Diode question 12V DC application

[Flyin900]

I need to restrict a 12VDC wire feed to passing voltage in only one direction. The current draw on the 12V feed is 20 watts, so I think a small diode should fit the bill.
I usually think of a diode for chopping an AC waveform, yet I know they are used in DC circuits too. Will a small 1/2 amp or 1 amp size work? I am not sure of how the current measurement of diodes are done in a circuit.

[J-T]

12v at 20w you'll need at least a 2 amp diode.

God I miss Radio Shack.

[jeepnjohn]

Not sure what you are trying to accomplish and why, maybe you could explain. However, Based on your info you are dealing with P = EI, So the current you are dealing with is 20/12 = 1.7 amps. So no, 0,5 or 1.0 amp diode will not be Technically speaking you would want about a 4 amp device. and if you want to be real safe about a 8 amp device to load it at 50% or 25% respectively.

Also, your resulting voltage will be 12-.7 = 11.3V after the forward biased voltage drop of the diode, So whatever you are dealing with will need to work with that reduced voltage.

Also, Voltage does not "FLOW", so there is no direction to Voltage. Only current flows, so your terminology is not corret. .

Again, If we knew what you are trying to accomplish and why we might be able to give you some better answers on what to do. The only thing I can gander is you are tryiing to incorporate two power sources (batteries) and you want to block current flowing from one battery or the other.

[Flyin900]

Thanks guys I didn't initially want to get into a long explanation of what I am doing, so I will gang 3 one amp diodes to make sure the load isn't too much on the circuit. It is in reference to the CL77 and the removed lighting switch in the headlight bucket. The old circuity was a three way design with the handlebar headlight switch (hi/low) and another headlight switch (2 position) on the headlight bucket and the ignition switch all interconnected.

Since I added turn signals and removed both the original handlebar switch and the headlight switch for newer handlebar controls the circuitry is now different. In order to get the taillight running light to work with the ignition on I needed to wire into the black 12VDC positive wiring of the harness. The circuit 12VDC passes through the ignition switch and splits into two wires, one to the ignition switch from the black 12VDC wiring in the headlight and the other to the taillight low filament running light. It works fine as I intended.

The issue becomes when you want to switch the ignition switch to the park light position which normally only powers the rear low filament taillight, because of the design of the ignition switch and the contacts I cannot get 12VDC just to the taillight only. The switch when turned to the park light position now sends power to the taillight (great) yet also sends 12VDC back up the circuit into the black wiring of the harness and powers the entire harness so everything is live.

If I put a diode into the headlight bucket (easiest access point now) it will allow the current to flow down for the running light operation when the ignition switch in in the on position, yet will block the 12VDC coming back up the circuitry to power the 12V entire wiring system when the switch is in the park position. It will still allow power down the circuit off the ignition switch to power the park position rear taillight filament, given how the switch contacts are designed.

Wasn't that easy

[WintrSol]

Putting 3, 1A diodes in parallel (gang) will not give you a 3A diode. Diodes connected like that will not share equally, as the junction Voltage differs a bit, and one may try to hog the current until it melts, leaving the other 2 to do the same in sequence. Diodes are fairly cheap - try NewEgg, or mayby Grainger supply. You will likely have to buy several, but they ARE cheap.

[Flyin900]

Ok good to know, as I have the 1 amp ones in stock. I will check our local supply house in town for a larger single diode.

[Flyin900]

I picked up a used diode from a TV circuit board. It is an IN5407GP which is a 770V by 3amp rated diode. Since the local electronics supplier is out of business it appears, so not parts there. The diode tests good forward and backwards. Yet after I fabricated a harness to plug it into the main bike wiring harness it doesn't pass forward 12VDC into the wiring circuit. It will block the return current and stop the 12VDC feeding back into the black wiring harness, so it partially works.

Is there an issue with this being an incorrect spec diode that won't work on 12VDC voltage asked to you electronic techs out there?

[mike in idaho]

If that doesn't work out, you could use a 4 or 5 pin relay in the circuit.

[jeepnjohn]

Not sure what bike you had, but I recently had to look quite extensively at my 71 CB350 wiring diagram. My bike essentially has 4 switch position OFF/ON with headlight/ON no headlight/Park. In the park position only the tail light comes on.

