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Connecting 7.5V Device to 12V?


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ckact 
Member - Posts: 5
Member spacespace
Joined: June 20, 2018
Location: Oregon, United States
Posted: June 20, 2018 at 4:11 PM / IP Logged Link to Post Post Reply Quote ckact
I need advice without all the technical home work, yea, basically I want somebody to just tell me the answer and not make me figure it out by myself.
I'm a hardware guy, I'm terrible with math, like legitimately dyscalculia.
I have a device that has its own batteries, there are 5 batteries about 1" long and the diameter of a AAA battery, the wrap says "AAA 2/3 250mAh" at full charge the battery cluster reads 7.xVDC
I want to attach this device to my car permanently preferably eliminating the battery cluster and having the device powered straight from the 12VDC car electrical system, so, I need the device to be supplied with no more than 7.x vdc @ 250mAh it would also be ok for the onboard battery to remain as it would do its job even in the event of the vehicle battery being dead or removed.
When I google 12v > 7.xvdc I come up with lots of products and a few people talking about resisters. Can I just get a few certain resisters, solder them inline and have what I need? I'm sure I could hammer out the voltage, but is it possible that the amperage might be excessive?
I've been thinking about just buying a bundle resistor pack and playing around with them to see if I can drop 12 to 7x, I'll need to do this project many, many times over if successful so having a stock of resistors isn't a burden.
Connecting 7.5V Device to 12V? -- posted image.
i am an idiot 
Platinum - Posts: 13,674
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 20, 2018 at 8:34 PM / IP Logged Link to Post Post Reply Quote i am an idiot
Following diagram plus a heat sink for a TO-220 case and 3 3 amp diodes. On the output of the device connect the 3 diodes in series. This will yield 7.5 volts
The tab of the regulator is the ground connection. You can mount it to a piece of aluminum flat bar and then mount the bar to the chassis. Monitor it for heat for 30 minutes or so.
http://www.circuitdiagram.org/12v-8v-converter-lm7808-regulator.html
https://www.the12volt.com/diodes/diodes.asp
eguru 
Copper - Posts: 340
Copper spacespace
Joined: February 04, 2018
Location: Quebec, Canada
Posted: June 22, 2018 at 4:30 PM / IP Logged Link to Post Post Reply Quote eguru
Something like this would be as easy to implement.
https://abra-electronics.com/voltage-regulator-modules/psm-160-5a-step-down-converter.html
ckact 
Member - Posts: 5
Member spacespace
Joined: June 20, 2018
Location: Oregon, United States
Posted: June 24, 2018 at 10:27 AM / IP Logged Link to Post Post Reply Quote ckact
4.50 aint bad, but I'd need 3 per vehicle and the amperage is very important, no idea what the result of device discharge over .25 amps.
While overvolts is likely to destroy the device, overamps is likely to do uhh more damage to the target of the device and while the target can't be 100% verified it would be a liability to have overamps.
KPierson 
Platinum - Posts: 3,527
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Joined: April 14, 2005
Location: Ohio, United States
Posted: June 26, 2018 at 3:35 PM / IP Logged Link to Post Post Reply Quote KPierson
The device will pull the current it needs - no more, no less (unless you can't supply the required current, then bad things happen).
If you get the voltage correct the current will take care of itself.
One issue I see, though, is that you don't have listed how much current the device pulls. Mah is a measure of capacity, not instant current. It basically gives you a rough idea of how long the battery should last on one charge.
Kevin Pierson
ckact 
Member - Posts: 5
Member spacespace
Joined: June 20, 2018
Location: Oregon, United States
Posted: August 05, 2018 at 6:02 AM / IP Logged Link to Post Post Reply Quote ckact
So as my work schedule sops up nearly all of my waking time, I've not been able to put much time in on this project, but I called out sick friday and went to work on it.
long story short I must have gotten a slightly different type than described above, cause I was getting like 6.8 from three diodes, the device was functioning at this feed but at about 50% of its full capacity, as of right now I'm using the original battery pack and having the circuit fed with charging power only in its ARMED state, to try and cut down on its charge cycles as the device will probably only do its thing once every month or so.
This build is only preliminary and I fully expect to perfect it.
Anyway I have about 7.8 open lead output from one TO-220 and one diode; in a charging state it drops to 7.37
With 2 diodes the battery pack never got higher than 6.8VDC
I'll probably soon just do away with this system when I discover a more stable, simple 12 volt friendly design.
This circuit is fed from a 30 amp relay tied to the ARMED output (-) of my alarm system, it also feeds to a second relay wired for NC, cutting fuel pump function.
Later I plan on installing another one of these devices in the drivers seat right about where a persons anus/genitals should be if they were to get all comfy in the captains chair without identifying the system and disabling it.
Connecting 7.5V Device to 12V? -- posted image.
Connecting 7.5V Device to 12V? -- posted image.
Connecting 7.5V Device to 12V? -- posted image.
eguru 
Copper - Posts: 340
Copper spacespace
Joined: February 04, 2018
Location: Quebec, Canada
Posted: August 05, 2018 at 11:16 AM / IP Logged Link to Post Post Reply Quote eguru
If your "device" is being powered by 250mAh batteries then it probably doesn't draw more than 20mA.
The PSM-160 buck converter which is rated at 5A is "over-kill" for your application as far as current handling capability If you are installing 3 of your devices per vehicle, it is more than adequate for powering them all.
However, none of the solutions presented here are suitable/safe for also acting as battery chargers for those 2/3AAA cells.
Suggest you change the batteries to a 7.4V LiPo battery pack and powering up the device with a LiPo charger.

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