Relays...

couchsachraga
couchsachraga Solar Expert Posts: 87 ✭✭
in Solar Beginners Corner #1
This is probably a fairly simplistic question, but I'd rather ask it than not.

I'll be working with some folks to install my Midnite Classic 200 this coming weekend. I'd like to have the equipment to potentially use one or both of the auxiliary functions:
1) Low voltage disconnect of loads (I've purchased a Xantrex change controller that will work for this, but if the Midnite will do it I'll return it)
2) Diversion load (when batteries are full heat water!)

It appears for either of these I need a "NC Relay" (Normally Closed Relay).

I'm familiar with automotive solenoids - I take it these are similar. Are they interchangeable, and if not, where might I order them? I tried a quick search from our fine hosts but couldn't find anything.

Both of these would obviously be fairly serious loads (heater water as a diversion load, or the normal relay of the entire load system (realistically under 30A at this point, but I'd go higher for safety).

All 12v DC...

Thank you!
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Comments

  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #2
    Re: Relays...
    couchsachraga wrote: »
    It appears for either of these I need a "NC Relay" (Normally Closed Relay).

    If you're going to switch AC, you will need a relay rated for 240VAC. Most automotve ones don't. If you want to switch DC, most AC relay will also be rated for 24VDC, but for 48V you will need a relay that is rated 72VDC or higher.

    IFAIK, for opportunity loads, Classic does relatively high frequency PWM called "waste not". Mechanical relay cannot switch that fast. So, you'll need a Solid State Relay (SSR).
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  • couchsachraga
    couchsachraga Solar Expert Posts: 87 ✭✭
    #3
    Re: Relays...

    The system is all 12v DC, including waste load.

    Sounds like I need 2 NC SSR relays then. Thank you! Now to find them...
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #4
    Re: Relays...

    With "digital" relays, mount them to a large metal plate/heat sink with good air circulation... They can overheat from internal resistance.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #5
    Re: Relays...
    couchsachraga wrote: »
    The system is all 12v DC, including waste load.

    Sounds like I need 2 NC SSR relays then. Thank you! Now to find them...

    For low voltage disconnect, a mechanical relay will work better, because SSRs always introduce some losses in the switched circuit, which can be very substantial at high currents. You may want to look for a latching relay, which would turn off and wouldn't consume power to keep it open afterards.

    For PWM, make sure you get SSR designed for DC and rated for at least 20V and your maximum amps.
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #6
    Re: Relays...

    I've bought SSRs on eBay. Very simple to wire up and use. Here's a few pics...

    diagram.jpg

    relay.jpg

    dscn4293.jpg

    dscn4298.jpg

    $11 on eBay, free shipping from Hong Kong. Takes about 10 days to arrive.
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