> 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?
Since you'll probably be looking for more than ten microfarads, your best bet is aluminum electrolytic capacitors. These are made in non polarized versions, which is what you want. You'll want a voltage rating at least as much as the peak voltage from your transformer. A 24VAC transformer would give 33.6 volts peak*, so a 35 volt capacitor would just barely be adequate - I'd use a 50 or 63 volt unit. They're available from lots of vendors, such as Mouser and Digikey in the United States. > 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? Right. The MAX628 contains two buffers, one inverting and one non-inverting. So if you feed the same square wave to both inputs (pins 2 and 4), you'll get opposite-phase square waves out of the two outputs (pins 5 and 7). That way, one end of the filament will be at the supply voltage while the other is at ground, then they'll swap, so on average the filament won't see a voltage gradient. With this lashup, you don't need the capacitors, but you may need a resistor if your supply voltage is greater than the filament voltage (this is common). Even better, the filament will have an average voltage of half the supply voltage, so if you turn off anodes and grids by pulling them to ground, they'll be negative with respect to the filament, which will give a good solid turn-off. If you pull up the data sheet for the LM9022 VFD filament driver chip, there's a nice circuit diagram and description of how this works. http://www.ti.com/lit/ds/symlink/lm9022.pdf We'd just use the LM9022, but it is no longer made, which is why the Kosbo circuit creatively uses what is intended to be a MOSFET driver chip to do essentially the same thing. Notice also that the LM9022 data sheet shows how to make the filament driver do double duty by adding a voltage multiplier to also provide the anode/grid voltage - nifty! I am planning on trying this with the MAX628, but haven't gotten around to it yet. > 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? Note, if you try this with a PIC, AVR, or Arduino, it's easiest to just use a PWM command to get a signal out, instead of doing it programatically by setting an output high and low over and over (that works too, but it's unnecessarily complicated). * An aside about AC voltages - the voltage out of a 24VAC transformer is about 67 volts peak to peak, which can surprise folks who aren't aware of it. It's easy to whomp up a power supply with a couple of diodes and capacitors and come up with enough voltage to mildly overstress a 63V capacitor with a transformer delivering "only" 24VAC. If a transformer is lightly loaded, it can deliver more than its rated voltage, so you can easily come up with 80 volts. - 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.
