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Isolation between primary & secondary



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Old 28th February 2008, 17:14   #1 (permalink)
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Isolation between primary & secondary

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... a simple resistor isolator should work fine with such low
currents. 10K load resistor across each cell output (R1, R2, R3), then 100K or so in series with each meter
input (R4-R9).



I built one on a scrap of perf board, then potted it in epoxy in a 35 mm film can
so it can live outside the loop (only one cable gland thru the head!)
In a previous thread (http://www.rebreatherworld.com/home-...-meters-8.html) I suggested a simple resistance isolation circuit to prevent a flood in one display from keeping the other from working. I finally got around to testing it, using my bar graph analog PO2 meter (see http://www.rebreatherworld.com/home-...2-display.html) as the primary and a triple panel meter (see http://www.rebreatherworld.com/home-...-meters-7.html) as the secondary. Sensor voltages are simulated with a millivolt source. A direct short between one sensor input and common on the primary (note the third bar graph is extinguished - e.g. sensor output is below 0.1 ata) had no significant effect on the reading of the secondary.



"Simple = good." -Dave Sutton


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Old 28th February 2008, 17:36   #2 (permalink)
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Re: Isolation between primary & secondary

aaaaa I am not shure you should be using a voltage source... because depending on the type it will always be trying to give a certain voltage so it might pump up the amps.

Why not try an old battery and a trimpot instead.

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Old 28th February 2008, 18:26   #3 (permalink)
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Re: Isolation between primary & secondary

Quote: (Originally Posted by teomannaskali) View Original Post

Why not try an old battery and a trimpot instead.

Teoman
Thanks. That's exactly what my mV source is - a 1.5 V battery , a resistor, and a ten-turn pot. I just wanted to indicate that I wasn't actually running the circuit off sensors.

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Old 28th February 2008, 18:36   #4 (permalink)
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Re: Isolation between primary & secondary

So you have to have potted all the way up to the cell. Or in other words, the system can tolerate a flood or short circuit if you please, after the 100k resistor right?
In case one cable gets compromised then you still loose both sensors?

Btw thanks on the design info. It is really very appreciated.

Now a question. Does the resistance value (100k) before the primary have to be identical to the one before the secondary?
What I am really asking is if say a VR3 or any other computer can be used with a primary isolated in this way.


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Old 29th February 2008, 10:40   #5 (permalink)
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Re: Isolation between primary & secondary

Quote: (Originally Posted by teomannaskali) View Original Post
So you have to have potted all the way up to the cell. Or in other words, the system can tolerate a flood or short circuit if you please, after the 100k resistor right?
In case one cable gets compromised then you still loose both sensors?
If a sensor or the cable between the sensors and the isolator shorts, then both displays will be affected. If one cable or display shorts, the other should continue to work.

Quote:
Btw thanks on the design info. It is really very appreciated.

Now a question. Does the resistance value (100k) before the primary have to be identical to the one before the secondary?
What I am really asking is if say a VR3 or any other computer can be used with a primary isolated in this way.
Teoman
I don't see any reason the resistances need to be identical. They are only there to limit current flow in case of a short. I'm not familiar with the circuitry of the VR3, but I think one could safely assume that it has a very high input impedance, and just reads (milli)voltage, so it should work with an isolator like this.

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