Diode Help

[pcross71]

Hi, I'm new to this site and hoping to get some help on a project. I'm working on installing something (heated grips) on a motorcycle that uses a 3 way toggle switch (for off, hi, low) and I am trying to add a single LED to illuminate when the switch is on (either hi or low). I know enough to know that connecting leads from the switch output prongs to a single LED will connect the two separate hi and low circuits together and believe I need to insert diodes on each leg in order to isolate the two circuits.

So I've run a wire from each switch output post (hi and low), each through separate diodes (a very small item from radio shack but unfortunately I tossed the packaging so not sure on specific model) and then connected the cathode side (black stripe) of the two diodes together to the LED annode, then grounded the cathode leg of the LED (forgive me if I've butchered terms, but think i got that all right).

So I hoped this would result in the lo side staying off when the hi side is switched on, and vice versa, but both would illuminate the LED. No such luck. The LED lights, but 12v is passing through both legs of the switch (i.e., when switched to hi, the low side is also getting 12v through the diodes circuit (despite the diodes).

So have I misunderstood how diodes are supposed to work? Is this the right approach but I've just got the wrong diode perhaps? The max draw on the main Hi side circuit is 35w but I would think much less than that would be pulled through the diodes/LED circuit as it is a dedicated circuit. I've seen some diodes with three prongs and wonder if these are effectively two diodes in one package that might work for my application.

Thanks so much for any help here.

 

[DerStrom8]

Welcome to Electro-Tech-Online, pcross71!
I think it would be most helpful if you could post a schematic of what you have done so far. Your description is rather confusing at first, so a diagram would help a lot.
Thanks, and keep posting!
Der Strom

 

[SABorn]

I think what you are trying to do is like this and if wired this way it should work.

You make no mention of a resistor in the circuit and suspect you have damaged the led with applying 12 volts to it.
The 1K resistor could be reduced to 560 ohms but 1 K should be bright enough and safe for your operation.

 

[Diver300]

It could be that the 12 V is coming through the heated grips. If you disconnect the diodes and check the voltages then you can see if that is the case.

SABorn's circuit is what I would have used.

 

[MikeMl]

How about using a Red-Green Bi-Color Common-Cathode LED. Then you don't need extra diodes, and you have an indication that tells you that nothing is on, or if LO(Green) or HI(Red) is on.

 

[pcross71]

you guys are the best!
Yes, that schematic is exactly what i've done. The LED i got from RS has a built in resistor. The LED lights fine, on both high and low, but i believe current is making its way back through the diode. Is it possible that I've got the wrong type of resistor (the ones i bought were the only ones in the parts drawer at my RS, and are about 3/16" of an inch long, 1/8" diam). I can isolate from the grips but it'll mean more cutting and soldering, oh well...
Thanks so much.

 

[MikeMl]

If the LED indicator lamp has an internal resistor, no external resistor is needed. Just three parts: two diodes (1N400x, where 1<=x<=7), and the indicator lamp.

 

[SABorn]

You have a little trouble with part names, as now you are talking resistors, when i think you mean diodes.

I cant see that the wrong diode would matter in this case as if you did have a zener diode instead of a standard silicon diode it would still function correctly for this circuit.

But when i read your post again you want the led on for the high side only and off for the low side the above circuit will not do that and it will need to be wired like this below.

Note there is no need for the diodes, and should you get feedback that lights the led on the Lo setting then you have a earthing problem that needs to be fixed.

 

[Reloadron]

Going back to here:

So I hoped this would result in the lo side staying off when the hi side is switched on, and vice versa, but both would illuminate the LED. No such luck. The LED lights, but 12v is passing through both legs of the switch (i.e., when switched to hi, the low side is also getting 12v through the diodes circuit (despite the diodes).

Possibly the trick or mystery to this is how the heating elements in the grips are configured. Attached is a rough guess at one possible configuration.

What I think is going on here is that there is not a high and low heater but rather two identical elements in each grip. You mentioned about 35 watts in high mode. Quite possibly each grip contains two heating elements of about 17 watts each. When in low mode only a single element in each grip is used but in high mode both elements are used similar to the attached.

This is where an ohmmeter would come in handy. Each grip should have a single common (ground) with two other wires (high and low). If you measure from common to each hot line and see about the same resistance, things likely work as I think they may. This would explain why when going from low to high there is still power on the low line.

<EDIT> Additionally please note the switch I used in the cartoon is purely for illustrative purposes. I seriously doubt they use a DPDT switch like that but more likely a slide switch which when advanced from OFF makes a first contact and then a second contact while maintaining the first. I believe this is why when the switch is moved to high there is still 12 volts present on low. </EDIT>

Just My Take
Ron

 

[SABorn]

Ron,

That theory dont work for me.

