I have a project that I designed to work with an standard LCD. It
works fine on a development board and on the prototype PCB.

I decided I wanted to change the display to a fluorescent type
(Futoba M202SD08GL in serial mode).

On my development board the original circuit with VFD also works
perfectly, though the 7805 regulator runs very warm to touch even
with a heatsink.

However when I connect it all up to the prototype board something
nasty happens, the 7805 (1A) regulator gets extremely hot to the
point of starting to give off a 'hot smell', and nothing works.

The mains transformer is capable of 4A  (it needs this to run heating
wire) and the bridge for the controller circuit is rated at 1.5A.

I first suspected that the tracks on the PCB may not have been up to
the current required by the VFD (1A surge and  340mA running), but
the tracks are a minimum of 0.254mm with the largest 0.865mm so they
should carry between 1.5 - 3A.

This leaves as suspect the 7805 regulator as being not up to spec, or
possibly the main capacitor, which is an aluminium type scavenged
from an old Philips mains shaver. I'm wondering whether the cap could
have a high ESR and producing the high resistance type conditions at
350mA current.

The voltage at the input to the regulator drops from 12.5v to 10.5v
and the output drops from 5.02v to 4.35v when the VFD is connected.
So something seems to be acting like short/low resistance.

Does anyone have any thoughts on this problem?

Colin

--
cdb, cdb@barnard.name on 31/03/2002

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