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Help! Display control board

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New Member
Hello, i'm planning to realize a board that could show on bargraph's display some data of my amplifier.

i need to control:
0-1A plate current
0-100mA grid current
100-0-100mA tuning
0-30microA output power

i'm thinking to use I/V converter and LM3914 for drive led's.
Any idea of built this board?any suggestions?



Well-Known Member
It's kind of a pain to do a bunch of current monitors on a high voltage line with LED bargraphs. Are you sure you wouldn't settle for a moving-needle meter? Heck of a lot easier to implement, and more precise besides.

Problem is your power for the 3914 has to be isolated from the plate and grid voltage. There's a couple of ways to do it, you can use a hall effect current sensor, a special linear opto (which needs 2 supplies), or isolate the 3914's with a separate transformer and power supply for each.

With a moving needle meter, it's just wired across a shunt resistor. That's it.


New Member
of course with moving needle meter is simply to realize but is not so beautiful :D

i have a separate transformer for the control board, i have to separate them anyway?optocouplers like CNY-74-4 or 4N25 are not suitable for this function?
thankyou very much



what about a LED dot matrix module,
either built from individual LEDs, or a ready-made module, together with a PIC?

for instance the 16f884 would be powerful enough, and it also has A/D channels.

I do not know the type of visual impression you are looking for, maybe just a small dot matrix, or LED chains like in the picture, covering the whole front plate. a small (green) 5x7 LED dot matrix would be very decent.
do you need to know exact current? maybe also add a small 8x2 LCD.


Well-Known Member
That does look pretty cool.

Problem with the CNY-74-4, 4N25 and almost every other opto is they aren't linear. Look at the hook in fig. 8 on the spec sheet -
- that's the collector current vs. forward current. Worse, the gain isn't repeatable from unit to unit.

Still, if you are only making one, maybe you could hand-tweak it, and try to use the more linear part of that curve.
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