I would recommend "attacking" this problem in two ways.
The first is to arrange all current metering so it takes place at
the lowest potential point possible referenced to DC and RF ground.
In a DC grounded screen circuit this point is the cathode, which for the
4CX1500B
would be at about -250V. This will not stress the insulation at normal
operation,
and if suitable means for limiting fault voltages are used, voltage excursions
during an arc or flash-over can be limited to +/- 1000V or so.
If operation of the whole LED metering board at -250V can be tolerated, it would
be a simple matter to use an isolated DC-DC converter to power the board, but
I personally have some misgivings about "floating" a whole metering board.
Another workable solution can be to use linear FET optocouplers in the circuits
(the H11F2 comes to my mind), connected across suitable current shunts and
protective diodes. The H11F2 and its "siblings" have 5000V insulation, and the
flash-over voltage is mainly limited by circuit-board design.
A drawback is the non-linearity at the beginning of the transfer function, as
the optocoupler LED will not light at voltages less than about 1,3 V, requiring
the shunt resistor to be dimensioned
so the current required to light the first segment will create a voltage drop
of 1,3 V.
At the output side of the optocoupler op-amps can be used to scale the current
from the FET into
more suitable voltages and/or currents to feed the LED bar graph displays.
And, as previously suggested, Hall-effect current sensors, at low-potential
points of the circuit,
also can provide isolated DC current readings. The scale factor and sensitivity
aspects could suitably be taken care of by op-amp scaling networks.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Patrick EGLOFF" <pegloff@gmail.com>
To: "Carcia, Francis A HS" <francis.carcia@hs.utc.com>
Cc: <amps@contesting.com>
Sent: Saturday, April 23, 2005 8:53 AM
Subject: Re: [Amps] RE : Led display problem
Hi All,
Thanks for all answers to my request. Nice feedbacks ! I think i found a
nice "hamspiritic" list.
Abt the utility of bargraphs compared to analog meters, i think that
after initial test during construction, they are better.
In the battle field, you don't need to measure with a high precision but
only "tune for maximum smoke" ! I don't care to measure my HV or current
with a high precision, they are always varying !
With LEDs you have no inertia and can easily measure peaks. It's far
better to put more protections to take care of the tube and operator.
Another good reason is that i broke several meters during
transportations... :-(
At last, it's a nice brainstorming ...
But, that's only my opinion.
Technicaly spoken :
Well, it seems that there is no simple solution to my problem. The
easiest is the solution produced by Allegro inc.
http://www.allegromicro.com
They produce very nice hall current sensors, but only for high currents.
The smallest one has +/- 5A range with a 800V isolation. All in one
small chip !
The problem is that it has a 133 mV/A sensitivity...
So, i have to switch to a 4CX100000 at least !
The nearest usable solution is to use a IL300 opto coupler that has a
very linear transfer by using a feedback control. But this solution
request at least one OP amp on the "hot" side. The "cold" side is
isolated at 5300 V max for 1 s.
Better than having all the display board with a HV rail but still not
perfect !
I continue to look for a hall effect sensor that has more sensitivity.
Several companies are producing hall detectors, but you have to build
you own core.
Anyone has a solution to build a current sensor as suggested by Mark
Moss on this list ??
73, and thanks again.
Patrick TK5EP
http://tk5ep.free.fr
Carcia, Francis A HS a écrit :
> Hall effect is an active current transformer device so it would be used to
> measure current.
> I think digi key sells them. It converts current to volts. The device fits
> over an insulated wire for high voltage isolation. It will measure DC
> current unlike a current transformer that only measures AC current. A simple
> panel mounted DVM could be scaled to measure current and be isolated from
> high voltage. gfz
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