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Battery DC converter
Battery DC converter
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Old 20th November 2017, 05:22 PM   #1
Fob69 is offline Fob69  France
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Default Battery DC converter

HI,

I supply my player (olimex) and an optical bridge with 6,4V and 3,3V lifepo4 battery.
The voltage is not exactly the good one but i don't want to use LDOs. At this time i use diods to decrease the 6,4V to around 5V and i need to decrease the 3,3V to 3,1V.

Is there a way to adjust the voltage without to loose the ultra low noise and impedance benefits of the lifepo4 cells ?

Thank You for your help

Bernard
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Old 20th November 2017, 05:29 PM   #2
miro1360 is online now miro1360  Europe
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use diode with forward voltage about 0.2V ... something like this: VS-10BQ015HM3/5BT
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Old 20th November 2017, 06:02 PM   #3
Fob69 is offline Fob69  France
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Hi thank you.

I thought diod gave only 0,6v down.
I have seen that with several diod in series, drop out on each diod was not constant and is decreasing with the number of diods. Is it normal ?

Are they other parameters i should take care on the diods if i don't want extra noise or impedance increasing ?

B.

Last edited by Fob69; 20th November 2017 at 06:05 PM.
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Old 20th November 2017, 06:11 PM   #4
miro1360 is online now miro1360  Europe
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yes, forward voltage is changed with forward current ... and it is why it is not very good regulator when current is not constant ... when you know average forward current, you can select best diode for your application, each diode have datasheet and tables where is forward voltage vs forward current in chart ...
and yes you can give more diodes into series



Quote:
Originally Posted by Fob69 View Post
Hi thank you.

I thought diod gave only 0,6v down.
I have seen that with several diod in series, drop out on each diod was not constant and is decreasing with the number of diods. Is it normal ?

Are they other parameters i should take care on the diods if i don't want extra noise or impedance increasing ?

B.
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Old 20th November 2017, 06:19 PM   #5
gentlevoice is offline gentlevoice  Denmark
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Battery DC converter
Salut Bernard,

May I suggest something a bit different ... LiFePO4 batteries typically are 3.3 to 3.4 VDC when float charged (i.e. constantly charging).

However, there is the option of only charging the batteries when you do not listen (seriously) to your system ... For example a couple of LM317T (with adequate cooling) may supply the batteries with e.g. 3.3 VDC - and you can just switch off the charger when listening. You do have to take care though that you do not forget to switch on the charger again - otherwise the batteries may be destroyed. However, depending on how skilled you are with electronics a suitable timer (e.g. a three hours switch off time) can be built with an LM555 timer IC.

Also I would suggest a balancing unit (e.g. a TL431) on each of the batteries in the 6.4 VDC voltage. Thus both battery cells remain at the same voltage.

If you would like 5 VDC directly from batteries I would suggest LiTO (lithium Titanate) batteries that can be charged to up to 2.8 VDC - and should be quite fine at 2.5 VDC.

Good luck with your battery solutions.

Cheers,

Jesper
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Last edited by gentlevoice; 20th November 2017 at 06:22 PM.
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Old 21st November 2017, 06:41 AM   #6
PRR is offline PRR  United States
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Common Silicon diodes are "0.6V" (0.5V to 1.2V depending on current).

Germanium diodes (now rare) were in the 0.2V-0.4V range.

Copper-Oxide (now even rarer) could be under 0.1V to over 1V depending on size and current.

Vacuum diodes can be well under 0.1V or over 50V, depending on current, which is usually much lower than modern electronics need. (They also need a heater power supply.)

Schottky diodes tend to about half the drop of Silicon; they are semiconductor-to-metal instead of semiconductor-to-semiconductor. This is the type that miro1360 is pointing you to.

In this application, diodes are not noisy.

Last edited by PRR; 21st November 2017 at 06:43 AM.
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Old 21st November 2017, 06:54 AM   #7
Fob69 is offline Fob69  France
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Quote:
Originally Posted by miro1360 View Post
when you know average forward current, you can select best diode for your application, each diode have datasheet and tables where is forward voltage vs forward current in chart ...
Hi thank you
Is Vf Vs Cf linear ? The current is stable in my case (450 mA).


Quote:
May I suggest something a bit different ... LiFePO4 batteries typically are 3.3 to 3.4 VDC when float charged (i.e. constantly charging).
This is allready what i do. The goal is to get the low noise, the low impedance and a full isolation.

The load cut itself if the voltage become too low (6,5V - 4,9V for my olimex player given for 5V), (? to 3V for my delock MFC given for 3,3V).

I charge it once a week. and disconnect everything from AC after load but i don't disconnect the batteries from the loader so i have very small leaks. I don't know if it is supposed to have impact on noise or output impedance. i could change this.
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Old 21st November 2017, 09:19 AM   #8
gentlevoice is offline gentlevoice  Denmark
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Battery DC converter
Hi again,

Quote:
I charge it once a week. and disconnect everything from AC after load but i don't disconnect the batteries from the loader so i have very small leaks. I don't know if it is supposed to have impact on noise or output impedance. i could change this.
I am not quite sure what you say here ... Do you mean that you don't disconnect the charger from the batteries even if you disconnect the charger from the AC mains? In any case if you want to "completely" isolate your player/battery setup from the charging circuit you might place a relay between the charger & the batteries. Make sure to disconnect both the ground & the + voltages, and also to ensure that the relay will sustain the max current that may go through the relay when it is switched on (remember here that capacitors may discharge with significant currents).

I don't know though if this will be audible ... but it will isolate your playback part from the AC mains.

Cheers,

Jesper
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Old 21st November 2017, 10:33 AM   #9
Fob69 is offline Fob69  France
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Hi,

Sorry, made a mistake. Yes i disconnet battery from loader when i use it. And disconnect loaders from AC too. I first wanted to use relays but did not at the end (i use mecanical switchs) because i did not want to supply them with an external supply to avoid HF noise transfert througt self. I am paranoid for this. I don't even use a switch for AC connect but a plug with no physical contact when unplugged (an of course disconnected on both poles for common mode curent blocking).
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Old 21st November 2017, 01:13 PM   #10
miro1360 is online now miro1360  Europe
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Quote:
Originally Posted by Fob69 View Post
Hi thank you
Is Vf Vs Cf linear ? The current is stable in my case (450 mA).
is not and change with temperature ... you must find the best characteristic for you ... for 450mA maybe these 2 diodes in series can do stable drop 0.2V
C3D03060A WOLFSPEED(CREE) - Diode: Schottky rectifying | TME - Electronic components
but you must try and measure it, maybe add heatsink for more stable temperature ...
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