Isolation between primary & secondary

DanFountain · 28th February 2008, 17:14

Quote:

... 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

-Dan

teomannaskali · 28th February 2008, 17:36

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.

Teoman

DanFountain · 28th February 2008, 18:26

Quote: (Originally Posted by teomannaskali)

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.

-Dan

teomannaskali · 28th February 2008, 18:36

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.

Teoman

DanFountain · 29th February 2008, 10:40

Quote: (Originally Posted by teomannaskali)

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.

-Dan

28th February 2008, 17:14	#1 (permalink)
DanFountain Analog Luddite Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Dec 2005 Location: Lake Superior Posts: 156	Isolation between primary & secondary Quote: ... 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 -Dan
(Offline)

28th February 2008, 17:36	#2 (permalink)
teomannaskali Dalış Bahane Alem Şahane Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 441	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. Teoman
(Online)

28th February 2008, 18:26	#3 (permalink)
DanFountain Analog Luddite Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Dec 2005 Location: Lake Superior Posts: 156	Re: Isolation between primary & secondary Quote: (Originally Posted by teomannaskali) 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. -Dan
(Offline)

28th February 2008, 18:36	#4 (permalink)
teomannaskali Dalış Bahane Alem Şahane Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 441	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. Teoman
(Online)

29th February 2008, 10:40	#5 (permalink)
DanFountain Analog Luddite Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Dec 2005 Location: Lake Superior Posts: 156	Re: Isolation between primary & secondary Quote: (Originally Posted by teomannaskali) 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. -Dan
(Offline)

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