EMRFD Message Archive 13228

Message Date From Subject
13228 2016-10-28 15:22:26 Ravi Miranda 13.8V 12V and 5v supply question
Hello

Been meaning to ask this question for a long time.

Let's say that I have 13.8v or 14V supply to Transceiver. Most circuits require 12v so using a 12V LDO would be ok I guess.

However, if I intend to use this as a mobile unit then I would need to power it with a 3.7x 4 = 14.8V LiPo.
So a drop of 14.8-12V 2.8V LDO would be ok I suppose?

Assuming I have a relay driver that would switch 2 or 3 relays T/R + LPF relay, the current consumption of each relay is 11.7mA @12V or 28.1mA @5V.

What would be the best way to drop 14.8V or 12V to 5V via an LM1117-5V and would prevent significant heat dissipation.

I hope I've explained it right?

Thank you in advance.

Ravi Miranda
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13229 2016-10-29 00:34:02 Ashhar Farhan Re: 13.8V 12V and 5v supply question
this assumes that the battery will stay at 3.6v, it could discharge to, as low as 3 volts.

- f

13230 2016-10-29 03:52:05 AncelB Re: 13.8V 12V and 5v supply question
If the 5V is for the relay drive, just use a dropping resistor
(12-5)/.028 =250 ohms, so a 240 or 270 ohm unit rated at a 1/4watt is
ok. No V reg required.
13231 2016-10-29 07:17:43 Ravi Miranda Re: 13.8V 12V and 5v supply question
Thank for the inputs, forgot to mention that the 5V is required for the PCF8575C which will drive the ULNs. There are 2 relays for each filter Sorry, I was wasn't clear on that earlier.

13232 2016-10-29 09:14:46 Eamon Egan Re: 13.8V 12V and 5v supply question

Hi Ravi.

The so-called 12V that transceivers are designed to run on is in reality an automotive supply that's often given as a nominal 13.8V. Check the transceiver specifications. It may be rated to take up to 15V or maybe a bit more. If you're running off of batteries there is a hard limit to how high the voltage will go, determined by the battery chemistry so you don't really need much margin on the top side.

If you're concerned about overvoltage a quick and dirty approach is to put rectifier diodes in series which will drop about 0.6V each. Of course this won't help regulation by dropping out on the low end.

I hope this at least partly addresses your question

Eamon

13233 2016-10-29 09:51:47 Andy Re: 13.8V 12V and 5v supply question
Ravi,

   "Most circuits require 12v so using a 12V LDO would be ok I guess."

Almost everything that says "12V" really means 13.8V nominal.

But you can't go to your local Sears auto center and ask for a "13.8V car battery".  They may think you are crazy.

I have heard of only one device designed for "12V" that really meant 12.0V, and could be damaged if powered by a "12V" car battery or electrical system (cigarette plug power).  That one device, I assume, was designed by a victim of the same misunderstanding about car batteries.

Andy


13234 2016-10-29 15:01:08 Sandeep Lohia Re: 13.8V 12V and 5v supply question

What would be the best way to drop 14.8V or 12V to 5V via an LM1117-5V and would prevent significant heat dissipation.

can send POE module ( power over ethernet )  small downconverter 5V, got so many in my junk. 1 amp max...

BTW, works on some kHz !
 



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13235 2016-10-29 16:52:39 Frank Dinger Re: 13.8V 12V and 5v supply question
In a recent QST article voltage from 4LiPo cells in series up to 16V  was dropped through  diodes to a suitable  transceiver supply voltage . A simple multiple  transistor relay circuit would shorten the diodes  in steps of  approx 0.8 V  when battery chain voltage drops during discharge.

Frank  ,  GM0CSZ / KN6WH    in IO87AT