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GreekMan:
seasons greetings to all!

I am moving to my next radio power project but I have reached a point of electronics help.

- First of, the project requires a DC-DC LM-series converter and a battery pack feeding the same radio contacts. But not at the same time.
In effect when power is drawn by the battery the DC converter board will be seeing voltage on its exit.
I have been told that it will probably will be alright, but I want to check again.
Else i think I should be adding a diode on the +exit of the DC converter.

- Assuming a 8.4V and 2A current, what kind of diode? (type/value etc)

- While googling for a diode calculator i only found Zener diode diagram calculators. They say it offers voltage regulation.
Is that a good option for bringing a Li-Po battery voltage down from 14.4 to 13.6 (or lower), or will just a diode will suffice (I know they have a 0.7V drop)
(burning a battery's voltage on resistors and diodes is robbing its capacity, so what method would consume less?)

that's all for now....GM

spacecase0:
so you are tying to drop the output of a 4 cell lithium ion battery to not fry a typical ham radio ?

depending on the diode and the current the forward voltage will change, I have some that are about 0.2V...
best way to find out about a diode is to download the data sheet on it
this may or may not be the kind of diode you are looking for
www.diodes.com/_files/datasheets/ds28007.pdf
but you can go look at the forward voltage VS current graph to see what I mean about the forward voltage changing
 
on the DC-DC LM-series converter
a switching one I assume ?
I run the LM2596 with battery voltage on the output and they seem just fine with it, (I have never looked at the data sheet to see if it should be ok or not)
but they do take some current that way, so you might want the diode there just to save your battery power.
if you are running an LM318, or any of that type, use the diode or make sure you properly set up the chip or you will just be getting lucky if the chip does not fry.

hope I answered your question,
but can do better if I know all of what is going on.

Tevin:
Your description is a little hard to follow.

The LM2596 spacecase0 mentioned works for just about anything related to ham radio. The data sheet even includes a sample schematic.

http://www.ti.com/lit/ds/symlink/lm2596.pdf

Using diodes and resistors for the sole purpose of dropping unwanted voltage is really poor engineering. Besides the wasted energy, the value of the diode/resistor depends on the current draw of the device. And once selected, will work only at the current it was calculated for.



GreekMan:
this is what i am trying to do.......
http://i176.photobucket.com/albums/w192/greekpreparedness/web-IGP0876-diagram_zpsxub2azpk.jpg~original
Chip is an LM style, but i cannot hunt for the datasheet right now.

BTW with the help of the PDF file i found the diode type, but 1volf forward voltage? that will be 10+ % losses! (output is set to 8,2V)

I smell the need for a switch.....

GreekMan:
as ime passed by I decided against a diode...It would not work since the radio would consume various currents.

So...been hunting for small Slide switches and here are the types I found:
- on-off
- on-on
- on-off-on
sample:
http://www.ebay.com/itm/10Pcs-Black-Mini-Size-SPDT-Slide-Switches-On-Off-100V-2A-DIY-Material-/141855128497?hash=item2107397bb1:g:cI8AAOSwSdZWcp6w

I would like to have your opinion. Since I do not mind if one of the two voltage lines is active any time, what kind would be good for the application?
I am tending towards an on-on SPDT slide switch

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