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SoftStart Main Page | Circuit Description | Board Assembly | Programming the Microcontroller

SoftStart Schematic

Let's take a look at the schematic as a whole, then discuss each of the parts. You can click on the schematic to see a larger version instead of squinting at the tiny one below.

On the left of the schematic is the heart of the circuit - a microcontroller. There are a multitude of ways that this function could be implemented, but we chose a microcontroller because many functions can be realized in the one piece of hardware:

The microprocessor chosen is the Cypress CY8C24123A. It is small (8 pin DIP), inexpensive (under $3), and requires no external crystal or components.

The two outputs of the microcontroller drive transistors which in turn drive relays. One relay routes the AC through current limiting resistors. The other relay routes the AC directly to the transformer. In normal operation, the device is waiting for the switch with both relays off. When a switch ON is detected, the current limit relay is turned on. After a short time delay (a few AC cycles), the direct relay is turned on and then the current limit relay is turned off. When the OFF action of the switch is detected, the direct relay is turned off.

Flyback diodes are placed across the relay coils to shunt flyback current spikes. AC line capacitors are placed across the relay contacts to reduce EMI from contact bounce.

The microcontroller does require 5V power to continually monitor for switch events and drive the transistors. Only a very small amount of power is required (under 20mA), so we selected an integrated and encapsulated 5V supply that runs from the AC line.

If you are interested in using these boards to make your own SoftStart, they can be purchased from the Store.

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