Where can I find a relay??

Discussion in 'General Electronics Chat' started by talco92, Jul 16, 2013.

  1. talco92

    Thread Starter New Member

    Jul 15, 2013
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    Hi everyone,

    You can read the last part as some background for what it's for, but basically I need to find/harvest a relay.
    Where can I find one that's rated above 10A from old electronics?? I have an old Playstation, old computer parts etc.
    Do you think these are salvageable?

    Also if you look at the video below, what is the best way to include the relay into the circuit?
    -----
    I've started a project (building a temperature controller for home brewing beer) and I want to include a relay in it to avoid frying the controller which is rated to 10A. The issue is that when a fridge is connected to the controller and the fridge gets turned on it can draw a current of above 10A for a split second, then drops to 5A or whatever it runs at.

    Here is a video of what the controller will look like (i'm using the video as a guide) http://www.youtube.com/watch?v=30TvX1Zz1-Y




    Cheers!
    Tal
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    I would use a 230Volt coil relay,on the heater output, and let the slave relay connect the heater on using its own N.O. contacts to the mains supply. Ps what beers are we making


    Ebay #130562133118
     
    Last edited: Jul 16, 2013
  3. wayneh

    Expert

    Sep 9, 2010
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    They're everywhere, including auto junk yards. Old TVs and monitors (CRTs) have some, as do old electromechanical stuff like tape players, fax machines, printers, shredders etc. The more modern the device is, the less likely you'll find a relay.
     
  4. talco92

    Thread Starter New Member

    Jul 15, 2013
    9
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    Thanks guys.
    currently brewing a Belgian Amber Ale. Should be delicious.

    What does an old relay look like?
    And if I do find one, can I just desolder it and put it directly into the circuit without any diodes or capacitors or any other components?

    cheers
     
  5. talco92

    Thread Starter New Member

    Jul 15, 2013
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    I found one in a CRT monitor identical to this:
    http://www.topchipcomponentes.com.br/fotos/SAM_1552.jpg

    I can't find any spec sheets on this type though because they're pretty outdated.
    Do you know what the '65A' means? or tv-3 or tv-4??
    I'm really new to all of this so I'll probably need it all explained :S
     
  6. Gibson486

    Member

    Jul 20, 2012
    199
    12
    Use a NTC thermistor?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    They're usually in a plastic box, more-or-less cube shaped or a bit rectangular, and seem to be mostly black these days. Since they're fairly large compared to other components, maybe 3/4" on a side, they usually have decent markings to quote their ratings. On the pin side, after removal from a circuit board, you'll often find the wiring diagram showing which pin is which. Just post a picture here if you have any doubt.
    Probably not. Pleas don't risk anything until you get more help. Sketch out what you want to do and post your diagram here. You'll get plenty of good feedback.
     
  8. Stuntman

    Active Member

    Mar 28, 2011
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    I've gathered all kinds of 1-5A relays IIRC, old modems were a hot spot for quite a few.

    As for the high current stuff, 10A isn't a deal breaker, but I don't recall ever pulling something quite that robust off of any old electronics (there was a time I was quite the scavenger... mostly old computers).
     
  9. talco92

    Thread Starter New Member

    Jul 15, 2013
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    0
    A thermistor is set to become a resistor at a certain temperature right?
    If so, it wouldn't help me as I want to set the temp controller to maintain varying temperatures depending on the stage of the beer making (I.e 70deg C when mashing, 20 deg C when fermenting, 4deg C when cold crashing etc. etc. etc.)


    EDIT: also, the temperature probe is about 2 meters long and so whatever it is measuring will have a completely different temperature to the circuit.

    EDIT EDIT: oops, just did some more reading:
    "Negative temperature coefficient (NTC) thermistors and fixed resistors are often used to limit inrush current." -Wikipedia
    and they must be placed in series, is that right??

    That's great, but where can I find one? and what specs should I be looking for?
     
    Last edited: Jul 17, 2013
  10. talco92

    Thread Starter New Member

    Jul 15, 2013
    9
    0
    I found this. http://www.topchipcomponentes.com.br/fotos/SAM_1552.jpg

    it looks as if it's rated to 5A but then it says 4A/65A on it which confuses me.
    what you think?

    As a recap: the temperature controller can withstand 10A being drawn from it, but a fridge briefly draws more than that on start-up. all I need is to stop that extra amperage going through the system. Maybe there's a better way to do it than using a relay????
     
  11. panic mode

    Senior Member

    Oct 10, 2011
    1,318
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    your posts are way too ambiguous and this thread goes nowhere, you will only get WAG (wild ass guesses). i see posts on AC relays, automotive relays, modems etc. (hence WAG)

    if you are controlling AC load running on hundreds of volts, you cannot use an automotive relay, they are only rated for VERY low voltage (but high current). why would you want to salvage one anyway? why waste time searching through junk only to get one of suspicious "ripeness" (relays do have tear and wear) instead of buying a brand new one for as little as $1..2? this is not something to be cheap about, failure can cost you way more than price of a new (and properly sized) relay.

    first reply should have been: what do you really need? what exactly you need to accomplish?

    control heater? fridge? pump? what voltage (12VDC, 120VAC, 230VAC...)? what power? what duty cycle?
    what control voltage? mention some numbers... then we can start looking into selection and sizing of components.

    once you know what you need (relay, triac, whatever), read datasheets. stop guessing obscure marking when part number is clearly readable - lookup datasheet. that relay is a small one rated up to 5A.

    the first step is to define problem and that's how you make progress...
    how would you control this relay? this will tell if the coil is supposed to be AC or DC and what voltage.
    if you are switching something really fast or if you want silent operation, SSR or Triac would be better option.
     
