Today I am working on a circuit board from a Dental Statim Sterilizer. This sterilizer is unique in that it doesn't work by heating water inside the sterilization chamber but rather it has a separate boiler. Water is injected into the boiler and then the steam and pressure that is produced goes into a sealed tray, called a cassette, that contains the instruments to be sterilized. The advantage is the it is a very fast process and can potentially sterilize instruments in a fraction of the time needed using a regular steam sterilizer. The Statim itself would make an interesting blog post but today I am going to explain how to build one of the components that I use to test and analyze the Statim's circuit board.
As you can see I am treating this board to an out-of-body experience. As much as possible I want it to think that it is installed in a sterilizer. The board has 4 Triac controlled outputs. The largest current draw comes from the boiler which draws about 10 Amps at 115 Volts AC. To simulate that load of the boiler I have the heating element from an old clothes iron with a 60 Watt light bulb in parallel. The light bulb is important as it lets me know that the circuit is energized. The other three outputs control a solenoid that lets steam from the sterilization cassette to vent to a waste bottle, a water pump that is used to inject water into the boiler as it is needed, and a compressor that pumps ambient air through the cassette during a drying cycle. As you can see I am using 25 Watt light bulbs to simulate the loads normally produced by the solenoid valve, the water pump, and the compressor. I have written the application that each bulb is simulating on the white PVC base so it is easy to associate a lit bulb with its application. Now that you have the background on why I need the light bulb loads I wanted to show you a neat way to make these light bulb loads in case you can use one in your own shop.
Here is the material that will be used to make a couple of light bulb loads. Since my Statim Test Bulbs are already constructed I have decided to rebuild a couple of other 60 Watt bulbs that I use for other circuit testing. Light bulbs make excellent circuit loads for certain applications. They are fairly safe as the heat produced is isolated from the bench and the light they produce is an immediate indication that they are doing their job of loading the circuit. The limitations are that they are not a constant load and their actual resistance varies considerably depending on the temperature of the filament.
Here we have a couple of regular incandescent 60 watt bulbs, some alligator clips, pieces of Pomona Test Lead wire, and a section of 1 inch inside diameter PVC. I have marked the PVC with cut points at 1.25 inch intervals. Here is what the PVC should look like after cutting.
The next step is to solder the red wires to the bottoms of the bulbs. A higher wattage solder gun works best as there is a fairly good heat sink that has to be overcome.
Next about a half inch of insulation should be cut from one end of the black wire. You can see an example in the picture above. The aluminum screw threads of the bulb will not accept solder so we are going to use the tight fit between the bulb threads and the inside of the PVC to make the connection. Thread the red wire through the PVC and position the bare end from the black wire so that it is pinched between the PVC and the bulb threads as indicated in the picture.
Once the bulb is seated into the PVC it makes a very good solid connection with the black wire. The assembly of the units can now continue with the installation of the alligator clips to the wire ends.
The PVC is a great insulator and isolates the line voltage so that the load bulb is safer to use than it otherwise might be. If you want to be even more cautious the bottom of the PVC can be covered or sealed in some way.
Just a word of caution that has been repeated many times on this forum. Line voltage is very dangerous to work with and needs to be treated with great respect. However as technicians and engineers we often need to work with it to do our jobs. In many circumstances the load bulb provides a fairly safe way to monitor a line voltage output and provide a load to a circuit under test. If you are working with line voltages please be safe.
The PVC that I used is One Inch ID PVC and is readily available at hardware stores here is the US where we are hopelessly locked into our inch-foot measurement system. I am not sure if the same sizes are used in the rest of the world or if those of you under the metric system will be able to find a metric sized PVC that works as well at the 1 inch ID does. Let me know as I am curious about this.
John
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