Hi Prashant,
> >The drive is now referenced to the 'ground' potential and consists of similar
> >segment and digit drivers. They are made to work at 0 .. 12V positive these
> >days while in the past the voltages were higher.
>
> I hope it is a 12V display. Anything higher would be a pain to use.
The FUTABA displays you have will most likely work fine with 12V.
> >You need to have some form of intensity balancing in long displays if you
have
> >a DC fillament voltage as the drive voltage varies from 12V at one end to
>
> It may be worth driving the filament through some kind of an H bridge.
H-bridge won't be much help except to swap the polarity and not much need
of that on a small display. What would work nicely is to have a regulator
that supplies 5V across the fillament and then has a floating ground
that you can adjust to a value from 0V to 5V, you could then choose
the fillament reference at 4.5V and make sure the fillament is at
that voltage infront of the digit you are displaying by adjusting the
'ground' reference. Digits at the higher voltage end of the filament
will be close to 4.5 V if low end is near ground and digits at the
low voltage end will be near 4.5V if you raise the ground to 4V and the
other end will be 9V. This would give you an even digit brightness
over the whole display with equal multiplex time. You must not go below
0V or else the Anode/Grid voltahe will be more positive and turn segments
on weakly.
The multiplex duration intensity control is something I thought of, I have
not seen it used and the characteristics of the Phosphor may preclude
the use of it as brightness may be a function of voltage only and
not time, I may try it out one day. The above sliding ground
method should work but means that you need a analog signal for the
fillament supply regulator ground reference.
> >The drivers on a Futaba type LT display I have are NE594 and have 8 High side
> >drivers with TTL inputs. With your 5 digit display you would need 5 + 8
> >drivers if you want to use the decimal points and the colon, just the
> >numbers would need 4 + 7 so you will need 2 ICs 8 or 7 drivers per
> >IC would be enough. You could suffer the high side drivers with 2
transistors
> >each but a lot more work.
>
> With 5 + 8 drivers, I would be using up all the PIC 16C84 I/O lines. A
> 1-of-n decoder for the digit drivers will save 2 lines.
If you leave the colon permanently on or off you save one line. If you
decide to use a BCD-7seg decoder you save 3 more lines, if you ignore
the decimal points you save one more so you can get away with 2 + 4
on the PIC, you still need 4 + 7 for the display.
> How much current does the grid and anode take? An alternative to high side
Not sure, at a guess Grid 0.1 mA and all the Anodes together say 1.0 mA.
{Quote hidden}> drivers could be using open collector TTL gates with pull-up resistors. A
> 7405, depending on the max voltage it can take, can be used to drive 6 lines.
> That would mean 3 of these chips. Other possibilities are an LM339(4 lines,
> 4 chips) or CMOS gates operating off a 12V supply with a 0-5V signal on
> their inputs. The decoder could be an extra chip unless I find an open
> collector decoder like the 7441 or a CMOS decoder. I wonder what the input
> characteristics of CMOS gates operating on a 12V supply are. If it is
> suitable, I could use 2 hex inverters and one decoder(3 chips). I might even
> be able to cobble together some kind of an H bridge for the filament with the
> spare gates.
You cannot use 5V levels to switch CMOS inputs running at 12V, the threshold
is near 6V so you would need to use a level shifter.
SPRAGUE and other no doubt have decoder driver IC's part number
UCN4805A might work, it sinks current so I'm not sure if the polarity is
correct but it has a latching 4-7 line decoder and an extra driver for
the decimal point and can work upto 60V. Polarity is the only concern
and you will probably need pullup resistors. A UCN4807A is similar
but has a 3-8 line decoder instead. With these two ICs you could
probably interface in minimum mode with 4 + 2 lines or in maximum
mode with 5 + 3 lines. I don't have datasheets so cannot verify that
they will work...... Having a better read of the section, they probably
won't work as VF displays are not mentioned but LED & filaments are.
The polarity of all the segments would be wrong :-(
Sprague do also have plain drivers, decoders required extra, with
6 and 8 gates per IC with numbers in the range UDN6116 UDN6118 UDN6138
The UDN6118 is probably the same function as the NE594.
Cheers
--
Kalle Pihlajasaari .....kalleKILLspam
.....ip.co.za http://www.ip.co.za/ip
Interface Products P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750 Fax: 402-7751 http://www.ip.co.za/people/kalle
DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer
