Electronics has started up and I’m already behind.  Chris decided to get our learning juices flowing by starting with a power source tester.  This is terrific for me since I have been assuming available power to “just work.”

I find it is very hard for me to follow along with someone’s circuit design unless I have a pretty good grasp of what led to the thought process. It is typical for me to totally miss the point – or valuable insights – at the beginning of a project.  I understand best if I am able to treat each component in the circuit as a character in a story.  The challenge is to understand the motivations and role the character/component plays. This post is my attempt to provide my learning techniques in order to clarify any misconceptions I have regarding the power source tester circuit Chris is helping us build.

My focus has been on measuring voltage.  After all, measuring small changes in voltage is what provides insight into reactions happening in the environment.  In the case of the Ladybug Shield, measuring the change in voltage is how to figure out the pH and/or EC of a nutrient bath – necessary for the healthy growth of hydroponically grown plants.

The need to characterize a power supply becomes clear when using batteries.  If the Ladybug shield was powered by batteries, how long can it run before the batteries need to be replaced?  How efficiently does the Ladybug shield use the battery?

To answer these questions, Chris is having us build a power supply tester.  Dave Jones’ EEVBlog #102 describes this circuit:

I can see the brilliant simplicity of this design (after :-)) Dave and Chris explain it.  A POT is hooked up to a +5V voltage source.  This is fed into Vin+ of an op amp.  The “golden rule” of op amps always applies – the op amp will do whatever it can to get Vin- to = Vin+.  Now the excitement begins.  Set up the the op amp as a voltage follower.  If 1V goes into Vin+, 1V will follow back into the op amp through Vin-.  The MOSFET controls the flow (current) since a MOSFET’s output current is controlled by the voltage at the input.  By inserting a 1Ω resistor, a constant current sink serves to flush the energy out of the power source (I noted a battery in the picture but this could be any voltage source like a DC-DC converter).

Dave has a plot that I found extremely helpful:

The Y-Axis is the amount of voltage applied to the MOSFET’s gate.  The gate stays shut until the voltage is at least 1.5V. As the voltage is increased on the gate, the MOSFET starts to let current through.  Since the current sink resistor is 1Ω and V=IR where R=1, V = I – so the X-Axis can be interpreted as the amount of constant current at a given Vin+ provided by the dummy load.

Now the question of what does a given load do on the capacity or other characteristics of a power source?  Can be measured.

OK, so all of this was obvious to you.  It wasn’t to me.  That’s one of the things I enjoy about Chris’s style.  These are useful circuits that use common components in ways that bring out the key characteristics of the components.  The circuits tell a story with characters – whose characteristics like the op amp’s golden rules or the MOSFET’s ability to act like a variable resistor to current flow – are pivotal in telling the story.

On to catching up with more of the design and what parts to use!