Go Down

Topic: 3phase mains faliure circuit (Read 1 time) previous topic - next topic


hi there
We have needed to monitor 3 phase supplies and have been using these or similar to


Now we are looking at doing something like this on own PCB. I know the obvious is to use 3 relays each supplied on a different phase and put one of the Arduino input pins through 3 contacts. I think there must be a better way of doing this,  any one have any suggestions?


Jan 02, 2014, 09:54 pm Last Edit: Jan 02, 2014, 10:00 pm by jackrae Reason: 1
2 phase-to-phase opto-isolators with outputs connected in series feeding a single input pin.

If you want to monitor phase-to-neutral then you'll need 3 detectors, again connected in series

If you want to identify which phase is lost then you'll need 3  detectors and 3 input pins whichever method you monitor


phase-to-neutral  would be good, what sort of thing should I be looking at?


Jan 03, 2014, 05:46 am Last Edit: Jan 03, 2014, 05:49 am by lemming Reason: 1
Have a look at a PS2505.  http://www.cel.com/pdf/datasheets/PS2505.pdf

Search the forum for threads on how to wire up the input of this unit.
You can get 3 single input units(PS2505-1)  but you should go for the PS2505-4 so you can use one unit for all your phases.


ok thanks for that, I have had a bit of a look about
so correct me if I'm wrong, I can get away with just using a resister on the diode side?
calculating the resister size do I do the same theory as I would be calculating the resister for an LED?


Using resistors could create heat problems since a dissipation of around 2.5watts will be experienced with a 10mA current flow

Since capacitive line impedances do not dissipate heat (current and voltage out of phase) you might be better off using table X capacitors rated at 400v or above.  Using a simple calculation and assuming 10mA flow through the diode section you are looking at an Xc of around 25Kohms.  This equates to a capacitance of around 0.1uF.

Line fuses in the sense circuit are a must


Jan 12, 2014, 08:31 am Last Edit: Jan 12, 2014, 08:39 am by TomGeorge Reason: 1
Hi, I'd say buy the RS one , even at 51pound, or 51/.65 (I think)= $78 AUS is pretty good.
You are also detecting low voltage as well not just phase failure, a low phase will make a mess of an induction motor without any obvious symptoms until it too late.
Phase sequence monitoring is also important if motor direction is important.
If you have them continually failing then you need to get the failed ones to an industrial electronics service workshop  :), to find out why.
I have had a number of 3phase monitors die, usually after they are 3yo, the problem is dried out electrolytic capacitors, and failed X2 MKT type caps.
One particular brand has a comms bus in it so some major electronics, the 5V supply is full of 6V electrolytics, they all leaked or suddenly GREW UP.(This customer has 139 of them in use, and keeps 50 in stock)

Tom..... :)
Everything runs on smoke, let the smoke out, it stops running......VK3DMK


hi there
We have needed to monitor 3 phase supplies and have been using these or similar to

Use 3 current transformers. Use an appropriate iron core toroid and wind it to give you maybe 10 to 20 milliamps at the load's normal current. Rectify and filter the output of each CT, then monitor the DC signal with the Arduino.

Using a current transformer on each leg is the safest method as it provides complete electrical isolation from the mains and it provides small, safe current levels to work with.

The only down side is that you need to disconnect each leg of the 3 phase power (only once) to install the current transformer.

If that's not an option, look for a "Split Core Current Transformer" which is just an ordinary CT with part of the core hinged and movable (to snap around the conductor).
Gentlemen may prefer Blondes, but Real Men prefer Redheads!


But a CT system only tells you the voltage isn't there (to drive the current) WHEN you need all three phases, not Like a voltage failure detector which tells you at all times, even with no, or extremely low, current flowing.  However if that's good enough and you have a neutral (star point) available in your critical load, and you only want a "Go-No Go" indication then you only need one CT fitted into the neutral line.  With any phase or  two phases missing, the currents will be imbalanced and detectable neutral current will flow.


CT does not suit my application.
So are you saying picking up a phase putting it through a resistor into an opt out through a capacitor?
Could I not have two 100k .25w resistors in series?


It all relates to how much current your opto-isolator requires.  My fag-packet calculation was based on 10mA. 

If you use a suitable size a type X capacitor you don't need any resistors.

If you are looking at 200k resistance, the opto current will be around 1.2mA on a phase-to-neutral test and the heat dissipation will be just over 0.3 watts.   That's pushing the resistors close to their rating  -  not always a good idea for something that's operating continuously.


so what size capacitor would we be looking at?


Jan 13, 2014, 07:44 pm Last Edit: Jan 13, 2014, 07:46 pm by jackrae Reason: 1
Read previous response of 11/1/14 (circa 0.1uF)

Note that phase-to-ground requires 400v X-rated capacitor,  phase-to-phase requires 600v X-rated capacitor

See attached for example


And don't forget the fuses !!


ok thanks for that.
I am putting the x capacitor on the phase line not the ground?


So where are you intending to connect the AC side of the opto.
I get the impression you are not fully aware of what you are endeavouring to build and therefore withdraw any suggestions or guidance I have previously made.
I respectfully suggest you back away from this project as you are dealing with seriously dangerous ziggies.

Go Up