diodes sizing

Electrical & Ignition

  1. saylor

    saylor Senior Member

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    o wise and venerable forum!

    calling any available E.E. or anyone running diodes on their ignition box or ALT or anywhere -

    is there any reason to NOT insanely oversize the diode?

    like say im only sending/trying to block .5a 12v or even less, but i use a 10a 1000v rated diode- way overkill. does it matter? i know you should allow 20% overage buffer for failure, but how bout like a 2000% overage buffer?

    Your Pal -

    - saylor
     
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  2. Joseph James

    Joseph James Well-Known Member

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    No reason to do it, but an oversized diode shouldn’t hurt anything.

    10 amp-how much current diode can safely handle

    1000v-should be the peak inverse (PIV) which is how high a voltage the diode can block.

    Most power supplies I have dealt with even in cheap consumer electronics use a 1000v PIV diode set although current rating is lower (because costs less).

    Note-I am not an EE but worked for years doing component level repairs. Diodes were a routine failure item from voltage spikes, lightning, etc. I have some 60 amp diodes in the basement if you really want to go crazy
     
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  3. 68_fury_ragtop

    68_fury_ragtop Member

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    The extra current rating on the diode shouldn't hurt. Then physical size and cost will just increase. Generally when doing designs, I double the max current rating the circuit will see, and then then go with the next standard value of the diode type I want to use.
    Are you using the diode for half wave voltage rectification or to block the DC voltage from feeding back to something? Just be aware that depending on the type of diode you use, you will probably also see 0.3 to 0.7 of a volt drop across the diode from your source voltage.
     
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  4. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    A larger diode will not hurt anything, but you are limited by the amount of space you have to install it. As noted above the .3-.7 voltage drop is converted to heat in the circuit. Over sizing the diode will result in about the same amount of heat but distributed over a larger unit so in theory at least it should last longer. Do not wire diodes in parallel as one diode will wind up doing the work and the other will heat sink the first one and do no work. When wiring diodes be sure to install them in the right direction as they are a one way switch.

    Dave
     
  5. saylor

    saylor Senior Member

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    thanks forum for the replies!
    the latter, like a gate. in fact if there is something better than a diode to do this let me know! i want a 1-way current flow. I want to trigger a relay #1 and have it trigger another relay #2, but it CANNOT reverse order where relay #2 could ever trigger relay #1.
     
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  6. saylor

    saylor Senior Member

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    noted. thanks!
     
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  7. Gerald Morris

    Gerald Morris Well-Known Member

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  8. saylor

    saylor Senior Member

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    preciate it. we both know theres a twenty9 cent diode under there :) i aint givin em 8 bucks :)

    im gonna get like a PIC roll 10-piece of them there is a component store in DFW called altex they will have something.
    i want to make a few up with some kind of spade terminal something and reinforcement and shrink wrap the whole business.
     
  9. Joseph James

    Joseph James Well-Known Member

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    Mouser is inDFW and has no minimum.
     
  10. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Are you talking about your head light relays? If so, a larger diode to trigger the second relay isn't buying you anything. All you need is enough current to make the relay coil energize and that's it.
     
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  11. 78Brougham

    78Brougham Deplorable FCBO Gold Member

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    I've got a real good solution to all this.... don't drive at night and if you do make like it's 1967. That's what I do.
    If it ain't broke don't fix it. Your still running the same amps regardless to the bulbs. Just pushing it thru a different way. You may be making it easier on the switch and dimmer, but in the end the head lights are still using the same power....
     
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  12. saylor

    saylor Senior Member

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    yes thanks Mr. big John, but the reverse of this i am worried about the current going the other way and using hoover dam to only stop a piss stream as an example.

    allright. even though that argument is not even rational to me, let me present this for consideration. and i again state everyone should upgrade their headlight wiring per slantsix dan guidelines, regardless if you have current problems or not. buy his and/or crackedbacks stuff if you dont feel good about wiring.

    exhibit A :)
    20191228_095056.jpg

    exhibit B

    20191228_095010.jpg
    this is typical 50 year old wiring, as found in our cars, as found on the 68 fury i bought from pacific NW. notice the wire melt in pic #1 wavy wire, and the exposed wire / broken insulation in pic #2.

    A MELTED WIRE. YOUR CAR. THINK.

    In addition, the wire is 18 AWG stock, which is minimally capable for the amount of power our headlights can use (dont take my word for it see wire melt in stock wiring in pic above).
    Its 50 years old, shows breaks (see pic #2) in insulation. thats enough to reject it right there.

    As you said, in the stock configuration ALL HEADLIGHT POWER runs thru the light switch at the dash. so if you have 4 stack lamps like a 68 fury you could push close to 20A thru your dashboard thru this switch.
    same with the foot dimmer switch as you mentioned. all this power flows thru it. And the ammeter circuit. and your bulkhead - 4 times !
    In the stock configuration, the entire front light system is grounded by 1 ring terminal up in the front right that all the grounds feed to - 50yrs old and as skinny and brittle as the rest of these wires.

