13.8 to 7.5 power supply

[SCPD]

I want to run a 7.5VDC handheld from vehicle power.
Any thoughts the best/easiest way to do this?

 

[rescue161]

Voltage divider.

Scroll to the bottom of the page for the formula to figure out the output voltage.

http://web.mit.edu/rec/www/workshop/photoresistors.html

 

[slicerwizard]

Voltage divider.

You have got to be kidding. The OP wants a stable 7.5V supply, which a voltage divider ain't.

 

[rescue161]

OP wanted an easy way to convert 13.8 to 7.5. He said nothing about stability. How about give him an easy way instead of bashing me...

This is one is simple, but not as simple as a voltage divider and uses an 8V zener diode and a 270 Ohm 1/2 Watt resister to keep the voltage near 7.5.

http://powersupply88.com/simple-12v-to-9-7-5-or-6v-converter.html/simple-dc-to-dc-converter

 

[gtriever]

He also wanted the best way, which a voltage divider is not. I'd start with a google search for a "battery eliminator" for his specific model handheld.

 

[XTS3000]

Use a LM317 voltage regulator. It's a very easy circuit to build and you can set the voltage to a very precise 7.5V.

Rainbow Kits has a kit to make this simple voltage regulator for not much $$.

The LM317 can be configured as a voltage regulator OR a current regulator.

I recommend against using a voltage divider using 2 resistors because if your supply voltage changes, you output voltage changes as well. Voltage divider is unregulated circuit.



Above is a my LM317 voltage AND current power supply for powering laser diodes. For a scanner, you would not need the 2nd current limiting LM317.

Another option I would do myself (but not recommend it to anyone else) is to simply use a LM7808 - 8V regulator. It's 0.5V higher than the scanner calls for, but scanners have their own built in voltage regulation circuits.

 

[prcguy]

LM-317s were popular during the dinosaur era but today you can get a DC-DC switcher for about the same price as an LM-317. The drawback of a linear regulator like the LM-317 is if you need say 1 amp at 8v and your supply is 24v, it still draws about 1a from the 24v supply.

A switching regulator will supply the same 8v at 1a but the current draw from the 24v supply will be around 350ma or so.

Here is a source for switching regulator replacements for an LM-317 that is adjustable from 1.25 to 35v with an input voltage up to 40v and its assembled and tested for a buck each! New Mini 5X LM2596S 3A DC DC Buck Converter Step Down Module Voltage Regulator | eBay
prcguy

 

[slicerwizard]

I recommend against using a voltage divider using 2 resistors because if your supply voltage changes, you output voltage changes as well.

More importantly, the output voltage varies as the load varies, which it surely would if the load is a radio.

 

[n0nhp]

This is a bit of overkill but is one of the RF quietest switching regulators I have found.
Automatic Buck-Boost Module - Red (8A) - Free Shipping - DealExtreme
It is un-branded Chinese but they take Pay-Pal so you can contest the charge if you don't get what you ordered.
I have ordered a number of gadgets from them and the cheap ones are really cheap, the items like this one are pretty well made.
YMMV
Bruce