Hi John,

Thanks for the answer and suggestion.

I have the LM3914 datasheet and since my first post, i've had a serious
brainstroming about the way to do that.

The best solution is to use a HALL current detector, then the rest of
the measure can be completely isolated. ALLEGRO makes very nice small
things but only for higher current. The smallest is for +/- 5A with
133mV/A sensitivity.

Otherwise i could use an OP amp followed by an optocoupler HCNR200 or
IL300 with a feedback. Linearity is then almost perfect But i still have
to supply the OP amp on the "hot side". The display part would be
completely isolated.

Last guess is to use as you suggest an opto coupler via a shunt on the
hot side. Nothing to supply and everything is isolated. Best is a FET
opto coupler like the H11F1

The problem is to know if i will be able to measure low currents. The
screen has max a few ten of mA. At what current will the opto coupler
start to light and what the transfer is?

Did anyone play with this ?

Patrick.


John Walton a écrit :
National Semiconductor has a high voltage application note in the PDF which
describes the product.

Or you could use an opto-isolator (an analog version like the HCNR200) to
read the voltage across a shunt. You will need a drive transistor because
the usual opamp suspects won't be able to provide enough current for a LED
bargraph.

"Patrick" wrote in message
...

Hi all,

I started a new amplifier project, a 4CX1500B 2m amplifier.
Wanting to make it a bit more state of the art, i want to replace the
old meters by Led bars...
The simplest way to do that is to use a LM3914 bar display which can
measure voltages with a ground reference.

My design will use a cathode driven, DC grounded screen. So measures
must be make "floating" as the ref is the cathode .

How can i achieve this in a simple way ??

73, Patrick TK5EP
http://tk5ep.free.fr