Notes on the B7971 tubes MDB Ventures is offering When operating as a clock, a maximum of seven segments and a minimum of 2 segments will be used. This limits the dynamic range requirements of the gas ionization. More segments being on means greater problems with evenness of segment ionization. This can be compensated somewhat with more current which can hide the problem to some extent. Also with more segments on, there is a greater likelihood of leakage to other segments causing them to partially ionize. Thus for that operation lower voltages should be used to minimize the possibility of ghost ionization. The tubes are rated for 170V supply and are rated to have an end-of-life with the ionization threshold at 170V. So you would not want to go lower than 170V. The normal run voltage is 140V once the ionization occurs. Once one segment has been ionized, the others will trigger at lower voltages due to plasma leakage from the ionized segment(s). How much this occurs is somewhat variable and will depend on a multitude of factors, including the number of segments on, the closeness of the segment in question to already ionized segment(s), the amount of usable gas left in the tube and the temperature being primary effects. Using the tubes for letters as well as numbers will require more segments to be on for some letters. This will cause more current to flow through the ionized gas in the tube. This causes two effects. One is that there is more ionized gas in the tube making it more likely for a leaky segment to partially turn on. The other is that the life of the tube is primarily determined by the amount of gas left in the tube that can be ionized. The more gas that is ionized, the quicker it is used up. The less available gas that can be ionized, the higher the turn on threshold voltage until the threshold goes above the available supply voltage. (That is why neon bulbs flicker as they get old, the ionization threshold of the gas has risen to the available voltage being supplied to the bulb, so sometimes it can turn on and sometimes it can't as the supplied AC voltage waveform cycles between 0v and the peak voltage. Hint: you can "fix" this by adding a 0.01uF cap across the lamp's load resistor. The capacitor charges up and gives a boost to the voltage supplied to the lamp.) The absolute max and suggested currents for the segments is: K1, K4, K15 : max 6mA - suggested 1.5ma K7, K9, K11, K13: max 5.5ma - suggested 1.25ma K10, K14: max 3.5ma - suggested 0.5ma K2, K3, K5, K6, K8, K12: max 5ma - suggested 1.0ma maximum total current allowed at anytime through the tube 21ma Max leakage on any "off" segment 5ua. mimumum segment current 50ua. Below the minimum segment current, the segment may or may not ionize. However the ionization may not be evenly distributed across the segment at that low of a current and the glow will be very dim. When looking at the tube, you can easily identify the anode pin. It has a small glass tube around it inside the tube (don't confuse this with the ceramic spacer on the pin on the other side of the tube). The small glass tube around the pin wire keeps the localized gas near the anode pin from ionizing due to the closeness of the adjacent segment pins. --- The driver boards have not been tested. We have no way to test them. As stored, the boards have the tubes installed. Prior to shipment, the tubes are removed from the boards, cleaned and tested. The boards are shipped without the tubes installed. The tubes are shipped separately packed to reduce the potential for breakage. First we do a visual check on the tubes to make sure they are not broken. The electrical testing consists of applying 200VDC through a 22K ohm resistor to each segment to verify that they ionize properly. The tubes will then be wrapped separately in packing material to protect them during shipment (rather than installing them back in the boards as they were originally packed). Our experience with these tubes is that they are in good condition. I know of one clock that was built with tubes from this batch that has been in operation since 1978 (from the same estate where the boards came from). I know nothing of the driver boards. People who have bought these boards buy them for the sockets and tubes. So I have no information on the status of the driver electronics. When using these tubes, be sure to keep on the low side of the stated specs of the tube (no more than 50% of maximum rating). Perferably no more than the currents noted above. I've seen some clocks that have failed early because they used too much current (to get a brighter display). Remember that these tubes have become nearly impossible to find, so you will probably not be able to find replacements once the tube wears out. The various segments have different current requirements, so make sure you deal with that as well. If you are operating this only as a clock, you can increase the supply voltage as much as 25% to 50% (200V to 230V) with an associated increase of the series limit resistors for the segments to keep the current in the desired target area. This gives you longer life as the end-of life ionization voltage is higher. Usage of higher voltages will work, but may cause uneven ionization of the segments and a greater chance of non-active segments ionizing due to leakage currents. Operating at lower current also significately increases the life of the tube. An average current of 1mA per segment (adjusted to compensate for segment size) works well with clocks. You can increase this to 2mA to get more brightness, but with an associated reduction in life of the tube. At 1mA, the tube will last over 50,000 hours (this is the current that the aforemented clock operates). Operating at currents below 1mA may cause segment ionization to be uneven. Another aspect to keep in mind with your design is that you should not leave a segment floating or it will pick up leakage currents and turn on partially. It is best to leave an "off" segment at a voltage somewhat below it's turn on threshold (about 50% is a good selection so 90V would do nicely). The normal run voltage of an ON segment is 140V. However this varies from segment to segment and rises as the tube ages (as the neon gas gets used up). These tubes require the anode to go to the positive side of the supply and the segments to go to the negative side of the supply. Note: One thing to watch for is that because of the high voltage these tubes operate at, the tubes tend to collect organic dust from the surounding air (like what happens on old CRT based TVs and monitors). That can cause the display to dim over time due to the gunk that collects on the tube. But also, the gunk can collect around the pins of the tube resulting in a leakage path that can cause partial ionization for inactive segments. You should keep unionized segments (even the segments you aren't using) at the 50% hold voltage noted above. Also it helps to keep the tubes in a sealed case to keep the dust from getting onto the tubes. Another thing to keep in mind is that as the tubes get used up, there is a slow metal migration to various parts inside the tube. The most common cause of failure is the metal migration causing an internal short between segments. When this happens, the two shorted segments will ionize when either one is turned on. This problem is usually the cause of the end-of-life for the tube. I have heard some people try to clear the short by applying short pulse of a high voltage between the two shorted segments. If it works, it can possibly extend the life of the tube by a few more hours. You should keep the end-of-life short situation in mind when designing your circuit. You should design it with the expectation of a short in the tube which can potentially damage your circuit if it is not designed to allow for a short to occcur between the various parts of the tube, including between the segments, and between the anode and cathodes (segments). You will also see some metalization occuring on the inside of the tube glass. This is usually most pronounced near the edges of the display where the metal is closet to the glass. This is a normal effect from the operation of the tube and does not affect the operation, although a well used tube will eventually start to darken as the metal migration will eventually collect on the face of the tube as well and darken the display. There is nothing really that you can do to fix that. However, typically before that becomes a concern, the tube has already reached end-of-life.