As I continue to study this stuff, I have more questions...Thanks to all for your patience and generosity in answering them...
In terms of the capacitors, is there a type you recommend if I am going to go the capacitive reactance route to step down the AC? Besides using a non-polarized one, is there any material that is preferable to others? If so, any particular recommendations from online sources? I also wanted to revisit the use of the MAX628 to get AC from DC for the filaments that was offered as a suggestion earlier in this thread. I know I originally asked for a non microcontroller based approach, but I am open to that or the use of a 555 or a PIC/ATMega. I looked over the Kosbo circuit diagram and the datasheet. I gather I feed a square wave into pins 2&4 of the MAX628 and connect 5&7 to the filament? This is all still pretty new to me, and although I know what a square wave is, especially after playing around with CMOS and 555 based noisemakers, could someone spell out in detail how I would use the MAX628 with let's say a 5V DC input and either a PIC/Arduino or 555 to get AC out? Best, Dylan > > > I am also pretty unfamiliar with capacitive reactance but did some > googling after your reply. I will do some continued follow up, but if you > have a second to look at this attachment, is the idea here the same concept > as the section related to the capacitive transformerless PS? > http://www.electro-tech-online.com/custompdfs/2004/09/00954A.pdf I > realize I am using a transformer, but I figured the step down concept was > the same. > > Yeah, it's essentially the same idea. Luckily, for VFD filament supply, > you have a constant load and don't need regulator circuitry, just a fixed > drop. > > Let's assume we have a 24V transformer, and a VFD that has a filament that > takes 1.5 volts at 150mA. What size capacitor would we need? The first > step is to figure out what value to use if it was a resistance. It would > need to drop 22.5 volts (24 - 1.5) at 150mA. Ohm's law says R=E/I. Note > that I do all my calculations in basic units (volts, amps, ohms, farads) > and apply milli- and kilo- and so forth later. So 150mA is 0.15A. Back to > R=E/I, that's 22.5 volts divided by 0.15 amperes, for 150 ohms. Now to > calculate a capacitor that would have 150 ohms of reactance. The formula > for capacitive reactance is X = 1/(2 pi f C). Rearranging to solve for C, > I get C = 1 ( 2 pi f X). As I live in the United States, f is 60. So I > need 1 / (2 x 3.14 x 60 x 150) or 0.000 017 7 farads, which is 17.7 > microfarads. > > My original idea was to make a symmetrical circuit, with the filament > completely DC isolated from the transformer: > > http://www.vitriol.com/images/tech/vfdps1.gif > > I've deliberately ignored most of the details of the VFD drive, as this is > just a power supply illustration. > > Note that in this case, two capacitors are used, effectively in series, so > they would each need to be half the reactance, which is twice the > capacitance, or about 35 microfarads apiece. Then I realized I didn't > really need to isolate the filament, I could just decide that one side of > the transformer winding is "ground", and reference my grid/anode supply to > that: > > http://www.vitriol.com/images/tech/vfdps2.gif > > This uses one 18 microfarad capacitor instead of two 35 microfarad ones, > which is nice. I used a different version of the voltage doubler for the > anode/grid supply too, but this seems like it ought to work. > > Note that you'd probably want a third supply to power the logic, there are > a few different ways to address that (more if your transformer offers a > center tap). > > - John > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To post to this group, send an email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/neonixie-l?hl=en-GB. > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
