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Calling electronics experts ...

Discussion in 'Home Cinema Building DIY' started by MarkHudds, Jun 17, 2004.

  1. MarkHudds

    MarkHudds
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    I'm a complete novice when it comes to basic electronics so go easy with me!

    I've got a 12v DC fan that I'm running from a mains power adaptor. I'd like to stop and start the fan via the 12v trigger socket on my projector however I'm not sure what I need!

    The projector supplies 12v when on and then 0v when off (not enough amps to actually power the fan directly though), so I just need something which can detect the 12v or 0v and switch the mains adaptors 12v power on/off to the fan.

    Some quick 'googling' brought back results including 'relays'? Is this what I need?

    Any help very much appreciated!

    Cheers
     
  2. mister_choos

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    You do indeed need a relay. A 12V single pole double throw (SPDT) relay. This is basically an electrically operated switch. You need to buy one where the switch contacts are able to handle the fan current (most will manage that easily) and one where the 12v from your projector is able to drive the relay coil. If you give me the details of the fan and 12V projector supply, I will happily source one for you if you don't feel confident enough yourself.
     
  3. MarkHudds

    MarkHudds
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    mister_choos - thanks for the reply!

    Here's the details:
    Projector 12v trigger: Power on: DC 12V (4.7 kohm output impedance) Power off: 0V

    Fan: 12V 0.07A - 0.09A

    If you can give me an idea of what relay I should be getting that'd be great!

    Also, do you know where I'd getting a simple wiring diagram?

    Thanks a lot!
     
  4. mister_choos

    mister_choos
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    You can get a suitable relay from www.maplin.co.uk

    The part no. is F43W.

    The connection is quite simple. You connect the 12V from your projector to the coil. (The pin connections should be marked on the top of the relay.) The fan supply should have its +ve to one side of the switch contacts, and one side of the fan should be connected to the other switch contact. Connect the other side of the fan to the supply -ve.

    I hope that is clear. If not, give me your e mail address and I will send you a picture.
     
  5. MarkHudds

    MarkHudds
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    Hi, thanks for getting back.

    I actually bought a relay from Maplins today, before I read your message. I got part no JM67X. I've not had any success getting the coil to trigger! I've used a meter to test the projector trigger and it says it's 11.50V, however when you connect up to the relay, the voltage across the coil pins is only 0.930V ?!?!? Am I missing something here? Relay seems fine if I connect the fan to 'N/O - Normally closed', so the power is getting through.

    Anyway I've attached a diagram to show my connections ...

    http://fp.mhalton.f9.co.uk/new_page_6.htm

    Do you mean part FJ43W? (F43W doesn't exist). Isn't that a double pole, double throw?

    I'll PM you my email.
     
  6. siwatkins

    siwatkins
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    Sorry to jump in here, but I see what the problem is.

    Your projector provides 12V but its output impedance is 4k7 ohms.

    Your circuit diagram is fine in principle, but the projector cannot source enough current to develop 12V across a low impedance relay like the one you are using (which has a 400ohms coil )

    Effectively, you have the 4k7 of the projector in series with 400 ohms of the coil giving a combined effective load of 5100 ohms. From Ohms law, with 12v across that combined series load, the current flowing is V/R = 12/5100 = 0.0024A.

    Taking the relay coil in isolation, with 0.0024A flowing through it, the voltage being developed across it can again be found from ohms law. IxR = V = 0.0024 x 400 = 0.96V which is (within rounding errors and component tolerances) exactly what you are seeing and is insufficient to switch the relay.

    In simple terms, the 12V trigger does not deliver sufficient current to drive devices like a 400 ohm relay directly. You'll therefore need to use a buffer transistor or similar to detect the voltage from the projector, and drive the relay.