From your explanation, I am envisioning you removed the handlebar controls and replaced the ignition switch with a newer switch that does not have the 4th park position.

Again, I do not fully understand, but on my bike, something similar would say you wired it into the wrong place. If your ignition switch does indeed have a Park position, The only thing it should supply power to is the tail light, It is a completely seperate circuit for that purpose. On my bike there are black wires all over the place with colored wires at the switch, TO me it sounds like you just have a wiring issue and you should not need a diode or an electrical device to do this. On my bike, the park function only has to do with the ignition switch so the fact that I might remove the handlebar switches would not impact that.

By crossing black wires, it sounds like you inadvertently combined two separate circuits into 1

[KenCM450]

Yet after I fabricated a harness to plug it into the main bike wiring harness it doesn't pass forward 12VDC into the wiring circuit. It will block the return current and stop the 12VDC feeding back into the black wiring harness, so it partially works.

Is there an issue with this being an incorrect spec diode that won't work on 12VDC voltage asked to you electronic techs out there?

If the diode is blocking current in both directions, it's burned out. Bench test it first with a resistor in series against a 9V transistor battery (hard to burn it up with that). See if it passes current in one direction, but not the other.

Keep in mind the INS1407 is a power rectifier sized diode, in one direction it will lop off 1.2V and everything downstream will get the rest, so if your diode connection on one side has 13.8V to ground, the rest of the circuit will be biased at 12.6V.

In the other direction it might leak a few mA with a few picoFarads of capacitance, and then pretty much shut down.

Not sure if this link will work, but here is the spec:
https://www.datasheetarchive.com/pdf...=P&term=IN5407

[WintrSol]

FWIW, the diode is not the problem. If you can, try drawing a wiring diagram of what you're trying to do - sometimes pictures tell a better story.

[Flyin900]

Ok I appreciate the replies and your efforts to help. I have enclosed a drawing and tried to explain what I am attempting to do here. The switch setup is from the original parts (ignition switch only now) which all others have been removed. It relied on a complicated routing of various wires between the 3 components that were originally in the circuit used to control the headlight/taillight and rectifier feeds etc.


Jeepnjohn it isn't an issue with what you noted. This is a 1966 CL77 Honda 305, which as I noted had 3 different devices from the factory to control the headlights. The park position is a separate circuit on the ignition switch. The fused hot from the battery bypasses the black 12V feed regular from the fused hot when in the last #3 park position on the ignition switch. The 12V hot battery input is fed directly to a white wire on the ignition switch which then feeds the taillight only. This is an emergency position when your leaving the bike broken down at night and can put only the taillight on and remove the key from the ignition switch.

The diodes check fine with only one way feed when tested. I can check the diode harness I fabricated hooked up directly to the bike battery and it will pass 12.4VDC. When the battery alone tests 12.6 VDC, so only a minor drop. When I insert the diode into the wiring harness, it will only pass one third of a volt??? 00.30, so no taillight in the on position on the ignition switch.

As previously noted the circuit work fine without the diode until the park position is selected then the taillight lights, yet the 12V also travels back up the white wire into the black feed wires of the harness and powers everything back up on the bike. I want to block the 12V travelling back up the white feed wire to the black wiring harness system. The diode does stop the power from passing back up the system, as I intended and only the taillight comes on in the park position. I am just not getting a 12V feed now down from the black wiring harness to power the running light in the on position of the ignition switch. The power (12V) is present at the connector yet is not passing through the diode only .3V barely anything.

There is no other way to wire this using the ignition switch internal connections I have looked and tried. I don't see any other feed off the new Honda control switches on the handlebars without turning on the headlight to gain a 12V feed there, which I don't want the system wired to the taillight and where the headlight needs to be on the feed the taillight running light.

wiring.jpg

[Flyin900]

Mike, if I can't solve this with the diode the relay is a great solution. It is more of a real estate issue with not any room down there inside the frame area. Inside the headlight bucket now has all the extra wiring harness wires from the new handlebar controls. I have the Probe Engineering box and the Kohler rectifier and the flasher can all crammed down around the old rectifier area.

See what the brain trust come up with, as the diode is very compact and a simple if it works solution.

