EMRFD Message Archive 13228
Message Date From Subject 13228 2016-10-28 15:22:26 Ravi Miranda 13.8V 12V and 5v supply question I hope I've explained it right?What would be the best way to drop 14.8V or 12V to 5V via an LM1117-5V and would prevent significant heat dissipation.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.So a drop of 14.8-12V 2.8V LDO would be ok I suppose?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.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.HelloBeen meaning to ask this question for a long time.Thank you in advance.Ravi Miranda--I'm here to add more value to the world than I'm using up. 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
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
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 can send POE module ( power over ethernet ) small downconverter 5V, got so many in my junk. 1 amp max...What would be the best way to drop 14.8V or 12V to 5V via an LM1117-5V and would prevent significant heat dissipation.
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