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perro
Frequent Contributor
Username: perro
Post Number: 76
Registered: 10-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 04:26 pm: | 
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Hi,
I acquired some Russian IV-18 VFDs ('cos they look fun). It seems I need a grid supply of 50-70V.
A few searches suggest people have successfully driven these with PICs and used "charge pumps" to achieve the required drive voltage.
Unfortunately, I have no experience with charge pumps (pulsed inductor, capacitor type arrangements?), and I can't find sufficient detail to give me the confidence to try this out (I only have a limited number of displays).
Does anyone have a firm opinion (or, better, experience) as to the best means of generating this sort of voltage?
If a charge pump approach is suitable, can anyone recommend a good website that has info on how to design & build such things.
I hope this isn't a dumb question, but I have no experience of driving any kind of tube, or of mixing microcontrollers with higher voltage components (except via relays or other isolation).
Thanks |
muskrat
Frequent Contributor
Username: muskrat
Post Number: 154
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 04:57 pm: |
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I will be monitoring this with interest. I also have a number of VFD displays I'd like to play with, but don't know where to start!
One in particular is an audio meter unit - VERY classy. I'd really like to incorporate it into an audio project. |
steerpike
Frequent Contributor
Username: steerpike
Post Number: 462
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 05:36 pm: |
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This page has some appropriate circuits using Cockroft Walton multipliers. These are very easy to follow & build, but use quite a lot of componens if the voltage multiplication needs to be large.
An NE555 astable can generate the AC input.
http://www.voltagemultipliers.com/html/multcircuit.html
More exotic are the inductive flyback boosters shown here:
http://www.dos4ever.com/flyback/flyback.html |
muskrat
Frequent Contributor
Username: muskrat
Post Number: 155
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 06:35 pm: |
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And now for one of the dumb questions I am (in)famous for:
How would I go about determining which pin does what on the VFD without destroying it in the process? |
magnum4
Frequent Contributor
Username: magnum4
Post Number: 787
Registered: 04-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 06:59 pm: |
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Have you got a type number Brian??
Also look for 34063 circuits. They are used a lot in nixie tube drivers.:-)
Regards,
Jim
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magnum4
Frequent Contributor
Username: magnum4
Post Number: 790
Registered: 04-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 18 January, 2010 - 07:21 pm: |
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Here is a link to a page. If you scroll down to the very bottom, there is a link to a diagram, it includes a circuit for a 34063 based 60V supply.
http://elbastl.sweb.cz/clock.htm
HTH
Regards,
Jim
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perro
Frequent Contributor
Username: perro
Post Number: 77
Registered: 10-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, 19 January, 2010 - 09:51 am: |
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Thanks magnum4 & steerpike.
That's great! I'll read in detail later, but this info seems to be just what I was looking for.
I think the flyback approach is cool, but I'm guessing maths & timing are fairly crucial ;-) It's always nice to learn something new...
Thanks, again, for your kind help.
Cheers |
stylers
Frequent Contributor
Username: stylers
Post Number: 173
Registered: 04-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, 19 January, 2010 - 10:14 am: |
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what I have done with one of my clocks is to use an MC34063 switcher chip in step up mode, using a small torodial inductor, and a fast schottky diode to produce the 30V or so I needed. then I wound a few turns round the inductor to produce the low voltage filament supply, and tied it to GND using a zener / capacitor arrangement. this allowed to get the small negative bias to turn off the anodes/grids completely, and removed the problem of the brightness gradient when using a DC filament supply. I used a PIC driving PNP transistors, with the +30V supply being connected to the pic +5V supply (the positive rail then being GND), effectively creating a -30V supply. this allows the anode/grid pulldown resistors to be a fairly high value, and reduces the dissipation of the circuit considerably, in comparison to a normal circuit layout. there's a calculator for the MC34063 on the web somewhere, and I can post up my circuit if anyone would like.. |
perro
Frequent Contributor
Username: perro
Post Number: 78
Registered: 10-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, 21 January, 2010 - 09:23 am: |
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@stylers
That's an interesting idea. I've read of the 'gradient' issue with DC, and it seems a common approach is to reduce this by using an AC supply, rather than DC, but I haven't seen your idea anywhere else.