[mike in idaho]

Basically, what he's done is to add a jumper wire between the switched power post(black) and the tail light post (white) on the rear of the switch. That gives you a tail light, with the key in run position. One needs to interrupt the power going through that wire, in park position, to keep from back-feeding current into the ignition circuit. That's why I suggested a normally open 4 or 5 pin relay in that wire, controlled by the ignition (black) circuit. Or, you could just connect that white tail light wire to the ignition post on the rear of the switch and eliminate the park light function, it's only real function is to run the battery down anyway.

[Flyin900]

AD thanks for the orientation correction. I did correct it then the forum software took it back to the sideways view.

Trying to keep the function intact with that park position. The diode should work, yet doesn't

The normal circuit works in the park position correctly, yet is feeding the 12V back up the wire into the black wire circuitry. I was hoping the green terminal on the ignition switch would be live off the park position, as that would simplify everything. The green terminal is part of the removed switch pieces and is a dead outlet off the switch in the present form.

The relay maybe the only option. I would like to find a small relay unit if I go that route, as the ones I have in stock are the physically larger 40 amp units.

[jeepnjohn]

Honda CB350 (k4) Wiring Diagram - Not Factory.jpg
Do a continuity check on your ignition switch and make sure in OFF/1/2 the connections work as described in this wiring diagram. All this shoiuld be taken care of in your ignition switch. It sounds like there is a short between circuit 1 and 2. You describe taking circuit 2 and connecting directly to a black wire and that should not need to be done if your switch is working correctly.

[jeepnjohn]

Here are the connections that should test out in the ignition switch position. The tail light (Brown WIre) Does not connect directly to the 12V Black black wire. It gets its power by being wired directly to the 12V in the headlight. The brown n white wire go to the black wire if you removed the headlight switch. The ignition switch is the only thing that controls where the tail light gets its 12 V depending on OFF/1/2.

Untitled.jpg.

[Flyin900]

Here are the connections that should test out in the ignition switch position. The tail light (Brown WIre) Does not connect directly to the 12V Black black wire. It gets its power by being wired directly to the 12V in the headlight. The brown n white wire go to the black wire if you removed the headlight switch. The ignition switch is the only thing that controls where the tail light gets its 12 V depending on OFF/1/2.

Untitled.jpg.

While I appreciate your help I feel your not aware of this situation, as this is not a CB350 or anything like that diagram. The early bikes are totally different than a 1970’s electrical set up. This system had the ignition switch an handlebars hi low switch and another two position headlight switch in the headlight bucket all interconnected. There was other positions on these switches and the ignition switch that I left out since they are not part of the issue.

Mike boiled it down pretty well in his above post. The diode needs to stop the feedback in the feed wire, or a relay as he proposed will work to isolate the issue. I could also wire off the headlight switch gear to run the rear taillight running light, yet the headlight would need to be on to feed the circuit.

I would like some feedback on why the diode isn’t feeding the 12V through the circuit as I have tested each part individually and they seem to work as expected until connected into the 12V feed system.

[Flyin900]

Jeepnjohn I am very good with electrical and reading schematics normally. I know what is going on with the problem, as I have had the ignition switch apart to clean it and determine the contact functions. Since this is a modified electrical system now and I am particular with getting the functionality back close to original.
The diode is what I would like to insert in the system to stop the feedback on the feed wire and need help in sorting out why it doesn’t work as I expected.

[ballbearian]

Having the taillight always on (running light) is an added function. Correct? And therefore getting power from the ignition instead of the lighting (switched) circuit. Dreams are not that way, nor most 60's bikes IIRC.

I admire your tenacity and efforts.

[WintrSol]

wiring.jpg

So, this is your new switch? Is the diode connected directly from the #2 black switch output directly to the #3 white switch output, forward in that direction? This should route current from the black circuit to the white circuit to power the taillight when in the ON position, and when the switch is in PARK, power only the taillight.
Like this:
wiring.jpg

[jeepnjohn]

Well, I guess a retired aerospace electrical engineer cant help you, LOL. The one hint I can give you is that someone has shorted either accidently or intentionally The parking circuit to the other circuit. Find that issue and fix it first.

Then, what ever it is you are trying to do in removing original switches with different switches (adding additional logic circuits using diodes/relays) to get a running daytime tail light (with/without headlight being on) should be done in the other circuit independent of the park circuit.