    By doing the crackedback harness and employing relays per slant six dans guidelines:
    we increase wire AWG from 18 to 14, which is proper AWG spec for this length of wire run at this amperage
    we entirely remove the headlamp power from traveling thru the inside of the car at all- no dash, no ampgauge, no headlight switch, no foot dimmer. we just removed ~20A from that entire harness that was already inadequate at best.
    we have added a level of redundancy by employing 2 power feeds and 2 relays driving hi / lo separately.
    we have added less potential draw on the battery by using the ALT stud as power source.
    new headlamp sockets v. 50 year old ones.
    new wire v. 50 yr old wire.
    potentially better connectors and connections than 50 yrs ago - especially if its handmade 1 wire at a time by a guy sitting here watching an internet car forum.

    ive actually been chatting with the professor about this, and have maybe come to running 3 relays total: 1 for low beam lo side, 1 for hi beam lamps, 1 for low beam hi side. i dunno yet im still kicking it around in my head.

    but to me the pro so many far outweighs the con. and a melted wire man. thats gotta go.
     
  13. saylor

    saylor Senior Member

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    and you end up with this to take to show n tell
    20191230_143218.jpg
     
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  14. 78Brougham

    78Brougham Deplorable FCBO Gold Member

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    My wiring is fine. My headlights work fine as they did 40 and 50 years ago. I don't drive my classics that much in the dark anymore to go through all this hassle. They are not daily drivers, and your still drawing the same amps to fire the bulbs, just rerouting it.. Plain and simple.
    Your wondering why you're having problems with your alternator. You're maxing it out with all the b.s. you have on your car. Have you fixed that yet?
    Many have suggested on how to fix it, now you're off on a tangent doing mods that will do nothing to lower your "end" power consumption. These relay mods are great to do, and commendable, but you're still pulling enough amps, with all your other mods, to light up a small town no matter how you route it.
    Take a moment focus on your alternator problem.
    I'll leave you another one.....
    The more you over think the plumbing, the easier it is to stop up the drain.
    Just trying to keep you on track:poke:
     
  15. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    You aren't going to have that problem.

    The relays you are feeding don't draw much current. I think you understand that.

    The wire feeding the relay just needs to be large enough to trigger the relay. I think you understand that too.

    But, worrying about the power "going the other way" isn't something you really have to worry about. You could run #2 cable from a giant battery to a small relay and the current needed to engage the relay is still going to be the same as if you had #18 wire coming from a small battery. Same for the current trying to flow back to your first relay. It won't be any more than the relay will draw... and a Tyco relay draws less than .2 amps when the relay is energized. Understand?

    Perhaps one of the EE's or techs can explain better than I can.
     
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  16. saylor

    saylor Senior Member

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    totally with you. the whole purpose of the diode in my design was to not allow the low beam relay to fire the high beam relay, but allow the high beam relay to fire the low beam relay.

    with moving to 3 relay method it removes the need for a diode as now each filament is on its own relay. and its slant six dan approved design, so im doing it.

    78brougham we should just agree to disagree and we approach our cars differently. i want 1300 watts of stereo. in fact i want more. I just got another set of 700w 6x9s to throw in. thats gonna take another amp.
    Im going to take an alternator off a 5th avenue and put it on here as soon as i grind the ear off the alternator. then Im probably going to get an isolator https://www.amazon.com/NVX-Mobile-B...ywords=Battery+Isolator&qid=1577765062&sr=8-7 and install a 2nd battery. im also gonna put in a couple of 1k caps. i tried to present the good scientific reasons to do a headlight wiring harness upgrade regardless of stereo or fuel pump or anything, so i dont know why you gotta be so slanted about what i do with my car. i post what i do to my car here so maybe other people can read about it and learn something, or take my idea and make their own like my A/C delete pulley. or how to wire a fuel pump with a safety kill switch so you dont die in a ball of fire. if people like me come here to ask questions about something like sizing a diode or why i got left hand lug nuts and they get salty ass answers like yours god help us all.

    i did fix 'all my problems with my alternator' it charges great and is more stable 14.5v than its ever been. however at idle speed my car requires the power equivalent of a modern car, with electric fans, fuel pumps, interior charging, navigation device, etc., etc. A mopar alternator from 1968 cant do that. its not 'b.s. on the car'. its upgrades from the 2watt stock stereo and mechanical fuel pump and mechanical fan.

    im gonna offload the headlamp power burden from the stock harness like slant six dan suggests. he is one of the foremost lighting experts on the face of the planet right now. and he knows mopar.
    when i cut on my lights i have tremendous voltage drop across the stock wiring. im glad your car does not. i need to repair that voltage drop/loss in my vehicle. regardless of any stereo or fuel pump or electric fan i may try to run. doing this headlight wiring upgrade is the most effective path to repair the voltage losses when i cut on my lights.

    try not to die -

    - saylor
     
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