"Patrick" bravely wrote to "All" (24 Apr 05 08:47:42)
--- on the heady topic of " Measuring current with LM3914 ?"

Pa From: Patrick
Pa Xref: aeinews rec.radio.amateur.homebrew:9454

Pa Hi John,

Pa Thanks for the answer and suggestion.

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

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

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

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

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

Pa Did anyone play with this ?


Patrick, optocouplers are as sensitive as the light from the internal
LED. I'd roughly estimate the lowest end LED current at about 0.1mA...
but would have to count on at least 1mA to be sure it was lit.

I'm looking at a spec sheet just now and that is roughly ballpark,
since the graphs start at 0.1mA. The one I'm looking at starts the LED
current vs collector current at 0.3mA vs 0.1mA. That is about 3 times
less than I expected. Keep in mind a minimum 3 volt LED voltage drop.
The isolation voltage is pretty good at from about 1500 to 3500 volts.

BTW I really like your brainstorm idea of using a Hall Effect device
but I don't know what the typical sensitivity of these can be.

I think the sure way is to have a floating source of voltage to power
the sending circuit, either a little battery or an ac transformer with
rectifier supply.

A*s*i*m*o*v

.... I think my learning curve is a circle!