Flock Applicator With the ever-expanding hobby craft market, fired to a large extent by Internet auction sites such as Ebay, I thought it was time to ressurect a project that I designed and built some years ago. In fact I made the first unit in 1980 and the second (having lost the first during house moves) in February 1996.
A flock applicator has a multitude of uses but the average hobbyist will use it to produce a grassy landscape for model railway use, "hair" on a ping-pong ball, a lawn for a dolls house or decoration on a greetings card. It is also used to produce flock wall coverings and protective linings in jewellery and watch boxes. Flock consists of short strands of synthetic fibre such as rayon or nylon. Nylon forms a stiffer, more hard-wearing brush-like covering.
You can buy an electrostatic flock applicator for just over £300 but this is rather expensive for a hobby. I decided that I could base my design on a vehicle ignition coil and save a few pounds. The original design used a coil from an Austin Mini but, for the later one, I reduced the size by getting one from my local motorbike breakers yard.
The Circuit (see magazine) For safety, a 240:240 volt mains isolating transformer is used. This could be replaced with a 240:350 volt transformer to increase the output voltage but you would have to make other changes to compensate. The isolated AC mains voltage is full-wave rectified by diodes 1 - 4 to produce positive going sine waves at 100 Hz. Each one charges up capacitor C1 and , as the voltage reaches its peak, the thyristor switches on to discharge the capacitor through the ignition coil primary. The rapid change in current induces a voltage of around 30kV peak on the coil primary. This voltage pulse is connected via 30 diodes to a metal plate which acts as a capacitor and holds a positive charge with respect to ground. The diodes each have a reverse breakdown voltage of 1000 volts minimum. Resistor R1 limits the current flowing through the thyristor when it turns on. The resistor chain formed by R2, R3 and R4 sets the point on the sine wave when the thyristor turns on. Ideally this should be close to the peak but some allowance must be made for locations where the mains voltage is less than the nominal 230 volts. The value of R4 can be altered to change the switching point. If a 350 volt transformer is used, the value of R4 can be reduced accordingly, so that the thyristor switches closer to 340 volts than 230. Please note that resistors R1, 2 and 3 are high voltage resistors rated at 350 volts. You should use the ones specified as ordinary resistors could break down. C1 is chosen for its ability to withstand the rapid change in voltage (dv/dt). If you use a different type from the one specified, it may not survive. C2 and L2 are included to minimise the "noise" going back to the electricity supply. Neither these, nor the transformer, T2, are required for correct operation but they are essential to meet the best safety and emission requirements. (Note that they may not be sufficient to meet CE and other legislation so please do your research carefully if you plan to manufacture equipment based on this prototype. Switch S1 is a press-on release-off type for safety. This ensures that you always have one hand on the switch while the unit is running. The switch has an insulating plastic cover fitted.
Construction The unit was built into a sturdy plastic "lunch box" with a recessed lid. All screws were plastic for added safety. The charged plate was cut from a piece of brass sheet but almost anything will do. (In the days of tinned steel oil cans I would have used a piece cut from one). The metal plate is insulated by covering it with a piece of black plastic.
This was held with four plastic rivets in the prototype but you may prefer to hold it with glue or tape. Black plastic is preferable because it usually contains a small amount of carbon, which makes it slightly conductive. A good source of black plastic is your local builder who uses rolls of it as damp-proof course material.
The diodes are soldered end to end, all facing the same direction, and threaded into a length of polythene tube which you can buy from most car accessory shops. In the prototype the diode cathodes faced the charge plate to produce a positive charge but some people think that negatively charged ions are beneficial to health. For the purpose of charging flock, it really doesn't matter.
The ignition coil came from a scrap motorbike. I had no trouble in buying two from a local motorbike repair shop in Crewe. A car ignition coil will work just as well but takes up a lot more room and is heavier. The diode tube is held to the coil "nose" or output lead by heatshrink sleeving.
The smaller components were soldered to a small printed circuit board that had been made for another purpose but you can use perforated stripboard such as "Veroboard". Be careful to leave sufficient gap between high voltage conductors, otherwise arcing could occur.
A toggle switch rated at 250 volts was used as the on/off control. A biassed (spring-loaded) switch was used in the prototype and a waterproof cover was added to give extra insulation.
DANGERS Holes for the switch, screws and mains cord were simply melted into the plastic box using a previously discarded soldering iron tip. If you use this method, do it outdoors because the fumes may be noxious. As with all such projects you should use common sense, remembering that soldering irons can burn and mains voltages can kill. The complete project should be finished so that it is impossible for any person (especially a child) to come into contact with any part that can carry a high voltage. If the unit is constructed as suggested, it will still be possible for someone to draw a small spark from the charged plate and get a nasty shock, albeit indirectly. This presents a similar risk to that of a car ignition circuit. In other words, the risk of injury or death is extremely low unless the shock recipient has a heart problem. Eye protection and a face mask should be worn while using the flock applicator because the tiny fibres can cause bad eye and lung irritation. IN USE The flock applicator is very easy to use and the results can be amusing, surprising and useful. Apply a suitable glue, paint, varnish or resin to the surface of the object. The adhesive should preferably be of a similar colour to the flock. Sprinkle a little flock onto the lid, hold the object above the flock and press the switch. Bring the object lower and the fibres will leap up and embed themselves, end-on, into the glue. The effect is more powerful if the object is grounded but, in practice, the presence of your hand is usually sufficient. The flock can be ejected in a powerful spray so you should use it in a place where the resultant mess is easy to clean up and won't cause problems. Do wear eye protection, a breathing mask and clothing that can be discarded. Although flock is not quite as bad to work with as glassfibre, it can be unpleasant. If humidity is high, the charge is quickly lost and results can be very poor. The flock fibres need to be completely dry - store them with a dessicant and heat them up if necessary to drive out moisture. For best results, sprinkle only a very small amount of flock on the lid and replenish as necessary.
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