Buck-Boost Wrangling I: Part Selection and Testing

When I joined the project on June 1st, Lindsey had already been working on the mechanical aspect for a couple of weeks.  The electrical system had only been discussed in very high-level terms.  By this point, Erica had suggested using a buck-boost converter circuit.  A buck-boost circuit takes some voltage input and spits out some voltage output.  The natural acceleration involved in riding a scooter would result in variable voltage when backdriving the motor; the buck-boost would act as a voltage normalizer.  For context, a cell phone charges on 5V and anywhere from 0.5-0.9A.

This naturally leads to the QUESTION: Which buck-boost converter should I use?

METHOD:

Choosing a buck-boost:

Quick Google searching resulted in a handful of options, but I decided to test use the LM2596 from Amazon because:

  1. Two-day Amazon Prime shipping = reduced lead time.
  2. Not only would it come in two days, I would also be getting a 2-pack.  Spares are always a good idea.
  3. The input/output voltage range and output current seemed appropriate.  Spec sheet here.
  4. There was a potentiometer onboard so I could customize Vout. Hooray for debugging tools!

Testing the buck-boost:

I didn’t have high hopes for the buck-boost converter, but testing was particularly frustrating.

This was the setup:

WP_20150618_001     WP_20150612_003

 (Right: The computer is not the power source; the power source unit is outside the picture frame.)

I set the power source to 5V and… nothing happened.

Debugging Step 1:  Tweak parameters and probe

I trust the power source to be outputting the correct voltage, so I connected a multimeter to measure the current drawn by phone.  I slowly tweaked the voltage supplied to 12V.  The buck-boost did its job and outputted 5V all the way, but still no current was drawn.

Debugging Step 2: Return to known system

I know that if I plug my phone directly into a computer, my phone charges.  I disconnected my male / female USB ports from the protoboard and reconnected to the computer’s USB, adding the multimeter to get a reading.

WP_20150618_003

Plug in phone, and… no dice.

Disconnect multimeter, plug in phone – still nothing.  Conclusion – borked USB cable.  Find another male/female, strip, splice, heat shrink.  Repeated this process twice until I found a working USB.

The multimeter read 5V and 0.4A.  This is my baseline.  The question now: was it simply the USB cord that was messing up?

Debugging Step 3: Add a layer of complexity

Move system back to protoboard.  This time, the multimeter registered a current draw of 0.1A, which stayed consistent no matter how I tweaked the voltage.  The USB cable is functional; therefore it must be the buck-boost converter that is limiting the current draw.

I was baffled, though, because the LM2596 chip was spec’d to allow a draw of maximum 3A.  I was at an impasse, and decided that I would pick Erica’s brain about the matter the next day at the National Maker Faire.

Continued: Buck-Boost Wrangling II

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s