what benefit using 2 diodes back to back in analog sinals 4- 20 ma
what benefit using 2 diodes back to back in analog sinals 4- 20 ma
(OP)
dear all,
in the pic attached ,any one know what is the benefit of using 2 diodes back to back in front of the analog signal input 4-20 ma before connecting it to the analog card???
in the pic attached ,any one know what is the benefit of using 2 diodes back to back in front of the analog signal input 4-20 ma before connecting it to the analog card???





RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
Keith Cress
kcress - http://www.flaminsystems.com
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
A more common arrangement is to connect them in parallel with one diode forward and one diode the other way. Usually, there is just one diode, but when there are long cable runs and a noisy environment, using one diode can result in noise being rectified and added to the original 4-20 mA signal. Using two diodes eliminates that risk.
Why? Because it is a convenient way to measure actual loop current. Kust connect a milliampmeter across the diodes. Most meters have a voltage drop less than the diode's threshold voltage and will therefore take over all the current so it can be read on the meter.
It is a safe way of checking mA signals without breaking the loop.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
mr Skogsgurra your answer look helpful but if diode is putted to measure milliamperes across it without interrupting the circuit , why then put 2 diodes ... i think one diode is enough right???
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
if the diode putted in this way to measure the milliampere across it , then it will not measure the actual current in the current loop by the transmitter bcz the diode will drop 0.7 volt and the transmitter will have 24 - 0.7 = 23.3 v only across it and will not give the same milliampere that i have measured when i shorted the diode with the ammeter
is it right??
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
I explained that in my answer. If you put one diode there, you will risk rectification of noise and that adds to the mA signal. If you put two diodes, as shown in your picture, there will not be any net rectification. It may be a little hard to understnd this, but you can trust me. Been there lots of times.
Next: It doesn't matter at all if your voltage is 24V or 23.3 V. It may, in fact, vary from probably 12 or 15 V up to typically 30 V without affecting the signal. This is because the transducer outputs a current signal and its internal (dynamic) resistance is usually in the Megohms region.
Also, At the typical voltage drop of a multimeter (around 100 mV), there is no current (OK, a few microamps - perhaps - but no more) in the diodes. Those few microamps are typically .1 percent of the actual signal and will not be significant at all if you want to do a quick check of the signal.
If you want to calibrate the thing, you should do it out of circuit. Not that it is needed, but the routine is such.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
is it right??
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
To calculate the correct current, you first need to know the internal resistance of the meter and then calculate how the total current is divided between resistor and instrument. I do not find that practical in any way. Nor is it more exact than the diode method.
Just accept that this is a thechnique that has been used for a long time (probably before you were born) and that the accuracy is just as good as the instrument you use to measure current. The diodes introduce no error whatsoever.
Also, if you want to get anywhere in life - please try to use normal punctuation, skip texting and use capital letters in the beginning of sentences. That is all part of engineering. The sooner you get used to it the better.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
what is wrong ?? ..!!!
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
Keith Cress
kcress - http://www.flaminsystems.com
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
TTFN

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RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma
RE: what benefit using 2 diodes back to back in analog sinals 4- 20 ma