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Designing auto turn on circuit for sub amp

Discussion in 'Archived Threads 2001-2004' started by Loren Jones, Feb 20, 2002.

  1. Loren Jones

    Loren Jones Agent

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    Hi,

    I have a SoundCraftsMan PCR800 amp for my sub (10 year old pro sound amp, about 300 WPC at 4ohms) that I want to trigger with the remote trigger on my Outlaw 1050. I have forgotten any actual electronics knowledge I ever had. What do I need to do? Just get a relay that is switched by the voltage from the Outlaw trigger output and wire that into one side of the power line coming into the amp? Any suggestions on the specific part number and source of a relay I could use to do this? What side of the power wire should I put it on, pos or neg? Also, anyone see a problem with wiring a thermistor into the power going to the cooling fan on the amp so I can keep it quiet when I am not running it hard?

    Thanks,

    Loren Jones

    BTW, the Outlaw trigger provides 12vdc
     
  2. Robert_J

    Robert_J Lead Actor

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    I've seen this mentioned before.
    -Robert
    I just saw your location. I don't think they have Sears in Germany.
     
  3. Wayne A. Pflughaupt

    Moderator

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    Loren,
    You’re on the right track. You need a relay with a 12DC coil that switches (i.e., has contacts for) the correct voltage and amperage to power the amp.
     
  4. Loren Jones

    Loren Jones Agent

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    Hi,

    Power here is 220V 50Hz, but we use transformers to step the voltage down to 110...obviously the frequency doesn't change to the 60Hz of US power, but it works fine. So, a relay with a 12vdc coil and able to handle 120 volts, and say 20 amps should more than cover it. Thanks. As far as the thermistor, I was just going to put it inline in one of the leads going to the fan.

    Loren Jones
     
  5. Dave Milne

    Dave Milne Supporting Actor

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    Woah... hold on here! You CANNOT simply wire a standard thermistor into the power lead to the fan. By definition, a thermistor changes resistance with temperature. These devices are nominally 100ohms. Adding 100ohms in series will have one or more of the following effects: 1. (likely) Little or no change in fan speed because you're changing the voltage by less than 10% regardless of the temperature of the thermistor. 2. (possible) Fan failure if its a synchronous AC motor drawing excessive current trying to maintain synchronism despite reduced input voltage or 3. (certain) Burn up the thermistor because you're dissipating roughly one-quarter watt in this tiny device.

    What you MAY have been talking about was a thermostatic switch. These are commonly available from electronics suppliers (Digi-key, Newark). The internal contacts close --providing full voltage to the fan --at some preset temperature.

    It is possible to design a variable speed temperature controller (if you have the proper fan type) using a thermistor, an op-amp or two, and a power transistor or triac.

    With regard to the relay: check the coil current and make sure it's within the rating of the switch output of your Outlaw 1050. Relays are available with coil currents of less than a milliamp to greater than one amp (1000 mA). The Outlaw is probably limited to 100-200 mA.

    Good luck
     
  6. Tim Kilbride

    Tim Kilbride Stunt Coordinator

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    I agree with Dave...a thermistor might be more work than you are willing to do...it will not be able to control a fan, you will have to wire it into a relay or transistor/triac setup. You will also have to figure out at what temperature it will drop resistance and throw a relay/triac setup... the thermostat would be a much easier/reliable way to go...

    TK-
     
  7. Jeff Rosz

    Jeff Rosz Second Unit

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    heya,
    about your relay,
    20A and above rated contacts is generally entering into the realm of "power" relays. automotive, air conditioning, motor control. you are looking at the $10-$20 range. so fyi..you arent going to do it with a pc board type or "mini" relay. your best bet is to use an automotive relay and probably the easiest to find. or something found here . type in 5Z550 in the search box. get a 3X748 for DPDT if you feel you would ever want to control 2 devices at the same time. gotta love upgradability. its an "open" design so mount it in a box lest you get bit.[​IMG] both have 30A AC contacts and 12vdc coils rated @169mA. put a 20A fuse in series with the output contacts too. can you confirm what the amperage is of the outlaw's control circuit? they should ship to germany...email them, click on their "worldwide" link.
     
  8. Loren Jones

    Loren Jones Agent

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    Thanks guys,

    I will definitely check things out based on the good advice. I looked at Digikeya and they had switches, but they didn't close until 100 deg centigrade which seems a little hot to me. I guess the amp has thermal protection on it anyway, but I wouldn't want to burn it up. The relay at Grainger looks just right, but I did look in my manual and I didn't see any spec on the current capability of the 12volt trigger, so I will email Outlaw and check.

    Does anyone have any idea if 100 deg centigrade would be ok?

    Thanks,

    Loren Jones
     
  9. Dave Milne

    Dave Milne Supporting Actor

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    Loren,

    100C is too hot. Your amp should shut itself down at that temperature. Come to think of it...100C switches are probably available because that's what manufacturers use.

    You should be shooting for roughly 50C outlet air --which translates to 60-70C on the amplifier heat sinks. Sorry if I steered you wrong... I thought Digi-key had everything. I'll see if I can find something, or if I get a few spare minutes, design a simple circuit and post it.
     

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