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-   -   Use Of Relays (https://www.cruisersforum.com/forums/f14/use-of-relays-250168.html)

Mark Thurlow 01-05-2021 09:48

Use Of Relays
 
I am on a long term full time cruiser monohull in the Caribbean.

I have been using a standard automotive 4 pin relay to switch through a 12vDC feed to my Freezer compressor so that I get the ability to provide a good 12v supply from my house batteries on a short run; whilst maintaining the switching capability at my control panel.

All should be well but recently I think the relay (it is new and I have swapped out with the same results) is occasionally opening even when the switch voltage is stable.

I am asking experienced users, if I am wrong to believe that a good quality and new relay can hold the 12v path open for 24/7?

Mark

Stu Jackson 01-05-2021 11:16

Re: Use Of Relays
 
Is it a continuous duty relay?

GordMay 01-05-2021 11:24

Re: Use Of Relays
 
Quote:

Originally Posted by Mark Thurlow (Post 3398749)
... I am asking experienced users, if I am wrong to believe that a good quality and new relay can hold the 12v path open for 24/7?
Mark

Relays generally have a specified "duty cycle", the maximum 'On' time, followed by a minimum 'Off' time.

Mark Thurlow 01-05-2021 15:49

Re: Use Of Relays
 
Gord and Stu,

I cannot find any markings or specs to show “continuous duty” or “up time” on the boxes or on the relays themselves. Does this they are NOT or the correct specification for my 24/7 use?

smac999 01-05-2021 21:25

Re: Use Of Relays
 
I would imagine standard auto relays are continuous. And should be fine.

Some high current solenoids are not. Like the 300a+ starting ones

PatB 01-05-2021 21:56

Re: Use Of Relays
 
Quote:

Originally Posted by smac999 (Post 3399051)
I would imagine standard auto relays are continuous. And should be fine.

Some high current solenoids are not. Like the 300a+ starting ones

Agreed. Something like a headlamp relay, for example, might be expected to be "on" for the longest expected journey of a vehicle, which might be anything from several hours and upwards. As far as the relay is concerned that is, effectively, continuous.

Tin Tin 01-05-2021 22:27

Re: Use Of Relays
 
Maybe you need a "latching relay" that only uses current to open or close the switch and does not need power to stay open or closed. Your on/off button on the console would need to change to a momentary switch and would need a separate LED to show fridge circuit was on. There may be more sophistcated versions around.
https://au.rs-online.com/web/general...g-relays-guide

xxxxx 01-05-2021 22:38

Re: Use Of Relays
 
An SSR for DC might be the solution you're looking for...or a power MOSFET depending on the load.


How much current?

Reefmagnet 02-05-2021 02:20

Re: Use Of Relays
 
This relay duty cycle thing is new to me. I'd suspect that the rating would apply to the contacts and not the coil if such a specification existed.


Perhaps the inrush current when the fridge or something else starts is causing the relay to drop out momentarily. This will certainly upset the refrigerator, especially if it has a 3 minute (or so) delay timer for compressor start on power up.


Ground connections can be a likely suspect in this instance, although for intermittent issues it can be like finding a needle in a haystack. You may need to check the relay circuit for high resistance joins, or otherwise cycle high draw equipment while monitoring the coil voltage of the relay.

Bycrick 02-05-2021 13:22

Re: Use Of Relays
 
Relays that draw a significant coil current might have duty-cycle specification. But all the 12v auto relays that I looked at had a coil current of about 100 ma. That shouldn’t cause heating problems.

Maximum current capacity is usually related to current flow through the contacts. The relay will get warm when carrying higher currents for long periods of time. But all the ones I looked at have a maximum temperature spec that includes the heat from the coil and the temperature increase in the contacts.

As the relay contacts age, the contact resistance increases which just makes the problem get worse. With higher current relays, the external connections become more important. A old, slightly loose slide-on terminal can lose its spring with age. So it gets loose. That makes it heat up and get still weaker. And so on, to failure.

Also, note that with most auto relays, the Normally-Closed contacts are often rated for lower current than the Normally-Open contacts. This apparently is the result of the contact pressure caused by the spring being less than the contact pressure caused by the coil.

I looked at a dozen different auto relays from 4 different manufacturers and didn’t see duty-cycle specified for any of them.

