Mechanical Relay Quality

[evilkitty]

Yesterday i had to replace a relay i installed about 2.5 years ago
This relay i used in my thermostat to use 5VDC logic to control 27VAC logic
I had used the cheap relays you can get on ebay for about $0.50 each (Songle SRD-05VDC-SL-C)
It failed cause the contact plate on the load side became dirty from electrical arcing
here is a video showing how they are designed: https://www.youtube.com/watch?v=KzR7EtPM15E

Would a 2.5 year life span be reasonable for a relay in this type of use? (defective?)
would one of these two last significantly longer (enough to justify 2 to 2.5 times the price)?
https://www.mouser.com/productdetail...wh-sh105d1f000
https://www.mouser.com/productdetail/JS1-5V-F

[stj]

Re: Mechanical Relay Quality

there should be no arcing, you need to add a snubber circuit across the contacts.

[PopcornMobo]

Re: Mechanical Relay Quality

There's a good video showing what the cheap relays are capable of.
https://www.youtube.com/watch?v=ftJ17Cp6itw

[evilkitty]

Re: Mechanical Relay Quality

there should be no arcing, you need to add a snubber circuit across the contacts.

So i need to do this (see attachment)?
What kind of cap would i need to use here? This is an AC circuit (meter says ~26.995V)
about how many ohms should i use? (5, 10, 50, 100, 1000, 50k, 1m)
* my on hand resistors only good for 0.25W, so going below 3k is probably asking it to burn
AC caps are not polarized right?

In this case Load B would be used when the unit is set to heat and Load A would be used if the unit is set to cool


When i designed this board i used the relay to replace mercury tubes, i think i have seen a small spark when they switch, this is why i assume it was arc damage

[stj]

Re: Mechanical Relay Quality

that circuit is nice.
caps should be film type,
analog is not my thing, but the current drawn by the load is usefull to know.

you can get snubbers containing the cap and resistor in a single package but most designers use seperate parts.
probably for cost reasons.

[evilkitty]

Re: Mechanical Relay Quality

I know the current is small, 26awg wires do not get warm
wonder what my clamp on meter says...
0.67A up to 1A then back down

[Curious.George]

Re: Mechanical Relay Quality

Yesterday i had to replace a relay i installed about 2.5 years ago
This relay i used in my thermostat to use 5VDC logic to control 27VAC logic
I had used the cheap relays you can get on ebay for about $0.50 each (Songle SRD-05VDC-SL-C)
It failed cause the contact plate on the load side became dirty from electrical arcing

Do any of:

• Add a snubber across the contacts
• Use Hg-wetted relay
• Exert more control over when the armature is moved wrt the AC waveform

[evilkitty]

Re: Mechanical Relay Quality

On the note of a snubber
Assuming the power is 1A @ 27VAC (60Hrz i assume)
what size/wattage resistor and film cap do i need?

The spark i saw when it used the tubes was like you would get from a little static (petting cat and touch it's ear near the fire place)

[sam_sam_sam]

Re: Mechanical Relay Quality

Yesterday i had to replace a relay i installed about 2.5 years ago
This relay i used in my thermostat to use 5VDC logic to control 27VAC

What is the amperage of the device that you are controlling

[stj]

Re: Mechanical Relay Quality

have a read.

[evilkitty]

Re: Mechanical Relay Quality

What is the amperage of the device that you are controlling

during the switching process my clamp-on meter measured 0.67, with a peak of 1A (slowly climbed and slowly dropped off)
*I need to get fuses for my other meter (in my mouser cart)

[evilkitty]

Re: Mechanical Relay Quality

have a read.

IL = Amperage, given my meter peaked at 1A , guess that is 1
E = ???
k= (unknown value from 0.1 to 2)
this is the issue i have when i go to look a circuit formula, i have no idea what the variables stand for...

[Curious.George]

Re: Mechanical Relay Quality

On the note of a snubber
Assuming the power is 1A @ 27VAC (60Hrz i assume)
what size/wattage resistor and film cap do i need?

The spark i saw when it used the tubes was like you would get from a little static (petting cat and touch it's ear near the fire place)

It's not the amperage or the voltage but, rather, the rate of change of voltage/current that drives the design.

The capacitor (in series with the resistor) ensures there is no DC path through the snubber -- you wouldn't want current flowing AROUND your switch contacts all the time as that would defeat the purpose of the switch! The capacitor ensures that only the "transient" (spark) finds its way through the snubber, being dissipated in the resistor.

With the mercury switches, the mercury acts to "clean" the electrical contact. Hence the use of mercury-wetted relays if you want to avoid the snubber.

[sam_sam_sam]

Re: Mechanical Relay Quality

Or you could use this

Crydom S3013A S3 Series Solid State Relay, PCB Mount, 3.5-8 VDC, 3A, 20-140 VAC

[eccerr0r]

Re: Mechanical Relay Quality

Had my microwave oven's relay go kaput recently, jury rigged a replacement until I can go order one (with other stuff).

It died when a capacitor died... Contact life exceeded/destroyed.

[redwire]

Re: Mechanical Relay Quality

I just did a snubber design for 24VAC zone-valve controller.
The values are not critical, I found 0.1uF-0.22uF 160V and 10R 1/2W worked fine.
This is to keep the back EMF from arcing relay contacts or damaging a triac.

The cap ends up being large physical size, I used junkbox X or Y-caps.

[evilkitty]

Re: Mechanical Relay Quality

I just did a snubber design for 24VAC zone-valve controller.
The values are not critical, I found 0.1uF-0.22uF 160V and 10R 1/2W worked fine.
This is to keep the back EMF from arcing relay contacts or damaging a triac.

The cap ends up being large physical size, I used junkbox X or Y-caps.

so these are good then?
https://www.mouser.com/productdetail...s2d031001a00ko
https://www.mouser.com/productdetail...os1-2ct52r100j

[stj]

Re: Mechanical Relay Quality

Tesla once said that too much theory and not enough experimenting will get you nowhere.

[redwire]

Re: Mechanical Relay Quality

I would use a higher voltage capacitor as 63VAC/100VDC is close to the part's limit.

The load R L values and math and what you can fit/have on hand- it's easier to just look at it with a scope. Zone valves have a lot of inductance 200mH. I can post SPICE sims.

With no snubber I got 300-500V spikes, and down to about 100V with a 0.22uF/10R

[evilkitty]

Re: Mechanical Relay Quality

well then guess i will go with a 200v cap 505-mks2g031001k00ko or 505-mks2g031001k00jo which ever is in stock...

In the interest of science would it be a problem if i used only one resistor with a double throw switch/relay
I know it would only save $0.10 but it is 1 less part to solder, now that the logical reasons are out of the way... i am really just curious about it if you can do that

EDIT:
Would i need a snubber for a relay that controls a incontinent light bulb and a florescent bulb?