very informative, thanks. I look forward to the next two installments!
On Thursday, September 24, 2015 at 3:52:06 PM UTC-4, gregebert wrote: > > [Forking a new thread from the partially-lighted nixie] > > I've used 3 different linear (ie non-switching) HV supplies in my clock > designs. I'll describe the basics of each in separate posts. > > #1: Voltage doubler. My first nixie clock has no transformers, so I use > the AC line directly (120V rms, 60Hz in USA). The real trick here is to > make sure the design is fail-safe. When the AC-line is rectified, it > produces about +170VDC. Nixie-tube datasheets, such as the Burroughs 5092, > specify a *minimum* anode voltage of 170, so that leaves no margin. With > a voltage-doubler, you get +340V and that gives plenty of margin, though at > the expense of more wasted energy (heat). If your tubes run at 2.2mA, > running at 340V instead of 170V will waste an extra 375mw per tube, or > 2.25W for a 6-digit clock. You will get reliable tube ionization at 340V, > and you will also have less variation in tube current as the tube ages, or > line-voltage variation. It's important to keep tube current in the correct > range, otherwise lifetime will suffer. To select the right anode resistor, > > calculate R = (AnodeSupplyVoltage - OperatingVoltage)/(OperatingCurrent). > > For an IN-18, I've measured the operating voltage 137V at 5mA of current. > Tube-voltage does vary with current. > In this example, R = 40.6K-ohms for a 340V anode supply. > > Referring to the attached schematic, the half-wave voltage doubler uses 2 > diodes (D6+D10, and D2), and 2 capacitors (C5 and C1). On the negative 1/2 > cycle, C5 is charged to the line-voltage. On the positive 1/2-cycle, C5's > voltage is "added" to the line-voltage, and you get twice the AC-input > voltage charging C1. There are also triplers, quadruplers, etc that can be > made. > > Safety is very important, so that's why there are so many 'extra' > components. Obviously fuses, and I put one on each AC line for redundancy, > and also in case hot/neutral get swapped. Resistors R15 and R16 are > actually 33 ohms each, and they have 3 functions: (1) limit inrush current > so the fuses dont blow when the caps are initially charged, (2) create an > RC low-pass filter along with C3 so that AC line-noise doesn't cause the > clock to "skip" time, and (3) act as redundant fuses. Hard to explain > without a picture of the PC board, but the fuse-clips are exposed metal and > it's possible a short could happen "before" the fuse. The varistor and C3 > suppress voltage spikes, and if spikes are particularly nasty enough to > endanger the capacitors, the fuses will blow. C3 is rated at 1kV; C1 and C5 > are rated at 450V. So why are there 2 diodes (D6 + D10) ? In case 1 fails > as a short, it will prevent AC from causing a catastrophic failure of C5. > BTW, bridge rectifiers dont have this vulnerability; they simply short-out > when a single diode shorts-out. > > Unique for this clock, there's R21 to current-limit the AC line on the PCB > to ~120mA before it's sent to the NE2 bulbs that I use for the colons; just > extra paranoia in case something causes a short on the PCB. Those NE-2s > each have a 220K series resistor. You can see 6 anode resistors (R7-R12). > R3 & R4 form a voltage divider so I can get the 60Hz line-frequency > reference; this gets clamped by diodes D3 & D4 before it goes to the > 400-series logic for the clock divider. The remaining devices (D5, R2, Z1, > Z2, C2) provide +10VDC for the CMOS clock logic. Why 10 volts ? It's plenty > enough to drive the NMOS driver transistors for the nixie tubes (all 45 of > them because it's direct-drive), and also improves noise-margin. > > As I mentioned, this clock has no transformer. Part of the reason is to > reduce power-consumption, but also because I had this silly idea that you > can reliably operate CMOS devices off the AC line with sufficient > circuitry. But, there's no isolation, so just about anything on the PCB can > give you a shock. > > > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/35ac895c-0a01-4c25-931d-beefba9caa8c%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
