Intro

I am working on circuit of a phase failure relay, which i will be going to use with three phase motor for protection.

I picked a simple circuit layout from the web and started building it. (Pictures of the layout and my circuit is attached)

The Problem

the problem i am facing is that the relay (24v, 0.45 watt) is not turning on except for the led which turns on.

I need help in finding and rectifying the problem

 

The relay coil is in parallel with the LED (Which you have marked as 3 volts on your schematic) so that will limit the voltage across the relay coil to 3 volts A 24 volt relay will not pull in on 3 volts. As you have a 300 ohm resistor in series with the LED and say the current you want through the LED is 20 mA the voltage across this resistor will be 0.48 volts. So the voltage you are expecting across the capacitor will only be 3.48 volts. So even if the relay coil was connected directly across the capacitor it would not pull in.

Les.

 

The relay coil is in parallel with the LED (Which you have marked as 3 volts on your schematic) so that will limit the voltage across the relay coil to 3 volts A 24 volt relay will not pull in on 3 volts. As you have a 300 ohm resistor in series with the LED and say the current you want through the LED is 20 mA the voltage across this resistor will be 0.48 volts. So the voltage you are expecting across the capacitor will only be 3.48 volts. So even if the relay coil was connected directly across the capacitor it would not pull in.

Les.

The relay is in Series with the 330R resistor and the 24V Zener diode, the led is in parallel with the 330R resistor. I think there isn't enough current to pull the relay in.

I would put the zener across the output of the bridge rectifier and also the relay with the led and series resistor.

 

The relay is in Series with the 330R resistor and the 24V Zener diode, the led is in parallel with the 330R resistor. Also the electrolytic cap is drawn the wrong way round,. I think there isn't enough current to pull the relay in.

I would put the zener across the output of the bridge rectifier and also the relay with the led and series resistor.

Thanks fellow... considering your reply...., I used a different relay (24v 0.22W) instead of of (24v 0.44W relay I was using before and it pulled in. Now can you help me finding the way to make the other relay work within this circuit by any chance.

 

I misread the schematic thinking the 300 ohm resistor was in series with the LED so every thing I said in post #2 is wrong. When it worked with the 0.22 watt relay was that connected as in your original schematic or in the way DD suggested ? Also what is the value of the resistor that is connected directly across the output of the bridge rectifier ? (I can't read it on your schematic.) Can you measure the voltage across the across the relays (Both relays.) and across the zener diode with the phases in the correct sequence and in the wrong sequence ? From your schematic it looks like it was designed for a 0.22 watt relay coil. Why can't you use this specification of relay ? You may have to half the value of all the resistors on the AC side of the bridge rectifier and double the value of the phase shift capacitor. We will know better whan we see the results of your tests.

DD, I thisn the capacitor is the correct way round.

Les.

 

I misread the schematic thinking the 300 ohm resistor was in series with the LED so every thing I said in post #2 is wrong. When it worked with the 0.22 watt relay was that connected as in your original schematic or in the way DD suggested ? Also what is the value of the resistor that is connected directly across the output of the bridge rectifier ? (I can't read it on your schematic.) Can you measure the voltage across the across the relays (Both relays.) and across the zener diode with the phases in the correct sequence and in the wrong sequence ? From your schematic it looks like it was designed for a 0.22 watt relay coil. Why can't you use this specification of relay ? You may have to half the value of all the resistors on the AC side of the bridge rectifier and double the value of the phase shift capacitor. We will know better whan we see the results of your tests.

DD, I thisn the capacitor is the correct way round.

Les.

Thanks LJ.. i used the same schematic when i used the 0.22 watt relay. The resistor just across the rectifier bridge is a VDR and is in parallel with the DC circuit.
Test Results
In Correct Sequence
with 0.44 watt relay volts measured is 12V across the rectifier bridge and
with 0.22 watt relay, volts measured is 19.5V
The 0.22 watt relay is not available in the market where i live. i desoldered 0.22 watt relay from another pcb for testing purpose.
I agree with u that the schematic may be designed for 0.22 watt coil. so i think the only way left is to follow your advise of doubling the capacitor and half the resistors on AC side. So, will it be correct if i use 41K resistor on R and only one 5.6K resistor on T. But what value of cap i should use? Should i have to use two 473J caps in parallel?

 

Have you considered using an optical coupler instead of the relay ? You would need a 2.7K resistor in series with the LED in the opto coupler so it looked similar to the 2618 ohms of the coil of the 0.22 watt relay. Another option may be to use a 48 volt 0.44 watt relay an possibly reduce the voltage of the zener diode.

Les.