# commutated dc current

#### andrea

Joined Sep 6, 2006
9
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

#### nanovate

Joined May 7, 2007
666
I may be wrong but it might mean that the current is pulsed or chopped. Some sensors use an AC-like current.

#### Dave

Joined Nov 17, 2003
6,969
Commutated DC is where you reverse the half-wave cycle of an AC signal to give you a unidirectional signal.

Dave

#### John Luciani

Joined Apr 3, 2007
475
What does commutated dc current mean?
How to build the commutated dc current source?
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 *)

#### recca02

Joined Apr 2, 2007
1,212
yes it should mean changing the direction of the current in the electrodes periodically.

#### andrea

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

Commutated DC is where you reverse the half-wave cycle of an AC signal to give you a unidirectional signal.
Does it use bridge circuit?

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.
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

#### beenthere

Joined Apr 20, 2004
15,819
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.

#### andrea

Joined Sep 6, 2006
9
Thanks a lot beenthere.

#### beenthere

Joined Apr 20, 2004
15,819
I'm not sure I follow your question. The use of AC on the electrodes is intended to avoid polarizing effects.