    Last edited: Jul 17, 2013
  12. talco92

    Thread Starter New Member

    Jul 15, 2013
    9
    0
    Firstly, thanks for the ass kicking.
    I don't really know much about circuitry or electronics but i can see how I have left out some pretty important info.

    I'm in australia, so the mains are 240VAC (but really anywhere between 200-240v) The temp controller unit is 220V and rated to 10A.

    Basically when the unit decides it's time to turn on the cooler, the fridge circuit will be completed and the fridge will turn on. This is where I ran into a potential issue: Fridges (as I'm sure other devices do too) draw MORE than their usual amperage as they turn on, maybe for a split second. If that spike is more than 10A, I'm worried the temp control unit will suffer some damages.

    That's why I want to put some sort of in rush limiter into the circuitry.
    This is what the circuit will look like:
    [​IMG]



    Sorry again if I'm missing some critical info. Don't know what 'duty cycle' or 'control voltage' means. I'm not lazy though, I'll do my reading after I post this!

    Cheers mate

    EDIT: during the stage of beer-making where the fridge is used the most, the duty-cycle could be around 90% (total period being 10 days-ish)
     
    Last edited: Jul 17, 2013
  13. wayneh

    Expert

    Sep 9, 2010
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    Look here.
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,088
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    OK, I finally understand what you want to do. Your Elitech temperature controller is not rated high enough (at 10A) for the devices you want to control, and you want to "enhance" it so you can switch a larger load without risk to the controller. The problem is not the continuous current but the startup load.

    If you find yourself relays that will meet your need (I'd look automotive), there are several options. If you have a 12V DC supply handy, you could use your controller to switch the 12VDC supply to your relay, and the relay would handle the AC load. That's probably how I'd do it.

    You could even consider hacking the relays of your controller. Use the signal that IT uses to throw its internal relay and just use that to control the external relay instead. But if you're not handy with such things, just use a 12VDC wall wart.

    I believe there is such a thing as an AC-powered relay but I've never looked into that.
     
  15. panic mode

    Senior Member

    Oct 10, 2011
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    do NOT use automotive relays on AC circuits, certainly not in case like this where line voltage is 10x the rating of the relay. if your temperature controller (can't see model number or link to a datasheet) has contacts rated 10A, use them to turn on an interposing relay or contactor, wire load (fridge or whatever) through contacts of that relay (or contactor).

    i still don't know what is this appliance rated for or at least what the AC outlet powering entire contraption is rated for. I am guessing 15-20A, at 230VAC that is plenty of juice. you can find tons of relays that will happily do this job.
    for example omron relay with 230/240VAC coil (so you don't need wallwart power supplies, you can have relay powered straight from mains through contact of the temperature controller). relay contacts need to be rated for:
    a) current (some 20-25A should do)
    b) voltage of 240VACalso AC has peak voltage significantly higher than "effective" or RMS value (so peak is some 340V). automotive relays are rated for no more than 30VDC so trust me and just DON'T use a DC relay in an AC circuit (or the other way around)

    for example consider specimen that is more like:
    http://www.digikey.com/product-detail/en/G7L-2A-TUB-CB-AC200/240/Z2937-ND/1813533

    more about contact rating:
    as the voltage in circuit is higher, so are the sparks when circuit opens. sparks are also accompanied by corrona which is basically conductor. automotive relays are meant to interrupt only very low voltage and opening contacts does not create nearly sufficient gap (distance) to reliably extinguish the arc. yes, it may appear to work on the bench for few minutes on testing but this is not how products get their ratings and you don't want to return home one day only to see that is in flames because of incorrect DIY wiring of some brewster contraption. insurance company would just laugh in your face...
    you can take a look at fuses too, compare automotive fuse, with one rated for 300 or better 600V and you will see noticeable difference - higher voltage rating means much longer fuse element. this is the distance we mentioned before.
    another issue with relays is type of loads. resistive loads are piece of cake since such loads do not store electromagnetic energy (technically heat is also in this category but getting electricity back from heat is not very effective). but inductive loads are another animal and stored energy is also released at the moment circuit opens. this means considerably longer sparks (need more gap). again, that is why nobody puts automotive relays in domestic appliances.
    contactors are basically big power relays, sometimes people use the term relay to refer to both relays and contactors -and while operation from user point of view is the same, there are some constructional differences. for example in contactors each circuit is made/broken not at one point but two (also effectively doubling the gap when contacts are open). also they are usually rated for much higher voltage and current than relays. they are also much louder in operation etc.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Yeah, my bad for bringing that up. I don't know what I was thinking.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,503
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    The TV rating of a relay is the UL etc rating that is determined by the cycle time/current usage etc, I have a chart somewhere.
    You can also get SSR's (Solid State Relays) Opto22 etc, fairly cheap.
    You would just need a DC source of 5-35v to switch it.
    Anything used in PC equipment is most likely not going to cut it for current.
    Max.
     
  18. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
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    Microwave control units usually have one for 500 watts ( and up ) at 120 volts. I doubt that a vcr, modem.... will have a really high current relay.
     
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