Lake-Effect 02-05-2021 13:43

Re: Use Of Relays
 
Relays usually require less current to stay closed than to initially close, due to the magnetic "load" being lowered once the armature pulls in. So I have trouble imagining a scenario where a closed relay with a solid coil voltage would drop out. Maybe some sort of circumstance where the startup current of the compressor momentarily reduces the coil voltage to a point where the relay drops out? [edit - as already suggested by Reefmagnet]

Is the relay +ve coil voltage sourced from right by the compressor? Maybe try sourcing the relay coil voltage from the DC panel.

AndyEss 02-05-2021 13:43

Re: Use Of Relays
 
Quote:

Originally Posted by xxxxx (Post 3399065)
An SSR for DC might be the solution you're looking for...or a power MOSFET depending on the load.


How much current?

https://lib.dr.iastate.edu/cgi/viewc...t=abe_eng_pubs

The EMR {electro-mechanical relay) is usually free from thermal heat buildup problems, but devices using semiconductors are not. Voltage drops across semiconductor junctions are on the order of 0.5 to 1.5 V. For every ampere of current, there- fore, 0.5 to 1.5 W of heat are created that must be conducted away. Heat buildup must be limited so that the junction temperature of the solid- state device stays within its ratings. Otherwise, the relay may not turn off. This necessitates heat sinks for....»

If your SSR relay drops 0.5 to 1.5 Volts across its semiconductor junction, in a nominal 12Vdc system you lose about 10% of your available power to creating heat, not refrigerator cold.

SSRs have their place in our toolkit of nifty ways to improve our boats’ electrical system, but this does not appear to be s good application for an SSR

Bycrick 02-05-2021 14:48

Re: Use Of Relays
 
Lake Effect is right. The power to hold a relay activated is small compared to the pull-in.

My Westerbeke Genset run solenoid takes about 40 amps to pull in and has an internal mechanical switch to reduce the power after it's energized. If you don’t adjust it correctly, the switch won’t activate and you’ll fry an expensive solenoid. Don’t ask how I know.

The new Blue Sea high -current relay apparently has an electronic version of the same circuit which reduces coil current for hold-in.

If the relay is dropping out, either it’s a bad relay, or the voltage supplied to the relay is intermittent.

Jammer 02-05-2021 19:18

Re: Use Of Relays
 
Hello Mark.



Quote:

Originally Posted by Mark Thurlow (Post 3398749)

All should be well but recently I think the relay (it is new and I have swapped out with the same results) is occasionally opening even when the switch voltage is stable.


The switch voltage isn't stable even though you think it is.


Somewhere you have a loose connection, bad crimp, failing switch, weak breaker, or corroded fuseholder. A visual inspection and some manual wiggling of the wires may isolated the problem, or you can try freeze spray.


Good luck

xxxxx 03-05-2021 13:21

Re: Use Of Relays
 
Quote:

Originally Posted by AndyEss (Post 3399358)
https://lib.dr.iastate.edu/cgi/viewc...t=abe_eng_pubs

The EMR {electro-mechanical relay) is usually free from thermal heat buildup problems, but devices using semiconductors are not. Voltage drops across semiconductor junctions are on the order of 0.5 to 1.5 V. For every ampere of current, there- fore, 0.5 to 1.5 W of heat are created that must be conducted away. Heat buildup must be limited so that the junction temperature of the solid- state device stays within its ratings. Otherwise, the relay may not turn off. This necessitates heat sinks for....»

If your SSR relay drops 0.5 to 1.5 Volts across its semiconductor junction, in a nominal 12Vdc system you lose about 10% of your available power to creating heat, not refrigerator cold.

SSRs have their place in our toolkit of nifty ways to improve our boats’ electrical system, but this does not appear to be s good application for an SSR

Respectfully, that's incorrect.

You've cited a 45-year-old paper about switching AC loads using TRIAC-based SSRs. Yes, they drop 1.5V, which is far less significant in a 110-240VAC systems, especially since the power requirements of today's AC systems are much lower than those of 1976. Consider an 100-watt equivalent LED bulb which only now draws 15W. I digress...

Modern DC SSRs are completely different. They are switched efficiently by low dissipation MOSFETs. This is a distinction with a huge difference. For example, a 10A DC SSR might have a maximum voltage drop of 0.17V.

Modern boat freezers draw as little as 3A. The corresponding voltage drop might be in the area of 0.08V. At 12VDC nominal, that's 99.3% efficiency.

Nonetheless, we still don't know the make/model/power requirements of the OP's freezer. All we know is that it is 12VDC.


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