Breadboard power supply (MB102)

The little green board that comes in the Elegoo starter kits and sits on the end of an MB-102 breadboard.

Aliases: breadboard power module, YwRobot 545043, MB102 power supply.

It plugs into both pairs of rails at once, takes a 9 V battery or a 6.5 to 12 V wall supply on its barrel jack (or 5 V from its USB socket), and puts 5 V or 3.3 V on each pair, set by a jumper per side. It is how a kit runs motors and servos off the board, so a stalling motor does not reset the Arduino.

Pins

Pin	What it does
VIN	The barrel jack's center: the battery or supply's plus.
GND	The jack's sleeve, and the board's ground.
5V	A header pin on the 5 V net, for a jumper wire.
3V3	A header pin on the 3.3 V net.
TOP+, TOP-	Into the first holes of the top pair of rails.
BOT+, BOT-	Into the first holes of the bottom pair.

Place it with TOP+ in hole tp1 of a breadboard and all four rail pins land in their rails. Every - pin is the same ground.

Properties

top and bottom are the two jumpers: 5V, 3.3V or off for that pair of rails. usb powers it from the USB socket instead of the jack.

While it runs

Click the module to work its power switch. The green LED is lit while it is on and its 5 V net is up.

Wiring it with an Arduino

Share the ground and nothing else. The Uno keeps its own USB power; the module powers the servo or motor; one wire joins the module's - rail to the Uno's GND, so the signal wire has something to be measured against. Do not join the module's 5 V to the Uno's 5 V.

What the model gets right

Two real regulators. The jack feeds an AMS1117-5.0 and the 5 V net feeds an AMS1117-3.3, both the bench's regulator model with the AMS1117 card: about a volt of dropout, an amp of current limit, 5 mA of bias. A 9 V battery gives a solid 5 V; four fresh AA cells (6.4 V) just manage it at a light load; three cells leave both rails short, because the 3.3 V side runs off the 5 V net.

The loads add up. Everything on the 3.3 V rail comes through the 5 V regulator too, and a short on either rail is held to about an amp.

The battery's own resistance. A 9 V battery is a volt and a half of sag at an amp, so heavy loads on a battery-fed module drop out sooner than on a wall supply.

USB puts 5 V from the socket on the 5 V net, and the 5 V regulator has nothing to do.

The switch cuts both the jack and USB, and the rails go dead.

What it does not model

Heat: the 5 V regulator turning 9 V into 5 V at 300 mA burns 1.2 W, and a long stall gets the SOT-223 too hot to touch, which here never happens. Reversed polarity on the jack, which on most copies destroys the regulators. The power LED is drawn as its current only, and the 3.3 V net carries a third of a milliamp of bleed so it always has a load.

Common mistakes

Joining the module's 5 V to the Uno's 5 V pin as well as the grounds: two supplies fighting over one rail.

Forgetting the common ground: the servo's signal then has no reference, and it twitches or does nothing.

Setting a jumper to 3.3 V for a 5 V part, or the other way around.

See it in action

Power module runs a servo off a 9 V battery through the module. Open it at /templates.