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Old 02-05-2003, 09:43 PM   #1
brunetmj
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Default Diodes

Diodes are frequently mentioned in a number of threads. I understand the general principle of the diode. A resistor which will allow for the flow of current in only one direction. My question is what kind or type are most commonly used in automotive applications. In what situations would you defintley use one? I mean if you went to radio shack what would you be looking for and why ? How are they physically applied? Soldered onto wire ends?
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Old 02-05-2003, 09:58 PM   #2
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A diode isn't a resistor. It's a semiconductor junction which allows only unidirectional current flow. They have many different uses in circuitry and wiring, but the most common uses mentioned here have to do with isolating different parts of a circuit to prevent interactions. They are also used across relay coils, connected in reverse polarity to the supply voltage, to prevent induced spikes when the coil is energized/de-energized.
The particular application will determine the specs for a diode to be used, such as current requirements and expected peak reverse voltage.
Yes the ones you would typically use are available at Radio Shack (most often 1N4001, 4002 etc.). Another thing you might want to pick up there is a book or two on basic electronics to help answer any other questions you have.
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Old 02-05-2003, 11:30 PM   #3
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Thanks mulder. I have actually purchased three books on basic electronics in a effort to learn. Unfortunately , more often then not, they discuss general principles which are sometimes difficult to generalize to specific tasks. If they discuss a diode they will not stop to say : “and here is how to wire one up and this looks like a good place for one in a car” . One could learn this generalization in a few years but this forum presents and opportunity to shorten the process. By the way if a diode is directional does that not imply it is resisting current in the opposite direction? i.e. a type of resistor? ahh a good try? lol
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Old 02-06-2003, 12:00 AM   #4
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lol, yeah, good try. It's not resisting current flow, it doesn't allow it at all. Think of it as a "road closed" sign. It's not resisting the flow of traffic, it just doesn't allow it at all. Hope this helps. I worked on F18 communication/navigation systems in the Marines and this stuff is second nature to me. I sometimes have a hard time explaining it, though. All in the way you look at it, I guess.
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Old 02-06-2003, 01:08 AM   #5
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Can someone tell me what EXACT resistor I may use to stop the flow of electricity from a stock headlight low/high beam wire....PLEASE. thanks. I am trying to wire a diode for my Morettes so that my lows do not flicker when I turn on my high beams.( FYI=I have HID's, and flickering is bad.)
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Old 02-06-2003, 02:33 AM   #6
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some cases not mentioned where you might use a diode:

if you do not have power locks and you're adding keyless entry, you usually use diodes to separate the left and right door open sensor.

if you're wiring up a warning buzzer to multiple gauges like a greddy peak/hold setup, probably want to connect diodes to isolate each data link unit so current only flows to the buzzer and not to a data link.


TaiChih, not sure what you're asking. if you want to stop flow of electricity, use the biggest resistor you can find. don't know how your morettes are wired right now, can you give some more details on that?
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Old 02-06-2003, 02:59 AM   #7
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Well, I do not have the morettes in right now, but essentially, if I have the low beams on(HID's) and I go to turn on the high beams, the lows will blink off for a second when the high beam engages. This is bad for the HID's especially if I am to flash someone for example. So, what Iwould like to do to prevent the momentary power cut is, between the low and high beam positive wires, I would like to bridge a wire with a diode to switch power between the two settings and still retain low lights constantly on. Does that make sense?
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Old 02-06-2003, 06:35 AM   #8
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TaiChih-
The problem has to do with the ground switching for the headlights. When you switch between low/high beam the ground is momentarily interrupted and this causes the HIDs to flicker and/or cut out. It's both annoying and not very good for them.
I have Morettes/HIDs and had the same problem initially. The solution is to bypass the ground switching altogether and connect the negative side of the ballasts directly to battery negative. This is a better all-around wiring solution for the HIDs anyway since the stock ground wiring is too weak for them to begin with.
Once you do this the HID/low beams will not be affected at all by the low/high switching and they should be more stable.
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Old 02-06-2003, 09:54 AM   #9
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As mentioned, a diode will conduct current in one direction and block it in the other. For current to flow, it goes into the anode and out the cathode. The cathode on most packages is marked with a stipe or band. There are numerous types of diodes for various applications, but the old 1N4001/4002 etc are good for general use and in automotive applications. Some things to be careful of. If you use a diode to make fog lights go on with high beams, the current in the high beam wire is increased by the amount going the the fogs. Usually not a good practice. You can avoid this by using a relay and getting the current through a separate fused wire from the battery, through the relay and then to the fogs. Your high (or low) beams can then be used to activate the coil portion of the relay. You won't need a diode for this implimentation....except for a reverse diode on the relay coil...anode to ground, cathode to activation end of coil. This gives current a path when the coil is de-energised. The collapsing magnetic field will induce a voltage to keep current flowing, so the diode allows it to do this without frying anything in your car.

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Old 02-06-2003, 01:13 PM   #10
TaiChih
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Wow, great advise from both of you, thank you so much for the advice.
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Old 04-14-2003, 08:18 PM   #11
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Just installed them. Mulder, you are the man. thanks
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