As there can be no feed back if the ground is a secure conection with no resistance.

But if the heater was configured like this (which is what i think you meant) both elements can be the same value, and feed back is going to happen.

 

[Reloadron]

SABorn

What I am saying and what I drew is that there are two identical heating elements in each grip that are identical that share a common ground. That was how I drew it and think it is configured.

When power is applied to the first element we get low heat, when power is applied to the second element along with the first element we get high heat. The switch matrix I shoved in there is set up that way. When power is applied to the first element one and only one element is powered, however when power is applied to the second element both elements are powered.

That would explain the symptom the original poster is seeing. You turn on low heat and the Low output has 12 volts off the switch, you advance the switch to high heat and the low heat still has 12 volts present.

What I am saying is that this would be normal by design and have nothing to do with feedback through any element. That all of the blocking diodes on the planet will not change things as to how the 12 volts is still present on low when high is selected.

Place a LED across the low output and place another LED across the high output of the switch. When low is selected a single LED will illuminate, move the switch to high and both LEDs will illuminate. See the attached where I added a LED to the high heat side.

Anyway, what I had in mind is how I drew the circuit. Generally when using heating circuits that have high and low heat it is done this way. Again, feedback of any sort is not why I think the original poster is seeing 12 volts present on the low power out even when high is selected. I believe that even with the heater elements removed from the switch the symptom would exist.

Just My Take
Ron

 

[crutschow]

But when i read your post again you want the led on for the high side only and off for the low side the above circuit will not do that and it will need to be wired like this below.

I don't understand why you say that. He stated in his first post that "I'm working on installing something (heated grips) on a motorcycle that uses a 3 way toggle switch (for off, hi, low) and I am trying to add a single LED to illuminate when the switch is on (either hi or low)."

 

[SABorn]

So I hoped this would result in the lo side staying off when the hi side is switched on, and vice versa, but both would illuminate the LED. No such luck. The LED lights, but 12v is passing through both legs of the switch (i.e., when switched to hi, the low side is also getting 12v through the diodes circuit (despite the diodes).

As he spoke of a "LED" i assumed only 1 led and it indicated Hi on only.

If he wants an indicator for both high or low with a single led than the first circuit i posted will work.

contray to a earlier post i made if the diodes were zener than that would explain why the circuit is not working, they need to be just a normal silicon diode.

I think the best solution was posted earlier with the use of a tri coloured led and use green for lo and red for hi as it solves all problems with 1 led and gives a indication of what is switched on.

 

[pcross71]

Wow - thanks for all the ideas - since several of you have been kind enough to chime in, let me respond to your comments by poster:

MikeMI: a "Red-Green Bi-Color Common-Cathode LED" would be perfect and in fact an improvement (separate colors for hi/low side). Radio Shack had a two color LED but it had only two leads and I suspect was static for one color depending upon how wired. Assuming the LED you reference would keep the hi/lo sides separate (3 leads, 2 anodes, 1 cathode?), then this would be ideal (I assume I'd need to add resistors). Would you be kind enough to point me in the right direction (ideally to a place where I can order just 1 or 2). That would be my fall back plan (ideally would like to get my current solution to work as I've already drilled for the LED (which sits in a 1/4" hole)). As to your second post, I have to admit I have no idea what you mean by "(1N400x, where 1<=x<=7)" - I'm trying to learn as I go here but just don't follow that. But as you say, the LED does have an internal resistor and is spec'd to 12V and I have just the three parts (2 diodes, 1 LED).

SABorn: First, thanks for the schematic - I believe it is exactly what I've done. Yes, I typed too quickly on my second post - meant to say diodes when I was providing the no-doubt useless size info. Sorry about that. To your next point, I do want he LED to be on either hi or low - sorry if I confused on that. If it was just mean to be on one side of the switch I don't think I'd be having any trouble. For that matter, if I were willing to connect two separate LEDs for hi/low I'd be fine. Its just having one LED without connecting the two separate circuits sent me down the diode path based on a very hazy recollection electronics. A two color LED that would preserve 2 circuits would be fine but I've got everything wired for a single LED and two diodes and it would be great to get that to work - I just want to make sure I don't accidently knock it on while riding, don't really need 2 colors.