BTW:
I looked back at the schematics for a CA77 and also back to all bikes back until 1963. Some use one combination switch, others break out the switches into multiple switches mounted in different places on the bike, HOWEVER - Electrically they all work the same way from what I saw from 1963 - 1972 at least.

The applications tend to use different color coded wires which can be confusing to some

Anyway, Good luck in your task - This is my last post in this thread.

[Flyin900]

So, this is your new switch? Is the diode connected directly from the #2 black switch output directly to the #3 white switch output, forward in that direction? This should route current from the black circuit to the white circuit to power the taillight when in the ON position, and when the switch is in PARK, power only the taillight.
Like this:
wiring.jpg

The diode is connected in that fashion I believe yet not on the switch. That is the original Honda ignition switch not a replacement. I have picked up the black wire feed inside the headlight bucket in a 3 way black female bullet connector. This is the end terminus of the white wire that feeds the running light filament in the taillight.
When I test the voltage output in that black connector there is about 12 volt with ignition in the on position, since the whole black wire system is active with the coils and front turn signal running lights, which are the two other wires plugged into that 3 way connector.
The diode isn’t passing the 12 volts when connected through into the white wire only one third of a volt registers on the meter, so no taillight. It does block the return voltage feed when the park position is selected on the ignition switch though so that part works as expected and the whole system isn’t powered up. The taillight does come on too in this park position, so the diode is doing the job of blocking the 12volts travelling back up into the whole electrical system.

Basically there is one white wire running from the headlight to the taillight with a split off to the white terminal on the ignition switch, then carrying on to the taillight. That one white wire is fed from two different 12V switch positions, but not simultaneously. It depends if the switch is in the on position or the park position. So the entire black wire system on the bike is powered up in either switch position since I am picking power up from the headlight black connector.

I am trying to stop the transmission of power back up the white wire into the black wire circuit with the diode when the switch is in the park position. Only allowing the power down the white wire to the taillight. The diode does do this function when connected into the harness, it just isn’t allowing the 12V to pass down through to the taillight when the ignition is in the on position. Only one third of a volt as measured?

[Flyin900]

Well, I guess a retired aerospace electrical engineer cant help you, LOL. The one hint I can give you is that someone has shorted either accidently or intentionally The parking circuit to the other circuit. Find that issue and fix it first.

Then, what ever it is you are trying to do in removing original switches with different switches (adding additional logic circuits using diodes/relays) to get a running daytime tail light (with/without headlight being on) should be done in the other circuit independent of the park circuit.

BTW:
I looked back at the schematics for a CA77 and also back to all bikes back until 1963. Some use one combination switch, others break out the switches into multiple switches mounted in different places on the bike, HOWEVER - Electrically they all work the same way from what I saw from 1963 - 1972 at least.

The applications tend to use different color coded wires which can be confusing to some

Anyway, Good luck in your task - This is my last post in this thread.

Apologies, as I know your trying to help and I an certainly stuck on this one. I am sure your correct in your assessment, yet you never know on a forum who is giving advice and their background.

[Flyin900]

Having the taillight always on (running light) is an added function. Correct? And therefore getting power from the ignition instead of the lighting (switched) circuit. Dreams are not that way, nor most 60's bikes IIRC.

I admire your tenacity and efforts.

Yes I am “ Like a dog on a diode” so to speak

Not sure what the original set up was, as this was a non working bike and no testing was done on the electrical switches or their functions initially. I would think their should have been a daytime running light switch function, yet in the 60’s maybe not.

[Richard Pitman]

Not sure if this is relevant at all, but my pic below shows how I wired the flasher repeater lamp, after fitting led flashers and matching relay. Standard lamp passes current in both directions, which upsets an led setup.

[Flyin900]

Well after jumping across the ignition switch with the diode wiring as suggested the system works as I wanted now. Every function is correct on the ignition switch with all rear running light positions correct in (on and park) selections. All electrical works as designed including the signals and neutral indicator lights (small LED's) and the front signal running lights etc.

All bulbs are stock with no LED's, yet no clue as to why the system was hinkey... it's beyond my understanding and pay grade.

A big thank you to all for the suggestions and solutions with my determination to get it right.

[WintrSol]

Glad you got it working! Post #23 implied you had a very high resistance in the connections to/from the diode. Now, there is none.