Clutch Switch Bypass For Big Blue

[Gary Lewis]

I assume you are not a fan of using add-a-fuse and just draw power from fuse 18 output.

When you are back, maybe I need to understand more about why the relay won't pull in when simply exposed to voltage. (It will draw the current it needs, right?)... See you soon!

Time for a break for water. You can see below how far I've gotten.

No, I'm not a fan of fuse taps. I want things done in a it-can-never-fail way, and that takes solder, shrink tubing, etc. This thing goes everywhere and I don't want things to quit in the back of beyond.

As for the relay's pull-in current, they are mechanical devices, not solid state. The coil moves contacts within the relay, which takes power: wattage = ExI. Small micro relays take a few ma, a Bosch relay might take 250 ma, and the fender-mounted starter relay I have on the bench pulls 3.5 amps @ 14v when pulled in. That's 49 watts. The LED only pulls 1/100th of that so the LED lights but the relay doesn't pull in.

[Gary Lewis]

Time for a break for water. You can see below how far I've gotten.

No, I'm not a fan of fuse taps. I want things done in a it-can-never-fail way, and that takes solder, shrink tubing, etc. This thing goes everywhere and I don't want things to quit in the back of beyond.

As for the relay's pull-in current, they are mechanical devices, not solid state. The coil moves contacts within the relay, which takes power: wattage = ExI. Small micro relays take a few ma, a Bosch relay might take 250 ma, and the fender-mounted starter relay I have on the bench pulls 3.5 amps @ 14v when pulled in. That's 49 watts. The LED only pulls 1/100th of that so the LED lights but the relay doesn't pull in.

Got the relay module tapped into Ckt 640 and tested it - perfect. So now it is going back together, as shown below.

And at the bottom of the dash you can see the module's box is open, ready for the diode - tomorrow. Gonna call it quits for the day as Janey and I are going out tonight.

[viven44]

Time for a break for water. You can see below how far I've gotten.

No, I'm not a fan of fuse taps. I want things done in a it-can-never-fail way, and that takes solder, shrink tubing, etc. This thing goes everywhere and I don't want things to quit in the back of beyond.

As for the relay's pull-in current, they are mechanical devices, not solid state. The coil moves contacts within the relay, which takes power: wattage = ExI. Small micro relays take a few ma, a Bosch relay might take 250 ma, and the fender-mounted starter relay I have on the bench pulls 3.5 amps @ 14v when pulled in. That's 49 watts. The LED only pulls 1/100th of that so the LED lights but the relay doesn't pull in.

I am the same way. I have used fuse taps, and do it very much so on modern vehicles where wires aren't accessible... but for anything mission critical or easily accessible, I either solder and heat shrink or crimp with marine grade water-proof heat shrink tubing that completely 'glassifies' and seals...

And have a great evening!

[Nothing Special]

.... As for the relay's pull-in current, they are mechanical devices, not solid state. The coil moves contacts within the relay, which takes power: wattage = ExI. Small micro relays take a few ma, a Bosch relay might take 250 ma, and the fender-mounted starter relay I have on the bench pulls 3.5 amps @ 14v when pulled in. That's 49 watts. The LED only pulls 1/100th of that so the LED lights but the relay doesn't pull in.

I understand Vivek's question, and I don't really understand Gary's answer. Not that I really need to, it seems to work now, and it definitely will with the addition of the new diode.

But after reading Vivek's question I see that there seems to be a circuit from VCC, through the LED, through the (closed) clutch switch to the starter relay. Yes the LED only draws a small amount, but it sort of seems like every time the clutch pedal is pressed (whether the timer is in use or not) the starter relay will draw its 3.5 amps through the LED (which would likely be pretty unhappy about it).

But as Vivek and I said, the addition of the diode will shut off that path, so there's no long-term concern.

[Gary Lewis]

.... As for the relay's pull-in current, they are mechanical devices, not solid state. The coil moves contacts within the relay, which takes power: wattage = ExI. Small micro relays take a few ma, a Bosch relay might take 250 ma, and the fender-mounted starter relay I have on the bench pulls 3.5 amps @ 14v when pulled in. That's 49 watts. The LED only pulls 1/100th of that so the LED lights but the relay doesn't pull in.

I understand Vivek's question, and I don't really understand Gary's answer. Not that I really need to, it seems to work now, and it definitely will with the addition of the new diode.

But after reading Vivek's question I see that there seems to be a circuit from VCC, through the LED, through the (closed) clutch switch to the starter relay. Yes the LED only draws a small amount, but it sort of seems like every time the clutch pedal is pressed (whether the timer is in use or not) the starter relay will draw its 3.5 amps through the LED (which would likely be pretty unhappy about it).

