Relay won't stay on (switches on then off instantly)

Hello,

This isn’t for an arduino, but for a relay control for lights at home that stopped working. I was hoping you folks can help me figure out what the problem might be.

When the power button is pressed, the relay switches, but turns off immediately. I replaced the relay with the same one hoping it was just a faulty relay, but the problem persists with the new one. My guess is that it’s another component on the control circuit that’s become faulty, but I have no clue which might be the culprit. I tested the switch with a multimeter and it’s working fine. The capacitors and resistors/diodes all look fine too.

The relay control takes in 220v (connected directly to the wires of an outlet). The relay is a DC12, so I’m guessing the circuit steps down the voltage.

The IC is HEF 4013BP (HEF4013BP datasheet(1/16 Pages) NXP | Tolerant of slow clock rise and fall times)

Now that I know it’s not a faulty relay, what could be causing a relay to not hold?

Any help would be appreciated.

Thanks.

scruffnut: The relay is a DC12, so I'm guessing the circuit steps down the voltage.

The circuit is powered by a capacitive power supply, so [u]the whole circuit is live[/u].

Problem #1 with a capacitive supply is the big series capacitor (330nF) going bad (lower value). But I've never seen any problems with these red/brown types.

Measure the voltage on the round electrolytic cap in the middle (or pin 7/14 of the chip), and see if it stays ~12volt when the relay kicks in. Remember that every part on that board is directly connected to mains power!!! Leo..

So where is the circuit schematic for this?

First thing needed to determine how it operates. :roll_eyes:

Less a schematic it's pure guess and speculation. The IC, a 4013 is a very common dual D flip flop. My guess is a button press applies a positive pulse (debounced) to a CLK (Clock) input of the 4013. Clock it once and an output goes high, clock it again and that high goes low. Looks like a small transistor beside the relay which is likely a relay coil driver. Again, less a schematic it's all purely a guess. A Google of 4013 circuits should get you an idea of what is going on and a look at a 4013 data sheet may help. That being about as good as it gets for now.

My advice would be replace the module.

Ron

Wawa: Measure the voltage on the round electrolytic cap in the middle (or pin 7/14 of the chip), and see if it stays ~12volt when the relay kicks in. Remember that every part on that board is directly connected to mains power!!! Leo..

Just measured the voltage of the cap like you suggested. When relay is supposed to be off (power button off), it reads around 10v. But when the relay is supposed to be on (power button switched on), it only reads around 1.6v. I can't get a reading of the moment the relay kicks in though.

I'm assuming the cap needs to be reading around 12v for the relay to stay on. I will replace the cap when I get my hands on the same one and see if that solves the issue. Hopefully it does =)

Thank you!

The round/black cap is a normal 16volt? electrolytic smoothing cap, probably nothing wrong with it. But the big red one is the actual power provider. That is a special one (330nF/250volt AC), and they sometimes go bad. Remove, and test it's actual value with a cap meter. Probably not easy to get. Leo..

Wawa:
The round/black cap is a normal 16volt? electrolytic smoothing cap, probably nothing wrong with it.
But the big red one is the actual power provider.
That is a special one (330nF/250volt AC), and they sometimes go bad.
Remove, and test it’s actual value with a cap meter.
Probably not easy to get.
Leo…

Yeah, the black cap is a typical 50v 47uF cap.

I was able to get a big red cap, but the local store only had a 224K 250v one. I’m wondering if there could be any danger associated with testing it out with the circuit. I could see why using a high capacitance one could potentially be dangerous, but other than the circuit not working, can something bad potentially happen if I hook up the 224K in place of the 330K?

The 224K I got is also 3~4 times larger in size than the original 334K, so that’s a bit concerning as well…

You should measure before getting replacement parts. It might not be the cap.

220nF is 2/3 of the value of 330nF, so 2/3 of the current capacity of the supply. That might or might not be enough to reliably operate the relay.

You should at least see a big improvement over that 1.6volt (post#4) if the original 330nF cap is bad. Leo..

Is that new cap really marked 225K? Maybe that is why it is much larger than the old one.

Good catch. 6.66 times the old value will create problems (magic smoke). Leo..

You need to replace with 330nF, not 2.2uF, that is a dangerous error that might burn things up. Replace like with like only - if you can't get the right one, you won't be able to repair this circuit.

Learn to read component codes - the local store were incompetent if they sold you a 2.2µF cap as a 220nF cap in the first place. Whenever I order in components, even from reputable supplier, the first thing I do is go through and check the markings on every component against the list, mistakes do happen and checking everything should be second nature and well worth it in the long run.