Author Topic: Relay clicking when load is applied  (Read 584 times)

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Offline okwTopic starter

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Relay clicking when load is applied
« on: May 24, 2024, 10:27:12 am »
I have a project where I relay control an external resistive load (1400W 240V). Between the relay and the load there is a thermostat. Relay is a Songle SRD-05VDC-SL-C.
I've tried to visualize it in the schematic. Relay coil is energized all the time. However, when the external thermostat goes from off to on (conductive, happens a few times a minute), the relay clicks a lot in the transition. I'm pretty sure it's in the transition, as the thermostat have a red ON / green OFF light, and the relay clicking always happens when the light changes from green to red.
It's like the coil pressure isn't strong enough to keep the relay closed. I use a 7002 (n-ch mosfet) and 1k base resistor. Base is driven by a 74HC02 (lots of logic behind it).
Would I be better off using another mosfet or reduce the base resistor? What mosfet would be a better choice?
« Last Edit: May 24, 2024, 10:31:43 am by okw »
 

Online themadhippy

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Re: Relay clicking when load is applied
« Reply #1 on: May 24, 2024, 11:06:08 am »
you sure its not the thermostat you can hear clicking
 

Offline okwTopic starter

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Re: Relay clicking when load is applied
« Reply #2 on: May 24, 2024, 11:20:15 am »
Yes, I see the 5V rail dip to 4.2-4.3V when it happens so it's obvious that the coil is trying force the relay reed in place.
If I buypass the circuit, the thermostat have a heavy low pitched "thump". When it's in, I hear the thump plus the relaywith it's high pitched "stressed" clicking.
 

Online wraper

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Re: Relay clicking when load is applied
« Reply #3 on: May 24, 2024, 11:25:07 am »
If power rail dips, it means you have a power supply problem. Likely heater somewhat sags mains voltage and your PSU is marginally operating to begin with.
 

Online wraper

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Re: Relay clicking when load is applied
« Reply #4 on: May 24, 2024, 11:27:34 am »
Quote
Would I be better off using another mosfet or reduce the base resistor
MOSFETs have no base and resistor be it 100k or direct gate connection to IC will make no difference.
 

Offline okwTopic starter

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Re: Relay clicking when load is applied
« Reply #5 on: May 24, 2024, 11:50:11 am »
If power rail dips, it means you have a power supply problem. Likely heater somewhat sags mains voltage and your PSU is marginally operating to begin with.

Wow, seems so. I just removed the PSU and replaced it with a lab PS. Relay completely quiet.
I thought the dip was due to inrush current of the relay engagement.

Then my second question is, what's the simple emergency fix for now? An X2 cap (xx nF?) close to the PSU? Or a rather large cap (xx uF?) on the 5V output?
It has this 5V 700mA Aliexpress PSU. I know, I know. But for now, it's the only one I have.
 

Online wraper

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Re: Relay clicking when load is applied
« Reply #6 on: May 24, 2024, 11:56:44 am »
X cap will have no impact on it. PSU likely is garbage, also it does not look safe.
 

Offline okwTopic starter

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Re: Relay clicking when load is applied
« Reply #7 on: May 24, 2024, 12:11:59 pm »
It's going to be inside a metal enclosure, out of peoples way. So even a fire or short circuit wouldn't pose any danger.
A cap over the relay coil took care of it.
 

Offline Coordonnée_chromatique

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Re: Relay clicking when load is applied
« Reply #8 on: May 24, 2024, 02:43:52 pm »
A cap over the relay coil took care of it.

Have you a trace of the coil current ?
Is the cap on the relay coil reducing the clicks frequency ?
 

Offline okwTopic starter

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Re: Relay clicking when load is applied
« Reply #9 on: May 26, 2024, 09:15:46 am »
A cap over the relay coil took care of it.

Have you a trace of the coil current ?
Is the cap on the relay coil reducing the clicks frequency ?

47uF reduced it, but 100uF removed it completely. So even with a 5V rail dip other places on the board, they relay stays properly energized.
 

Online ArdWar

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Re: Relay clicking when load is applied
« Reply #10 on: May 26, 2024, 10:12:02 am »
Huh, it's unusual for DC coil relay to chatter(?) like that. In fact a common power saving technique for non latching relay is to halve the drive voltage after initial pick-up.

Just how far your supply dips?
« Last Edit: May 26, 2024, 10:15:54 am by ArdWar »
 


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