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Having two power sources for a direct wire scanner installation.

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Mbk127k

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Hey everyone, i just have a few questions and wondering what peoples thought were or if it's possible to be done. I recently installed a BCT15X into my truck, i direct wired installed it due to saving cigarette plug space for other electronics such as my fire dept. light stick, bluetooth fm transmitter and phone charger.

Now i installed it where it went to the ignition/radio fuse section so when I shut the truck off the scanner goes off as well (so the battery doesn't die or use juice), it also has a delay when you shut the car off the radio and all stays on until you open the doors or wait like a few minutes and it shuts off.


I was trying to see if i could still have the same setup except possibly running a y off the power/red wire where it could still be used with the ignition but as well as having a secondary power source by switch so in case the truck is off and i still want to listen to it all i would have to do is turn the power switch and have power coming from there to the scanner. So pretty much the switch would act as a secondary power to the scanner when the ignition button is off.


Thoughts and Ideals are welcome as well with some help on the subject, wanted to hear what people thought or if this is good?.

Mark
 

QDP2012

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  • Many people have done something like this. An old fire chief I knew years ago did this with his transceivers in his personal SUV. The switch acts as a bypass around the ignition. He was an electical engineer by profession, and knew how easy it was to accidentally leave the switch in the bypass position, and thereby drain the vehicle's battery.

    If I remember correctly, as a visual-reminder that the switch was engaged in bypass mode, he installed one of the flip-up red rubberized switch covers, like you see in the movies. When the cover was down, the switch was in "ignition" position. The cover had to be up for the switch to move to the "ignition-bypass" position. Even if he was just listening, he didn't leave it in "bypass" mode more than a few minutes before starting the vehicle. He had several 1980's/1990's -era Motorolas that would kill the battery quickly.

    I don't know if he had any other circuitry to protect the radios, but sometimes he would bypass the ignition as he entered the vehicle, so he could monitor radio traffic uninterrupted, while trying to get the vehicle started.

    I have not installed this setup yet, so I will differ to the experts here on how to do so safely and correctly.

  • Also, in case it helps avoid dead car batteries, you might consider something like a "Battery Buddy" circuit-breaker type device that mounts to or near the car battery and auto-disconnects the battery from the car when it gets low enough to risk not starting the car. The idea is that when you return to the vehicle, you can still lift the hood, reset the breaker, and start the car, instead of being stuck somewhere with a dead battery. Maybe someone else here has used a device like this and can give a better idea of how well they work, or don't, and which one(s) to use or avoid.

  • (Other people with heavy demands, and with a suitable vehicle, sometimes choose to install dual batteries with isolators, etc. This also reduces the risk of not being able to start the car.)

Hope this helps,
 
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N0YFE

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You could run a bypass switch and wire it in Y configureration to power the radio. I would suggest to have a diode in line from the accessorty to the radio to avoide backfeeding the accessory circut when you are using the "bypass switch". I would suggest running power directly from the battery to a relay and then to your devices. You would still run a wire from the accessory to the relay coil and a bypass switch but there would not be as much load on the accessory circut as it is only operating the coil and I would also suggest to have a diodo on that feed. I like the suggestion of having a lighted bypass swtich, so you don't accidentally drain your battery.

Good luck!
 

ofd8001

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I'd discourage the use of a "Y" configuration (presuming I'm understanding what you are describing). The possibility exists that you'd end up backfeeding the always hot to the ignition hot.

I'm thinking your best bet is something along the lines of the following, which is a double throw switch.

Amazon.com: JT&T Products (2620F) - 20 AMP @ 12 Volt - S.P.D.T., Non-Illuminated On/Off/On Toggle Switch, Black: Automotive

You would wire the center connector to the power input of your scanner. Then the top connector would be "ignition hot" with the bottom "constant hot". Then you flip the switch as desired. If the vehicle is off, but you wanted to listen to the scanner, flip the switch and you're back listening. After you finish, flip the switch the other way and the scanner is ignition sensed.
 

QDP2012

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I'd discourage the use of a "Y" configuration (presuming I'm understanding what you are describing). The possibility exists that you'd end up backfeeding the always hot to the ignition hot.

I'm thinking your best bet is something along the lines of the following, which is a double throw switch.

Amazon.com: JT&T Products (2620F) - 20 AMP @ 12 Volt - S.P.D.T., Non-Illuminated On/Off/On Toggle Switch, Black: Automotive

You would wire the center connector to the power input of your scanner. Then the top connector would be "ignition hot" with the bottom "constant hot". Then you flip the switch as desired. If the vehicle is off, but you wanted to listen to the scanner, flip the switch and you're back listening. After you finish, flip the switch the other way and the scanner is ignition sensed.
This is the right approach. This is what the fire chief, mentioned above, did. I think he also included a solenoid so that the load through the switch was minimal. When the switch allowed power through, the solenoid would then power the radios. When the switch did not allow power through, the solenoid would not power the radios.
 

jim202

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I'd discourage the use of a "Y" configuration (presuming I'm understanding what you are describing). The possibility exists that you'd end up backfeeding the always hot to the ignition hot.

I'm thinking your best bet is something along the lines of the following, which is a double throw switch.

Amazon.com: JT&T Products (2620F) - 20 AMP @ 12 Volt - S.P.D.T., Non-Illuminated On/Off/On Toggle Switch, Black: Automotive

You would wire the center connector to the power input of your scanner. Then the top connector would be "ignition hot" with the bottom "constant hot". Then you flip the switch as desired. If the vehicle is off, but you wanted to listen to the scanner, flip the switch and you're back listening. After you finish, flip the switch the other way and the scanner is ignition sensed.
N0YFE had the right suggestion of adding a diode to prevent the backfeed of the secondary supply going back into the ignition system. I have done just this on a number of installs. Works great and you don't have to worry about the vehicle electrical system. Just put the diode in series with the ignition source. Place the diode so the cathode is towards the scanner.
 

Mbk127k

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Thanks a lot guys, i really do appreciate the advice and expertise on this. I am definitely looking forward to this project. I will most definitely update you all on the end results. Again thank you very much your time and dedication :) .
 

N9JIG

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I'd discourage the use of a "Y" configuration (presuming I'm understanding what you are describing). The possibility exists that you'd end up backfeeding the always hot to the ignition hot.

I'm thinking your best bet is something along the lines of the following, which is a double throw switch.

Amazon.com: JT&T Products (2620F) - 20 AMP @ 12 Volt - S.P.D.T., Non-Illuminated On/Off/On Toggle Switch, Black: Automotive

You would wire the center connector to the power input of your scanner. Then the top connector would be "ignition hot" with the bottom "constant hot". Then you flip the switch as desired. If the vehicle is off, but you wanted to listen to the scanner, flip the switch and you're back listening. After you finish, flip the switch the other way and the scanner is ignition sensed.
This is the exact approach I took on a prior installation, it allows one to switch from one mode to the other easily and safely.

Just make sure the switch you use has the capacity for the current draw you expect, since a scanner draws fairly small amounts of current this shouldn't be an issue but if you plan on adding more devices be sure to account for those.
 
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