In my latest box of circuit board goodies I came across this interesting circuit that has helped revolutionize dentistry.
However, before I get into the function of the board itself, let me digress and give you some back ground on its application. Up until about 20 years ago it was necessary for a dentist to shine light into a patient's mouth from an external light source in order to see what he was doing with his drill. This procedure required very precise positioning of the external light and even then it was often necessary to use a mirror inside the mouth to further direct light onto the drilling site. You can imagine the difficulty of working in the confined space between the tooth and the cheek at the back of the mouth and at the same time trying not to cast a shadow on the work area by getting ones body, head, or hand in the way of the external light. All this changed when fiber optic technology was introduced to dentistry. The drills, also called handpieces, were designed with a fiber optic or Lucite channel that brought light directly to the front of the drill. Now the dentist could put light directly on the area of the tooth that he was drilling without worry of having to direct light into the mouth from an external source.
Two main ways of getting light to this fiber optic or Lucite channel came into use. The first was to have an external light box and a long fiber optic bundle bring the light up through the dental unit and out the cord to the base of the handpiece where it was put in close proximity to the fiber optic channel of the handpiece. While this system worked well, it was heavier and more expensive than the second option which was to place a high intensity halogen bulb of about 4mm diameter in the handpiece cord to shine light directly into the light channel of the handpiece. This second type system was called a Power Optic System. The bulb itself was designed to produce and project the maximum amount of light forward even to the extent of having a lens built right into the glass of the bulb. These bulbs were rated around 3 watts and relied on the movement of air off the exhaust of the dental drill's turbine to cool them. Here is a closeup of one of these high intensity bulbs:
Power for these bulbs was supplied through 20 gauge multistrand wire (well over 100 fibers) so that 720 milliamps could be delivered to the bulb. The multistranding was important so that the wire remained flexible and durable while being flexed and moved around as the dentist drilled. The circuit I will discuss today is the power supply and control for these power optic bulbs.
Since it was possible for a dentist to have up to three of the power optic handpieces ready for use on different procedures this board comes with three power drivers. Each driver is capable of delivering up to 720 milliamps to the bulb attached to the handpiece on its circuit. The circuits are designed to be mutually exclusive so that the bulb on only one handpiece can be powered at any one time. In the case of this board the outputs are TIP 31C NPN power transistors. You can see the three TO-220 devices in the picture. Since the bulbs were such high wattage for their size it was important to adjust the voltage output to them to the correct value. The three small blue trimmers allow the voltage output to be adjusted between 3.5 volts and 3.8 volts for each handpiece.
The system would be activated when the dentist pushed on his foot control sending air pressure to the turbine in the handpiece that spun the drill. This air pressure would also go to one of 3 small pressure switches in the control unit and tell the circuit which bulb to illuminate. Another feature that was incorporated into the circuitry was a delay of the shut off of the bulb after the dentist released the foot control. This allowed the dentist to stop drilling and continue to have light for 10 seconds before shutting down. In this way the dentist could inspect the work site without having the drill spinning. If the dentist put one drill down and grabbed another the light in the first would shut off immediately on the turning on of the second bulb. I have made a short video to demonstrate how the circuit works. I have used small low wattage token bulbs to show the three different handpiece lines but I have put a real high intensity halogen bulb on line 1 so that you can see just how bright it is.
Please note that I misspoke in the video and the bulbs are 3 watts not 12 watts. Today the high intensity halogen bulbs have ceded some of their position in dentistry to new technology, namely LEDs. The LEDs can be mounted very close to the drill and produce an excellent light without the need for heavy wires and the problem with the heat that was produced by the halogen bulbs. The circuitry behind the bulb however, whether it is halogen or LED remains the same. Chances are that if your dentist has a light on the end of his handpiece there is a circuit of similar function to this circuit inside his dental delivery unit. Here is another picture of the test setup showing the cover that was over the board and the air pressure inputs.
John
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