    Additionally, you should not connect a relay coil directly to your projector like that (even if the relay had a high enough impedance) without some kind of induced EMF protection across the coil. Lets assume that your triggering device could supply enough current to energize the relay - when the triggering device (your projector) shuts down it stops producing 12V at its output, the magnetic field in the relay coil rapidly collapses an in turn induces a very high voltage at the terminals of the coil. This of course, would be fed straight back into the triggering device connected across that coil, potentially killing its switching stage. A reverse biased diode should provide suitable protection if connected across the coil. Connect the cathode of a diode - something like a 1N4148 to the switching output end of the coil, and the anode to the ground end. In normal operation - ie with 12 V applied to the coil, the diode will be reverse biased - no current will flow through the diode. When the 12V is removed, the induced EMF in the coil will forward bias the diode, effectively placing a short across the coil and limiting the high voltage spike that would otherwise occur.

    If you want to "feel" the voltage that can occur, energize the relay with 12V directly, stick your fingers over the coil terminals, and switch the 12V off. You'll jump ;) (Please only try this at your own risk!!!).

    If you want a hand designing a transistor buffer stage to drive the relay, give me a shout and I'll sketch you something up later. Off to Camden Market now to take some pictures, but will be back this evening.

    Simon
     
  7. MarkHudds

    MarkHudds
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    Simon ... wow, thanks for the lengthy post - guessed it must have been something like that.

    mister_choos - Tried the FJ43 relay this morning and that must have even more coil resistance (volts drops to 0.5V!). I did however add a diode between my coil terminals (1N4001).

    I'm confused, as I've read one or two posts here where people are driving their electric screens from 12v projector triggers using the same model relay ... e.g. :
    http://www.avforums.com/forums/showthread.php?t=134736&referrerid=13666

    My projector is a Sony VPL-VW12HT, so it shouldn't be out of the norm in terms of trigger output.

    If you can get back to me with help on the buffer transistor, then that'd be appreciated (in simple terms please!)

    Looking forward to getting this working!!!
    :lease:
     
  8. siwatkins

    siwatkins
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    Some devices will be capable of driving a low impedance load directly, but the 4k7 output impedance of your projector's switching stage means it will need buffering for driving a relay.

    Rather than knock up a schematic and try and remember my ftp password :) I just looked up an existing circuit. Types A or B here should be fine. For the transistor, use any General Purpose transistor to hand - it is operating in saturation, so just about anything will do. A BC107 or BC108 should be fine.

    http://ourworld.compuserve.com/homepages/Bill_Bowden/r_ctrl.htm

    You shouldn't even need a circuit board with such a low component count, but if you are a perfectionist like me, a small piece of veroboard would be ideal.

    Hope this helps, and if you get stuck, feel free to give me a shout.

    Simon
     
  9. MarkHudds

    MarkHudds
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    Simon

    Looking at example B in that link, it appears I can simply wire the transistor in-between my projector 12v + and the relay + coil terminal?

    Is it that simple? Here's an amended diagram to show my understanding:
    http://fp.mhalton.f9.co.uk/new_page_6.htm

    Are BC107 and BC108 Maplin's numbers?

    Cheers
    Mark.
     
  10. MarkHudds

    MarkHudds
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    Oops! I just realised transistors have 3 connections ... means I'm off-track!

    What is the 'input' in my scenario?
     
  11. siwatkins

    siwatkins
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    Transistors have 3 terminals - a Collector, and Emitter and a Base. The Base in your case is the input and that should go to the projector +, The emitter (the arrow in diagram B) should go to Pin 5 of your relay, and the collector (which you have shown going to the projector) should go to the +ve of your 12V supply (that you have shown going to pin 1 of your relay).

    As far as Maplin goes, it looks like they are running down their stocks of transistors based on the paltry selection available in stock.

    Either UL36P or QQ14Q (maplin codes) should do the trick. The BC107 I mentioned earlier looks like it is no longer available from Maplin, having been superseded by the BC108C (order code QB32K), which is on "whilst stocks last". For your application, it really is not that critical, so just about any NPN low power, low frequency transistor will suffice.

    Simon
     
  12. MarkHudds

    MarkHudds
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    Hi Simon, I promise this is the last question ...

