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

On Mon, 18 Apr 2005 15:19:14 +0200, Patrick wrote:

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 ??

Without more thought on my part I can't say whether this will help,
but it sure won't hurt to look.

http://wd7s.home.att.net/ds-1.htm

Hi Wes,

Thanks for the info. I've looked on the pages you gave me and see that
all measures are made with a ground reference...

I will drop a mail to wd7s and ask some suggestions...

Thanks for the answer !

73, Patrick

Wes Stewart a écrit :

On Mon, 18 Apr 2005 15:19:14 +0200, Patrick wrote:


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 ??


Without more thought on my part I can't say whether this will help,
but it sure won't hurt to look.

http://wd7s.home.att.net/ds-1.htm

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

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

"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!