If you're still happy to post your circuit, I'd be very grateful.
Many thanks. |
microguy
Valued Member
Username: microguy
Post Number: 13
Registered: 01-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, 22 January, 2010 - 02:28 am: |
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I've had some really nice VF displays in the past, but I never got them to work.
I do know, Pico is a company that makes the little round transformers that generates the power needed to get these things up and running.
The circuit itself is a pretty simple inverter. You might be able to purchase a cheap device that uses a VFD and use that Pico (style) transformer, and trace out how it's connected.
Good luck with this one! (they are nice looking displays, I've always liked them) |
zeitghost
Frequent Contributor
Username: zeitghost
Post Number: 1414
Registered: 01-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, 22 January, 2010 - 08:25 am: |
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I've got an OKI driver circuit in a notebook somewhere from when I was fixing these things for a living.
When I say fixing, I mean taking the glass off one & putting it on another.
And or replacing the OKI driver chip. |
stylers
Frequent Contributor
Username: stylers
Post Number: 174
Registered: 04-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 11:53 am: |
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Hello All,
I've drawn up a bit of a general schematic of the circuit I've used for driving multiplexed VFD's. The general idea is very similiar to the way its done commercially. most of the microcontrollers (TMP470Cxxxx etc) used in stuff with VFD's have open drain P-channel drivers arranged like this. the pulldown resistors if external are usually in SIL packs. usually the SMPS used in VCR's etc provides the -30V and ~ 2V AC heater supplies. the zener/capacitor ties the filament/heaters to float above the negative supply, and this ensures that the anodes cut off completely when switched off.
here I've shown a 4 digit display, and the HV generator. the advantage of this circuit is it only requires one transistor and two resistors per grid/segment. the pulldown resistors can be connected in SIL resistor packs at the pins of the display.
I used the web calculator at..
http://www.nomad.ee/micros/mc34063a/index.shtml
for the MC34063 to get rough values for the components but I found I had to play with them a bit to get it working right. The transformer shown in my schematic is actually a torodial inductor with another winding added, just a few turns center tapped. Note that this will of course affect the inductance and hence output voltage, so you may have to compensate and adjust component values a bit. don't expect to drive a huge display filament, as you'll have to design a proper inverter then :-). Also, the center tap may not be entirely necessary, but it will help to keep the brightness even across the display.
Individual displays will vary, but I find that you need just enough current to make the filament wires glow a dull red. So you will have to experiment a bit if you've got a display with no data for it. The resistors in the filament/heater supplys are used to limit the current when cold, and allow adjustment. they may not be entirely necessary. The zener voltage can be chosen to suit the grid cutoff voltage needed, but 3V or so should be ok.
The PIC just multiplexes the display similiar to driving a common anode LED display, so the pins must be driven low to turn on a grid/segment. I found it necessary to have a delay between turning off one grid and turning on the next, to allow for the RC delays of the wiring and display, otherwise you will get shadowing from one digit to the next. If the decay is very long, maybe the resistors need to lowered in value, but this will also increase the current consumption from the HV circuit. I've not shown the rest of the PIC gubbins, its just a general schematic.
It may be the case that you need to go higher (lower ?) than 30V to get the required brightness, if you do make sure that the transistors can handle it.
There are some maxim IC's suitable for driving VFD's but they are again shift registers and might not be as easy to use as a direct drive.
stylers.
p.s. sorry attachment is a zip, it actually contains a PNG from eagle and its bigger than the allowed image size here..
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perro
Frequent Contributor
Username: perro
Post Number: 85
Registered: 10-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, 29 January, 2010 - 11:14 am: |
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@stylers
Thank you very much for posting that. Most appreciated.
Cheers |
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general VFD PIC display driver schematic