    Do I keep the 12v power supply connected to pin 1 of the relay (common) as well as to the transistor collector pin?

    Amended diag:
    http://fp.mhalton.f9.co.uk/new_page_6.htm

    Thanks again and I'm sure I'll get there! :thumbsup:
     
  13. siwatkins

    siwatkins
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    Your amended diagram is correct - should all work OK :smashin:

    However, you may also need to connect the negative of your 12 V supply to pin 3 to keep the grounds common.
     
  14. Darren Blake

    Darren Blake
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    Just to stick my 2 pence worth in:

    Shouldn't there be a high resistance (>1KOhm) current limiting resistor connected to the base of the transistor, or is the high output impedance of the projector sufficient?

    Also, if the projector runs on a 12 DC transformer, it might be worth considering buying a different transformer that can supply enough current to supply both the projector and the fan, and simply piggy backing the fan supply off that. It would save having two separate PSUs and the grounds would be commoned together by default.
     
  15. siwatkins

    siwatkins
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    No, not required. The circuit is in emitter-follower configuration and thus presents a high impedance load to the projector. The (base) current flowing will be the collector current divided by the gain of the transistor. In this case, the relay is 400 ohms, the voltage is 12V thus the collector current will be 0.03 Amps. Divide that by the gain of the transistor, say 100, and the base current will be neglible, thus no current limiting resistor required at the input.

    Certainly possible, but if he were doing that, then he would have warranty issues to contend with. I wouldn't fancy the risk of opening up a projector unnecessarily either for risk of knocking something out of cal, or introducing dust where it's not welcome etc. However, if he were opening up the projector, then it is likely that the existing transformer (or switched mode psu) would have sufficient spare capacity to cope with an additional fan, thus all that would be required is to build the buffer stage at the 12V Trigger internally. Again, warranty and the aforementioned risks would make it less than attractive in my opinion.
     
  16. Darren Blake

    Darren Blake
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    Of course. DOH! :D

    I was thinking more along the lines of the PSU/tranformer being external. If it is internal then yep - too much hassle to open it up just to do this.
     
  17. MarkHudds

    MarkHudds
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    Just wanted to report back quickly to say that I connected a BC108C transistor and everything is working fine! Tested the voltage at the relay coil terminals and it's showing as 10.5V. :thumbsup:

    Many, many thanks to Neil and Simon for their fantastic help and patience :clap: This is a great forum!

    I do hope this thread is useful to others, especially electronic novices like myself. Saying that, I have learnt a lot this weekend!
     
  18. siwatkins

    siwatkins
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    Excellent, glad you got it going :thumbsup:
     
  19. klrbee25

    klrbee25
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    This discussion was some time ago, but hopefully one of you electronics whizzes is still out there :). Here's my scenario that has my stymied.

    I have an older receiver without a 12v trigger, but a new amp that has it. I've heard of people using an AC adaptor plugged into the switched outlet on their receiver to deliver a 12v signal to the amplifier, so when the receiver goes on the amp gets the 12v signal and goes from standby to on as well. Finding the adaptor is easy enough (many are adjustable from 3v to 12v), but there are many options regarding the amperage (300mA, 500mA, 1000mA). Seeing how I don't want to fry my amplifier, I'm concerned about what effect these values have if used as a 12v trigger. The specs on the trigger for the amp only say that it takes a 3v to 30v signal...nothing about amperage. Could someone give me a little guidance?
     
  20. mujiber

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    Hi, had the same problem at Mark, and did a search. Found a lot of posts and people having the same problems, but this one seemed the most informative out of the rest of the threads. Just want to say thanks Simon for taking the time to help out Mark, helped me out as well. I've got the Panasonic AE700, so I needed to have that relay in there. All I had to buy was a relay from The Source, and pull a transistor and diode from an old burnt out computer power supply, a 12V AC adapter from an old cordless phone, and put it all together. Works great! Thanks again!
     

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