I'm back again with my second installment for another type of homemade COP coil bench tester. This design is to control a 3 or 4 wire coil with an internal module- ie. the primary switching transistor is part of the COP unit. Now what I came up with is a small device that makes a 0-5V squarewave with 3msec on-time at 15Hz.[DIY mini coil driver] So you could just buy an adjustable signal generator that would do the same thing, but where's the fun in that? ;)

Most 3 - 4 wire COP coils function about the same; power, ground, and a control ckt (4th wire might be another ground or coil feedback ckt). The control signal is usually a squarewave (around 0-4V to 0-5V), when it rises the coil starts charging, when it falls the coil fires.

This build is based on another NE555 signal generator. This unit has an on-board output transistor that is rated at 500mA - higher than most other units that just use the driver inside the 555 chip(100-200mA). [NE555 squarewave] Sourced from ebay (a dozen sellers have this- I'm sure from the same factory)

So here is the block diagram. [block diagram for DIY coil driver] Battery power goes through a fuse (7.5A) & splits to the COP (+) & a momentary switch. From the switch power goes through a diode for reverse-polarity protection, then to a DC-DC buck regulator - converter [Mini DC-DC buck converter] - also ebay. The adjustable converter is set to reduce voltage from 12V to 5V. 5V into the NE555 generator produces my 5V squarewave. There is a 1/4W resistor (about 100 ohms) in the output ckt to protect the 555 unit from shorting the signal to ground (overload).

There you have it. I find it's a nice tool to have- not often but sometimes. For about $15 plus some scrounged bits. Connect the power, ground(s) & signal leads, add a spark gap tester, then hit the button.

Chris from New Jersey

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