Unique wiring challenge on travel trailer

Thread Starter

Farmaller48

Joined Jun 30, 2016
25
@crutschow ... I have built the diode/capacitor/relay circuit that you drew a diagram of for me, using the PWM trailer brake supply for the necessary energy to trigger the relay. The good news is that it works! I grabbed 12V power from the onboard 12V battery, routing it through the relay (into the "30" post and out from the "87" post on the relay) to the new LED 3rd brake light. When I press the brake pedal in the truck, the relay triggers and the new 3rd brake light illuminates as desired.

Now the snafu (because there apparently always has to be one!). The capacitor is releasing enough energy about every three to five seconds to trigger the relay on/off rapidly, even when the brakes are not activated. I went back and read through your posts and see that you asked for the output voltage with the diode/cap in the circuit. I will have to run that test tomorrow because it got dark on me before I remembered to test that voltage reading.

Any thoughts on the capacitor randomly discharging and triggering the relay on/off rapidly? In the spirit of full disclosure, I was unable to find (locally) the exact cap you recommended [2700uF 16V], so I got as close to that as possible. I am using a 2200uF 35V cap. I know I can order the 2700uF 16V, but wanted to at least test the theory as soon as I could.
 

GopherT

Joined Nov 23, 2012
8,012
@crutschow ... I have built the diode/capacitor/relay circuit that you drew a diagram of for me, using the PWM trailer brake supply for the necessary energy to trigger the relay. The good news is that it works! I grabbed 12V power from the onboard 12V battery, routing it through the relay (into the "30" post and out from the "87" post on the relay) to the new LED 3rd brake light. When I press the brake pedal in the truck, the relay triggers and the new 3rd brake light illuminates as desired.

Now the snafu (because there apparently always has to be one!). The capacitor is releasing enough energy about every three to five seconds to trigger the relay on/off rapidly, even when the brakes are not activated. I went back and read through your posts and see that you asked for the output voltage with the diode/cap in the circuit. I will have to run that test tomorrow because it got dark on me before I remembered to test that voltage reading.

Any thoughts on the capacitor randomly discharging and triggering the relay on/off rapidly? In the spirit of full disclosure, I was unable to find (locally) the exact cap you recommended [2700uF 16V], so I got as close to that as possible. I am using a 2200uF 35V cap. I know I can order the 2700uF 16V, but wanted to at least test the theory as soon as I could.
You have made a relay oscillator. Try without the cap.
image.jpg
 

crutschow

Joined Mar 14, 2008
25,686
@crutschow ....................

Now the snafu (because there apparently always has to be one!). The capacitor is releasing enough energy about every three to five seconds to trigger the relay on/off rapidly, even when the brakes are not activated. I went back and read through your posts and see that you asked for the output voltage with the diode/cap in the circuit. I will have to run that test tomorrow because it got dark on me before I remembered to test that voltage reading.

Any thoughts on the capacitor randomly discharging and triggering the relay on/off rapidly? In the spirit of full disclosure, I was unable to find (locally) the exact cap you recommended [2700uF 16V], so I got as close to that as possible. I am using a 2200uF 35V cap. I know I can order the 2700uF 16V, but wanted to at least test the theory as soon as I could.
The capacitor is not the problem since it can't generate voltage by itself.
There must be enough voltage from the PWM circuit to trigger the relay even when the brakes aren't on.
Measure the PWM voltage when the brakes aren't applied.
The 2200μF, 35V capacitor should be fine.
Where are you getting the power for the relay contacts that go to the LED?
 
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Thread Starter

Farmaller48

Joined Jun 30, 2016
25
The capacitor is not the problem since it can't generate voltage by itself.
There must be enough voltage from the PWM circuit to trigger the relay even when the brakes aren't on.
Measure the PWM voltage when the brakes aren't applied.
The 2200μF, 35V capacitor should be fine.
I thought you may ask for that measurement, so I did measure the voltage of the PWM (+) wire that I tapped into to trigger the relay. Without the brakes applied, the reading was 0.00 (multimeter set to 20V, then to 9V, then to 1.5V just to see if I could get any voltage reading at all). Double checked the multimeter contacts to make sure it was properly grounded, and it was. I'm baffled. :confused: I feel like there must be some amount of voltage coming through that circuit in order to load the capacitor.

I also ran a test where I removed the diode/cap from the relay trigger circuit and just powered the relay directly from the PWM hot lead, then applied the brakes to see if the relay would trigger, but it would not.
 

Thread Starter

Farmaller48

Joined Jun 30, 2016
25
Sorry, but that makes no sense. :confused:
The contacts only go to the power and LED, not back to the input as you show.
The capacitor is to smooth the peak voltage from the PWM signal.
Just to make sure we keep down any confusion, let me clarify that all of my PWM voltage measurements have been at the brake drum on the trailer. Let me know if I am taking those measurements in the wrong place.
 

