Testing Photodiode Tubes 24 May 2012 Mike at MDBVentures.com http://www.MDBVentures.com - Great prices on great tubes! There are two basic types of photodiode tubes, vacuum and gas filled. The vacuum photodiode tubes can handle higher voltages. They are usually operated from a 250 volt DC supply, although they can function at lower voltages with reduced dynamic range. Vacuum photodiodes have a wider frequency response and better linearity. Gas filled photodiode tubes are the other style. These operate at lower voltages and have higher sensitivity and gain. They are generally used in audio work, simple light beam interruptors or daylight detectors (for street lamps). Don't confuse photodiode tubes with photo resistors or solid state photodiodes. They all essenially do the same job, but use different technologies. A photo resistor is a resistor that changes resistance depending on the light exposure. The advantage of the photoresistor is that it does not require a bias voltage to operate and has very high sensitivity to light. The disadvantage is that it has poor frequency response. A solid state photodiode is just a solid state diode that has been optimized to respond to light. All solid state devices are light sensitive. Actually, they are sensitive to all forms of radiation, which can create problems. Like photodiode tubes, solid state photodiodes require a bias voltage to operate. Photodiodes are more expensive to make and use than photoresistors, but they have a much wider range of operation. PIN photodiodes can operate up into the GHz range, which is why they are commonly used in fiberoptic systems and laser based applications. Photocell is a generic term that describes a sensor that is light activated. It covers the photodiodes and photoresistors. It is also sometimes used to describe a solar cell. A solar cell is a device that emits electrons when exposed to light. All solidstate devices (which rely on the PN junction to operate) will emit electrons when exposed to light. Solar cell devices are optimized for this work, primarily by providing a very large PN junction area exposure to the light. Solar cells can be used as photodiode detectors, but their frequency response is poor, so they are seldom used that way. Testing photocells: Use an incadescent bulb style flashlight. The blue/white LED based flashlights will not produce light in the region that most photo detectors are sensitive to (primarily infrared - aka S1 spectral response). Testing the photoresistor: Testing photoresistors is easy. All you need is an ohm meter and a flashlight. You will need to have the specifications for the photoresistor to know what the resistance readings should be for the device. Depending on the construction, the resistance can be as little as a few hundred ohms or as high as a several hundred thousand ohms. When you shine the flashlight on the photoresistor, the resistance should be reduced. Testing the photodiode tube: Testing photodiodes requires a circuit to provide the bias voltage. While photodiode tubes are designed to be used with 90V or 250V DC power, most will work fine at much lower voltages. For testing a photodiode tube, a 9 volt battery can be used as the power source. Here is an example test circuit: http://www.fourwater.com/files/testingphotodiodetubes.jpg Use a strong incadescent flashlight with fresh batteries. Attach the positive terminal of the battery to the anode of the photodiode. Connect a 470K ohm resistor between the cathode of the photodiode and the negative terminal of the battery. connect a high impedance ohm meter (10M or more - most modern digital meters satisfy this requirement) across the 470K resistor. Cover the tube with a dark cover to keep light from getting to the cathode surface. You should read less than 0.1 volt on the meter. If the reading is higher, the photodiode probably has a resistive connection between the anode and cathode. Possibly caused by dirt or other contamination, or the photodiode may have had excessive voltage applied to it causing it to arc-over leaving a carbon track between the terminals. Now remove the cover and shine the flashlight on the photodiode. Move it around to obtain the maximum voltage reading on the meter. You should see a reading between 5 volts and 8 volts. If you don't, the photodiode may be weak. This is usually caused by an air leak into the tube which destroys the vacuum for vacuum based photodiodes, or contaminates in the gas for gas based diodes (or the gas is completely gone if the leak is large enough. A photodiode that has been overstressed from too much current or voltage can also cause the photodiode to give weak readings due to the damage to the cathode material. Testing solid state photodiodes: A solid state photodiode can be tested using the same circuit, except you attach the cathode to the positive battery terminal, and the anode to the resistor. Everything else is the same. Tube testers: Some tube testers, such as the Eico 666 tube tester have settings to test a photodiode tube. All they are really doing is configuring the circuit as a diode emission test and measuring the current through the diode. When you shine the flashlight on the tube, more current flows causing the emission current reading to be higher. Note: It is probably best not to test photoresistors or solidstate photodiodes on a tube tester as the voltages used by a tube tester will likely exceed the maximum ratings for the photoresistor or solidstate photodiode.