# commutated dc current

Discussion in 'General Electronics Chat' started by andrea, Aug 23, 2007.

1. ### andrea Thread Starter New Member

Sep 6, 2006
9
0
hi,

I am trying to measure the conductivity of liquid with electrodes. I found in an article, it tells that " to avoid polarization of the electrodes by the use of dc, it use commutated dc current "
What does commutated dc current mean?
How to build the commutated dc current source?
Thank you

andrea

2. ### nanovate Distinguished Member

May 7, 2007
665
1
I may be wrong but it might mean that the current is pulsed or chopped. Some sensors use an AC-like current.

3. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Commutated DC is where you reverse the half-wave cycle of an AC signal to give you a unidirectional signal.

Dave

4. ### John Luciani Active Member

Apr 3, 2007
477
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The word commutate means "to reverse". To make a commutated current source
you need to periodically reverse the direction of the current flow.

If your current source is unipolar you could use one or two solid state switches to reverse the direction of current flow to the electrodes.

(* jcl *)

5. ### recca02 Senior Member

Apr 2, 2007
1,211
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yes it should mean changing the direction of the current in the electrodes periodically.

6. ### andrea Thread Starter New Member

Sep 6, 2006
9
0
Thanks to all for your help.

Does it use bridge circuit?

Using solid state switches to reverse the current direction. How does it work?
Could you give me the example circuit or the links about it?

andrea

7. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The commutated DC current is confusing. If you are going to use electrodes in water to measure conductivity, though, you need to use AC excitations to avoid plating crud on your electrodes.

Water, when pure, is a pretty good insulator. Chemicals dissolved in the water dissociate into ions, and let electrical current flow by transporting charge carriers. If the applied voltage is DC, then the positive electrode (anode) will get coated with an insulating layer. Using AC voltage tends to keep the build-up from becoming serious.

There are many ways to provide AC to electrodes. Possibly the easiest, if you're in a lab setting, is to get a step down transformer. Something like 12 - 24 volts applied should be enough. Place a resistor in series with one electrode. A large value, like 100K might be right. Apply voltage to the circuit and measure the drop across the resistor. It's a series element, so that will let you calculate current, which will give you the water conductivity. What current flows through the resistor has to also flow between the electrodes through the water.

8. ### andrea Thread Starter New Member

Sep 6, 2006
9
0
Thanks a lot beenthere.

9. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
I'm not sure I follow your question. The use of AC on the electrodes is intended to avoid polarizing effects.

10. ### andrea Thread Starter New Member

Sep 6, 2006
9
0
Thank you for your help beenthere.