Reloadron: The switch is a simple toggle with three posts (middle off, down low, up high). Each grip has three wires (high, low, ground and the two sides are tied together (two high leads to hi switch pole, two low leads to low switch pole, two grounds to ground). I added separate leads from the high and low switch poles, inserted diodes, connected cathode sides of diodes to the anode of a LED (w/resistor) and grounded the cathode of the LED). I can meter around but it seems odd to me that, for example, the low post on the switch would show 12v+ when the switch is on hi (as it does per my metering as configured). And vice versa. Here's the product i'm using:
Heated Grip Kit
Not sure you can tell much from the site. I believe (based on nothing concrete) that there are two separate elements (or more likely the hi side hits 100% of the elements and the low side some lower %). If so, would the opposing circuit test positive when not switched on?? My concern is that I've either (a) put the low side to full power or (b) worse yet, connected two separate hi and low elements to create an ultra high (seems unlikely). I can test this but that will require me to cut out the work i've done - will likely do that tmrw.

Again, thanks so much for your expertise on this. And your tolerance of my ignorance.

 

[Reloadron]

My guess, and again it is purely a guess. is that there are dual elements for each grip. That low heat consist of running a single element. When high heat is selected the low element still runs but with the added high element. Thus once On the low will always be On. That is purely my guess but a common configuration used for heaters, including a little cheap 15/30 watt soldering pencil I have. One way to see for sure would be to disconnect the elements from the switch completely. Leave the switch 12 volt power connected. With the switch off both low and high outputs should have nothing. Advance the switch to low. Only the low output should have power, the high should be nothing. Now advance the switch to high. If both outputs from the switch are 12 volts then I figure it would be safe to assume it is configures as I think it is and as I drew it. Again, pure speculation on my part. I just figure it would explain the symptoms you see from one angle.

If that is true then two LEDs would be a solution as I last posted in my drawing. A single LED would be illuminated for low and both for high.

<EDIT> There are ways to get the results you want if it works the way I think it does but it gets just a little more complicated. </EDIT>

Ron

 

[MikeMl]

Here is how to wire the BiColor LED. Here is where to get it.

After thinking about it, putting just a single resistor from Cathode to ground will not work. The reason is that when one LED is on, the other LED is subjected to an excessive reverse voltage. Using two resistors and grounding the Cathode will work just fine, as the off LED is at zero reverse volts.

 

[pcross71]

OK, I'm get some time over the weekend to meter around.
If I go with the dual color LED linked above, is the following reistor appropriate for a 12v system? (used some web calculator).
"The wizard recommends a 1/2W or greater 330 ohm resistor."
And as if the above wasn't a dumb enough question, my middle of the night worry is that when i tested the voltage at the switch prongs, I may have left the cathode / ground side of my LED + diodes assembly ungrounded (I was half way through the install when i started testing) - would that lead to this problem (i.e., would two diodes with cathodes connected and not grounded fail to work as a current gate?). Again, I need to double check where it is (I certainly grounded the assembly to test the LED but may have just done so temporarily). The bike is at another location so can't do till tomorrow.

 

[pcross71]

I got some time to work on it and thought I'd report back.
Grounding the diode/LED assembly made no difference (which makes sense if you keep reading).
I confirmed that the switch hi lo sides are independent (when on hi, the high prong alone shows 12v+ and same for Low when I connect just the 12v main and nothing else to switch.
I cut out the diode assembly and it turns out you guys were right: when everything is wired in stock form (two leads to each grip, one high, one low, and 2 separate grounds from each grip), and switch set to high, the low side shows 12v as well. and vice versa. This makes no sense to me - i don't understand how there are two different heat levels if both sides test 12v+ at the switch regardless of whether switch is set to high or low.
But it works.
So, I eliminated the two diodes and connected the LED and resistor to just one side of the switch and it lights for either hi or low, ans stays off for off. I wonder if a bicolor LED would even work given that both sides are sending 12v whether on hi or low.

In any case, all is well, so thank you very much for all the help and education.

 

[Reloadron]

i don't understand how there are two different heat levels if both sides test 12v+ at the switch regardless of whether switch is set to high or low.
But it works.

If I read everything you posted correctly it makes sense to me. You have a low and high range. When in low a single element is working and the Hi out at the switch is off. However, when you advance from low to high then both elements are used. Thus at the switch both the low and high connections have output.

Let's say you have two elements of 17 watts each. When on low you are using a single 17 watt element. When you advance to high you bring in the other 17 watt element for a high total of 34 watts or so of heating elements. One element on for low and both elements on for high. That is my take anyway. Makes sense to me that way.

I don't see a simple solution for a bi colored LED short of possibly adding a logic gate chip. I can see a single LED on for low and two LEDs on for high.

Ron