Lestraveled

Joined May 19, 2014
1,946
@Farmaller48
@crutschow
I have a Prodigy brake controller and it periodically outputs a very narrow pulse to determine if it is connected, or not, to the trailer brakes. The pulse is not wide enough to activate the brake but it is wide enough for the prodigy to tell if it is connected to the trailer brakes by detecting the current. I think this is what is happening.
 
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crutschow

Joined Mar 14, 2008
25,686
@Farmaller48
@crutschow
I have a Prodigy brake controller and it periodically outputs a very narrow pulse to determine if it is connected, or not, to the trailer brakes. The pulse is not wide enough to activate the brake but it is wide enough for the prodigy to tell if it is connected to the trailer brakes by detecting the current. I think this is what is happening.
That would explain the relay and light pulsing.

Perhaps a small resistor (a few ohms) in series with the diode would allow the capacitor to absorb the pulse without energizing the relay.
 

Thread Starter

Farmaller48

Joined Jun 30, 2016
25
@Lestraveled - That makes perfect sense to me. My truck tells me when the trailer has been disconnected.

@crutschow - does it make any difference whether the resistor is placed pre-diode or post-diode in the current flow? What would you recommend on the max ohms rating of the resistor?
 
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Thread Starter

Farmaller48

Joined Jun 30, 2016
25
@Farmaller48
@crutschow
I have a Prodigy brake controller and it periodically outputs a very narrow pulse to determine if it is connected, or not, to the trailer brakes. The pulse is not wide enough to activate the brake but it is wide enough for the prodigy to tell if it is connected to the trailer brakes by detecting the current. I think this is what is happening.
@Lestraveled - I ran a test this morning to try to validate your theory. The result was that the PWM module of the truck is sending a signal of less than 1V to the trailer brake circuit every 4 seconds. I believe this is designed to validate whether or not a trailer is connected, then report to the info center on the dash the message "No Trailer Connected" as appropriate.
 

Lestraveled

Joined May 19, 2014
1,946
@Farmaller48
Have you inserted a small resistor in series with the diode like @crutschow suggested? Start at 5 or 10 ohms. The higher the resistance, the less it will be affected by short pulses, but will be less able to activate the relay. There will be a optimum resistance. It does not matter if the resistor is pre or post of the diode.
 

crutschow

Joined Mar 14, 2008
25,686
I ran a test this morning to try to validate your theory. The result was that the PWM module of the truck is sending a signal of less than 1V to the trailer brake circuit every 4 seconds. I believe this is designed to validate whether or not a trailer is connected, then report to the info center on the dash the message "No Trailer Connected" as appropriate.[/USER]
If your measured that with a multimeter you likely are not getting a correct reading since the meter can't respond to a short pulse.
You need to measure it with a series diode and a small capacitor to ground so you can determine the peak voltage (which I expect is near 12V).

Try what Les suggested.
 

Thread Starter

Farmaller48

Joined Jun 30, 2016
25
@Farmaller48
Have you inserted a small resistor in series with the diode like @crutschow suggested? Start at 5 or 10 ohms. The higher the resistance, the less it will be affected by short pulses, but will be less able to activate the relay. There will be a optimum resistance. It does not matter if the resistor is pre or post of the diode.
Went to Radio Shack looking for resistors. I could only find 10 Ohm and 15 Ohm, both in 1/2 watt. Since we didn't discuss wattage ratings for the resistor, just wanted to ask if 1/2 watt rating is too low?
 

crutschow

Joined Mar 14, 2008
25,686
You can put resistors in parallel to get lower values.
Two of the same value in parallel give 1/2 the resistance.
Three in parallel give 1/3 the resistance, etc.
Here's a calculator for arbitrary resistors in parallel.
 

Thread Starter

Farmaller48

Joined Jun 30, 2016
25
You can put resistors in parallel to get lower values.
Two of the same value in parallel give 1/2 the resistance.
Three in parallel give 1/3 the resistance, etc.
Here's a calculator for arbitrary resistors in parallel.
I have sat on this and just spend some time thinking about it for a week now, and I think maybe a different approach may work. If we were to try to power the relay from the 12 volt power source feeding to the stock tail lights on the travel trailer, obviously it would trigger the relay when you press the brake. However, it would also obviously trigger the relay each time you used the turn signal on that side. Since the turn signal cycles on and off about every half second, would there be anything that we could do that would create a delay of more than a half second, effectively suppressing the relay trigger event because there is not a steady 12 volt power supply going to the relay for more than a half second? Any thoughts?
 

crutschow

Joined Mar 14, 2008
25,686
....................
Since the turn signal cycles on and off about every half second, would there be anything that we could do that would create a delay of more than a half second, effectively suppressing the relay trigger event because there is not a steady 12 volt power supply going to the relay for more than a half second? Any thoughts?
The problem with that is, it will delay the turn-on of the brake light for at least 1/2 second, which is what you don't want to prevent a rear ender. :eek:
 
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