Relay for Tow Charge Indicator
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=145833
Printed Date: May 02, 2024 at 12:31 AM
Topic: Relay for Tow Charge Indicator
Posted By: jrschatz
Subject: Relay for Tow Charge Indicator
Date Posted: December 04, 2019 at 9:33 AM
Hi All - I am trying to create an indicator in my cab to show when my trailer is being charged (or when any power is being used on the 12volt terminal of the 7-pin adapter for that matter). I’ll share my thoughts on what I tried and ask for help as to why it isn’t working — I’ve tried searching for relay diagrams that may be of help, but I can’t seem to find out why it isn’t working.
My thought was to connect a relay in line with the 12v power lead going to the 7-pin trailer plug. I thought once a trailer was hooked up and the battery was properly charging, it would connect the circuit on this lead to ground and activate the relay. Then connect a small led indicator to 87, with power from 30, and I would know when it was on.
When experimenting with this set up, I found that the relay does activate when needed, but I am getting reduced power through the 12v circuit to the trailer. I was trying it out with just a test light and the bulb would barely light up — obviously I have something wrong in my plan here but I can’t seem to figure out where it is or if there is a way to actually accomplish this. Any help greatly appreciated!
Replies:
Posted By: jrschatz
Date Posted: December 04, 2019 at 9:50 AM
So as I tried to troubleshoot this is my thinking correct that my problem is I have this relay installed in series essentially with a high current draw wire?
Posted By: eguru
Date Posted: December 04, 2019 at 10:02 AM
Are you just wanting to know if the trailer side of the 7-pin connector is receiving 12V or do you want to know how much current is being consumed by the trailer?
Posted By: jrschatz
Date Posted: December 04, 2019 at 10:09 AM
Good question — I suppose either option would work. Two reasons why I want the info:
1- I want an indicator that my trailer is being charged because sometimes the fuse blows an I’m left with a dump trailer that has a dead battery
2- I have a small salt spreader that runs off the 12v power of the 7-pin and I’d like to know when it is running
Knowing amperage for #1 could be helpful
Posted By: eguru
Date Posted: December 04, 2019 at 1:57 PM
What is the current rating of the fuse feeding the trailer connector?
Posted By: jrschatz
Date Posted: December 04, 2019 at 3:42 PM
20amp
Posted By: eguru
Date Posted: December 05, 2019 at 9:00 AM
I was hoping to suggest a current measuring solution that was relatively inexpensive - such as this:
https://abra-electronics.com/test-equipment-tools/meters/digital-panel-meters/pm-50-voltmeter-and-ammeter-combination-panel.html
However, the setup requires that the shunt be installed in series with the ground wire. That is not practical with a trailer as there is usually a ground path through the hitch and safety chains. An alternate measurement method with a shunt in the +12V feed could be devised but that would probably cost about $75 to implement.
So my only suggestion (so far) would be to add an additional connector to the trailer that connects to Pin 4 on the trailer side and feeds an LED inside your tow vehicle. At least you will know that 12V is available at the trailer.
Posted By: jrschatz
Date Posted: December 06, 2019 at 6:44 AM
Would a clamp-style ammeter work?
Posted By: eguru
Date Posted: December 06, 2019 at 8:42 AM
jrschatz wrote:
Would a clamp-style ammeter work?
Yes a clamp-on meter would work. Just make sure it is rated for DC - many are AC only.
A cheap fix for making the item in the link work on the +12V side would be to power the display with a 9V battery.
Posted By: jrschatz
Date Posted: December 06, 2019 at 4:36 PM
Thanks for your help.
And for the sake of learning — is the reason my relay would not work because I was trying to run it in series with the 12v line?
Posted By: eguru
Date Posted: December 06, 2019 at 5:16 PM
jrschatz wrote:
Thanks for your help.
And for the sake of learning — is the reason my relay would not work because I was trying to run it in series with the 12v line?
Correct, the relay coil cannot be in series with a low resistance (high current) load.
Posted By: i am an idiot
Date Posted: December 07, 2019 at 4:17 AM
Have you thought about an amp meter installed in the truck? 15 bucks on amazon, you will have to put the shunt in series with the wire going to trailer. It will give you voltage and current real time.
Posted By: eguru
Date Posted: December 08, 2019 at 10:12 AM
i am an idiot wrote:
Have you thought about an amp meter installed in the truck? 15 bucks on amazon, you will have to put the shunt in series with the wire going to trailer. It will give you voltage and current real time.
Do you have a link?
Posted By: i am an idiot
Date Posted: December 08, 2019 at 12:49 PM
Posted By: eguru
Date Posted: December 08, 2019 at 7:18 PM
i am an idiot wrote:
https://www.amazon.com/s?k=12+v+dc+ammeter&ref=nb_sb_noss_2
At the $15 price point you quoted in your post, I don't see any products where the shunt is installed in the +12V feed - all are installed on the ground side. As I explained in my earlier post, monitoring the ground side for a trailer application is not feasible.
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