Franken Slurpee

I am proud to announce my very first electronics project - Franken Slurpee. I bought my son a slurpee drink maker for Christmas and was dismayed to find that it required four "C" batteries. The machine turns a metal drum full of ice and salt - which in turn mixes 12 oz of fluid into slush. I could picture constant trips to the store for new batteries, so I decided to convert the thing to run off of wall power. Knowing how to read a multimeter and not much more, I set off on my quest.

The first thing I needed to know was how much voltage the machine required and how much current it would draw. I got out my multimeter, opened the motor housing, and discovered that I needed to supply 6.4 volts of power using hardware that could handle up to 1.5 amps of current.

Next, I needed to build a circuit board that would provide 6.4 volts of regulated DC power. Using information from Bill Bowden I built a circuit with the following parts:

• 5 - 1N4003 rectifier diodes
• 1 - LM317T variable voltage regulator
• 3 - 100 ohm 1/2 watt resistors
• 1 - 330 ohm 1/2 watt resistor
• 1 - 1uF tantalum capacitor

These parts were assembled according to the following circuit diagram.


Neither the diagram nor the parts list includes a heatsink; however, it is very important to have one attached to the voltage regulator. I did not include a heatsink in one of my early prototypes. This would cause the regulator to quickly overheat and shutdown. Now, the machine will run properly. I used the biggest heatsink I could find although it is perhaps overkill.



Here is a picture of the completed circuit. You can see the resistors to the right of the heatsink and the bridge rectifier is sitting behind it. The voltage regulator is in front of the heatsink along with a diode and the tantalum capacitor.



Now that I finished my circuit board, I needed a power supply for it. I settled on a 12.6 volt center tap transformer. This transformer converts 120 volts AC down to a voltage that the regulator can work with. The transformed power is still AC, but the bridge rectifier takes care of the conversion to DC. Notice in the above picture that the center tap (black wire next to the yellow ones) is tied off. If you use the center tap and a yellow wire, then you will only get 6.3 volts of AC power as the output. This is not enough power to run the voltage regulator. The voltage regulator needs at least 3 volts of input higher than the desired output. So, for my application I want 6.4 volts of output which would mean I need at least 9.4 volts from the transformer. By using the two yellow wires, the transformer puts out its full capacity of approximately 14 volts.

Here is the transformer mounted to the bottom of the slurpee pedestal. The circuit board is sitting in front of it and ready for mounting.



The transformer and circuit board are in place, but it is still necessary to get power to the transformer. Drill a small hole through the back of the slurpee machine and feed in an ungrounded lamp cord. Make the hole as small as possible so the lamp cord fits tight. You don't want it moving around to much. You can also tie a knot in the cord inside of the pedestal housing so that it cannot pull out any farther than the knot. Strip back the lamp cord and connect it to the two primary wires on the transformer. It doesn't matter which wires are connected - just hook them up.

All that is left is to hook up my new power supply to the slurpee machine. When you very first open the slurpee machine, you will see a black wire and a red wire coming from the battery compartment. Clip these wires and strip them back. The red wire is attached to the output pin of the voltage regulator. The black wire is attached to ground.

Here is the circuit board and transformer fully mounted on the pedestal. Notice the black and red wires running from the board back around the transformer.

Now we can close everything back up. Front view of the Franken Slurpee. It is ready for business.

Back view of the machine. I wanted it to be as aesthetic as possible. No one should know that it ever ran off of batteries - but of course you know otherwise!