Diodes

Discussion in 'General Electronics Chat' started by Sheldon, May 29, 2005.

  1. Sheldon

    Thread Starter Member

    May 29, 2005
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    I'm looking for some diodes for use in automotive circuits, but haven’t got a clue about the specification side of a diode; does the diode have to be specifically for 12V? Or can you use any diode rated above 12V? Is there a specific part number/type/make......?

    Any help would be appreciated.
     
  2. n9xv

    Senior Member

    Jan 18, 2005
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    What is the actual application/purpose of the diodes?
     
  3. Sheldon

    Thread Starter Member

    May 29, 2005
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    My car has a boot popper which runs from the car alarm remote control (the engine has to be turned off to open it), but I want to install a button inside to pop the boot as well (will probably run it from the ciggie lighter), but I will need to install two diodes to aviod blowing my car alarm.
     
  4. Sheldon

    Thread Starter Member

    May 29, 2005
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  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    The voltage rating is not a problem - you have probably noticed by now that most diodes are rated above 12 volts. Your diodes have to be able to carry the current that will pull the solenoid that pops the boot lid.

    Individual diodes with ratings above a couple of amps can be fun to find. If your application has the diodes sharing a common line, consider a dual diode rated for the current. Even a full wave rectifier bridge can be had that is rated for 400 volts and 35 amps current for about $2.50 - no idea about the euros equivalent. That is almost certain to be less expensive than a pair of diodes. The excess voltage and current rating just means it will be very hard to blow it up.
     
  6. Sheldon

    Thread Starter Member

    May 29, 2005
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    Hi, thanks for the replies but im still non the wiser, I've enclosed a crude diagram ive created to give you an idea of what im trying to do, could someone have a look at this website http://www.maplin.co.uk and let me know if they do the sort of thing I need.

    The current circuit has a 10amp fuse in it so it would need to be at least 12v and 10amp.
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    Could you not use a double throw pushbutton instead of a diode? Then you could wire your Clifford alarm via the NC contact and it would still work normally.
     
  8. Sheldon

    Thread Starter Member

    May 29, 2005
    17
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    To be honest I'm looking for someone who knows what they are doing to basically tell me what to use to get the thing working, so if I can use a double throw switch instead of a push button and diode then so be it.

    What would I need a SPDT or DPDT switch?

    Would it work kinda like the diagram attached? (crude again)
     
  9. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Your circuit is spot on. You need a SPDT switch.
     
  10. Sheldon

    Thread Starter Member

    May 29, 2005
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    Nice one thanks, I've found someone on ebay who has some, so just to confirm I need a SPDT Push to Make Switch.
     
  11. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Yes, a SPDT is what you need. It's also referred to as a changeover switch. I'm not sure what you mean by a 'push to make'. The switch will have a common contact which you connect to the solenoid. Its 'Normaly Closed' contact connects to the Clifford alarm, and a 'Normally Open' contact which connects to the 12volt supply.

    I hope that helps.
     
  12. Sheldon

    Thread Starter Member

    May 29, 2005
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    Got my switch purchased this one http://www.maplin.co.uk/module.aspx?ModuleNo=35033&doy=1m6 ........at first I thought it wasnt correct as it has 6 terminals but I guess it just does two circuits at once.

    The switch is rated 250vac 3A will that be ok? I thought as im only passing 12v it would be ok but im not sure about the amps?
     
  13. pebe

    AAC Fanatic!

    Oct 11, 2004
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    You normally have to derate the AC spec of a switch when it is used on DC. It would have been nice to see the DC spec.

    But unless it's a very high current solenoid it should be OK. Heck, you've bought it so give it a try.
     
  14. Sheldon

    Thread Starter Member

    May 29, 2005
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    They don’t show the spec for DC current on the site, just for AC. I can take it back if it’s likely not to work or to start to malfunction; the last thing I want is to blow my alarm as that cost £600!

    The switch seems like good quality, and is described as so on the site.......

    Worst case scenario what could happen?
     
  15. pebe

    AAC Fanatic!

    Oct 11, 2004
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    The very worst would be that the contacts would weld up and the switch would stay in one position.

    The less serious would be that the contacts could erode with arcing and sometimes not switch the solenoid.

    In either case, your alarm would not be damaged.
     
  16. Sheldon

    Thread Starter Member

    May 29, 2005
    17
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    Could the switch then not get stuck constantly sending power to the solenoid and burning it out? ........infact I think I will install a 12V LED as well in the circuit so I can see when power is being sent to the solenoid (also help as a visual aid). I've emailed Maplin asking what the ratings are for DC and will see what they say, they usually reply fairly quickly.

    Thanks for all your help BTW and the other pps who have replied.
     
  17. Sheldon

    Thread Starter Member

    May 29, 2005
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    I was thinking about it last night and I think I'm going to take this button back and go down a different route. I'm going to purchase a SPST push switch and a SPDT automotive relay as the relays are rated upto 30a and I would feel far safer knowing that its not likely to blow up.

    What do you think?
     
  18. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I agree. That's a good decision.
     
  19. Sheldon

    Thread Starter Member

    May 29, 2005
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    Did you get my pm dude?
     
  20. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    Yes. And sent a long reply. Shout if it's gone astray and I'll do it again.
     
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