But as Vivek and I said, the addition of the diode will shut off that path, so there's no long-term concern.

Well, the schematic is now incorrect. It used to have a resistor in series with the LED. It is actually there but it is built into the LED's housing so I eliminated it in the schematic. My bad. I'll put it back.

But it limits the current through the LED to ~20 ma if you put the LED across 12v. I don't have any way of knowing for sure on this one, but most LED's are designed to run at 2v. Assuming that is the case then the internal resistor will be 500 ohms: R = E/I = 10/.02 = 500 ohms.

As for the primary coil of the starter relay, if it conducts 3.5 amps at 12v then it has the equivalent to 3.4 ohms.

I've drawn the circuit w/o the switch, or anything else, to keep it simple. But with the total of 503 ohms there would be roughly 20 ma, or .02 A, flowing through the circuit. Far less than the 3.5 amps needed to pull in the starter relay.

Does that help?

[Nothing Special]

Well, the schematic is now incorrect. It used to have a resistor in series with the LED. It is actually there but it is built into the LED's housing so I eliminated it in the schematic. My bad. I'll put it back.

But it limits the current through the LED to ~20 ma if you put the LED across 12v. I don't have any way of knowing for sure on this one, but most LED's are designed to run at 2v. Assuming that is the case then the internal resistor will be 500 ohms: R = E/I = 10/.02 = 500 ohms.

As for the primary coil of the starter relay, if it conducts 3.5 amps at 12v then it has the equivalent to 3.4 ohms.

I've drawn the circuit w/o the switch, or anything else, to keep it simple. But with the total of 503 ohms there would be roughly 20 ma, or .02 A, flowing through the circuit. Far less than the 3.5 amps needed to pull in the starter relay.

Does that help?

As I was thinking about it more I realized that I had to be making a bad assumption. I was thinking that an LED had the same 0.7V drop as a regular diode. But that clearly can't be right or it would act about like a dead short if it was put between 12V and ground. Having an internal resistor makes a lot of sense!

[Gary Lewis]

As I was thinking about it more I realized that I had to be making a bad assumption. I was thinking that an LED had the same 0.7V drop as a regular diode. But that clearly can't be right or it would act about like a dead short if it was put between 12V and ground. Having an internal resistor makes a lot of sense!

LEDs blow quite easily. One designed for 30 ma fails before you get to 100 ma. DAHIK. My memory is going, but that was only last week, so... But they are BRIGHT for a very brief period.

As for the resistor, there are LEDs with an internal one and LEDs w/o. I started this project with one that didn't have a resistor and quickly rued that decision as I blew one up and broke the leads off of another one.

So I quickly moved on to the one I have now which not only has the resistor built in but also has strain relief on the leads.

[viven44]

LEDs blow quite easily. One designed for 30 ma fails before you get to 100 ma. DAHIK. My memory is going, but that was only last week, so... But they are BRIGHT for a very brief period.

As for the resistor, there are LEDs with an internal one and LEDs w/o. I started this project with one that didn't have a resistor and quickly rued that decision as I blew one up and broke the leads off of another one.

So I quickly moved on to the one I have now which not only has the resistor built in but also has strain relief on the leads.

Addition of the resistor totally makes it clear for me..

But... Would this work ??? Takes out the LED out of the current path so you don't have to worry about voltage drop through a resistor or LED.

 

Edit: I don't think the voltage drop matters as that voltage drop won't affect anything.. anyways. Just add the diode Bob mentioned to your original schematic, should be good!

[Gary Lewis]

Addition of the resistor totally makes it clear for me..

But... Would this work ??? Takes out the LED out of the current path so you don't have to worry about voltage drop through a resistor or LED.

 

Edit: I don't think the voltage drop matters as that voltage drop won't affect anything.. anyways. Just add the diode Bob mentioned to your original schematic, should be good!

No, that won't work the way I planned it. That plan is to have the LED come on when the system is armed so people understand that danger is at hand.

The system works that way now with the exception of the LED coming on each time the clutch is depressed, which is annoying - especially at night. But Bob's diode should fix that. I'll add it tomorrow.

[viven44]

No, that won't work the way I planned it. That plan is to have the LED come on when the system is armed so people understand that danger is at hand.

The system works that way now with the exception of the LED coming on each time the clutch is depressed, which is annoying - especially at night. But Bob's diode should fix that. I'll add it tomorrow.

In that schematic, the LED would definitely come on when the system is armed. The LED is connected to ground on one side and 12V would be supplied via the bosch relay on the other side. But agreed that your current schematic would work as well with the diode!