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I was making PIR based light on/off using a TRIAC initially, but the LED flickers continuously when it is ON state although with an incandescent bulb there is no flicker.

Now I tried witha relay but once the relay is connected the voltage across the terminals is less than 1 (0.77) volt. After removing the relay and checking the voltage it is 12 v.

Why once relay is connected voltage is dropping? Because of this NO in relay is not getting closed.

Sorry for not providing circuit diagram.

My circuit is simple. 12V DC RO relay and 220V AC for the bulb.

enter image description here

enter image description here

enter image description here

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    \$\begingroup\$ You need to provide a schematic to get a sensible answer here. You can used the schematic editor button. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 11, 2020 at 14:28
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    \$\begingroup\$ Crop the photos, man. Crop the photos. \$\endgroup\$
    – Transistor
    Commented Oct 11, 2020 at 14:28
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    \$\begingroup\$ Size issue so I took screenshot for smaller image, sorry for messed up with images \$\endgroup\$
    – Sijith
    Commented Oct 11, 2020 at 14:37
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    \$\begingroup\$ @Sijith you added the pictures to Mohamed's answer ... the pictures belong in your posted question, not in an answer that somebody else posted \$\endgroup\$
    – jsotola
    Commented Oct 11, 2020 at 19:11
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    \$\begingroup\$ @Sijith: Never mind the photos and the voltage drop. Stop messing with that thing before you kill yourself. That is a capacitive dropper. You must regard all points in that circuit to be at line voltage. That's probably 220VAC where you are. That circuit is not safe. It is waiting for a chance to kill you. \$\endgroup\$
    – JRE
    Commented Oct 11, 2020 at 19:12

3 Answers 3

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With the circuit diagram you have finally posted, the problem is clear.

  1. You are using a capacitive dropper as a power supply.
  2. Current through a capacitive dropper is limited by the capacitor. At 50Hz, your 1uF capacitor has an impedance of about 3k ohms. At 220V, that's a maximum of about 70 milliamperes.
  3. Your relay has a coil resistance of 400 ohms.
  4. At 12V, the 400 ohm coil of the relay will draw about 30 milliamperes.

The coil of your relay draws about half the current your power supply can deliver. It is no surprise that the voltage drops.

Capacitive droppers are dangerous. You must regard every point in your circuit as being at line voltage. Any point in the circuit can kill you - even the supposed low voltage side.

Since you had to ask where the problem is, it would appear that you don't understand how a capacitive dropper works, which implies that you don't understand why it is dangerous.

Put it down. Unplug it. Don't touch it until you know why it is dangerous.

A capacitive dropper should never be operated in the open and should never be in a position for a person to touch any part of the circuit.

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I can answer this question:

Why once relay is connected voltage is dropping?

A relay requires a large amount of current to energize the coil and the coil can be modeled as a resistance, so when you are connecting a relay in series with other electronic components, the current passing through all of them would be high (current drawn from a voltage source has a relation with the load and the voltage supplied by the source is constant,) and the voltage on the relay terminals would be: I * R(coil,) and it would be a high voltage also.

This what causes a voltage drop.

In fact the voltage of the voltage supply never drops, the voltage on the thing connected in series with the relay is changing here (drops.)

enter image description here

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    \$\begingroup\$ Thanks for your answer can I connect a resistor to increase voltage \$\endgroup\$
    – Sijith
    Commented Oct 11, 2020 at 14:20
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    \$\begingroup\$ Could you please post a draw of your circuit diagram for further information ? It should not be perfect I mean , consider it a physics problem of resistors , graph it and upload it to get the maximum benefit . \$\endgroup\$
    – Mohamed Atef
    Commented Oct 11, 2020 at 14:22
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    \$\begingroup\$ Image I added but it's under moderator review \$\endgroup\$
    – Sijith
    Commented Oct 11, 2020 at 14:53
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    \$\begingroup\$ electronics.stackexchange.com/a/525899/264404 \$\endgroup\$
    – Sijith
    Commented Oct 11, 2020 at 14:54
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    \$\begingroup\$ You can edit, crop and rotate screen grabs too. \$\endgroup\$
    – Transistor
    Commented Oct 11, 2020 at 15:02
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Question

How come my capacitive dropper is not working?

pir 1

Answer

Update 2020oct13hkt0914

The OP seems using a relay switch without any flyback diode. The back EMF from the inductive coil of the relay switch might cause big trouble.

cap dropper

(1) Capacitor dropper is dangerous - DO NOT USE IT. To live longer, you can use a cheapie 12V wall wart.

(2) Capacitive dropper is only for small current apps, so is not good for hobbyist relays which usually takes up to 70mA.

References

(1) Capacitive Power Supply - Wikipedia

(2) Bob's Capacitor Project - BobParadiso, 2015feb03

(3) Youtube on Bob's Capacitor Project (5 stars; good explanation, including schematics, calculation of capacitor's impedance) - 2015feb03

(4) Transformerless Power Supplies: Resistive and Capacitive (with Vout and power calculations) - Reston Condit, MicroChip, App Note AN954, 2004

Appendices

Appendix A - Capacitive Dropper - Wikipedia

cap dropper

Appendix B - Songle Relay Spec

songle relay spec

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