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Forum Index : Electronics : Will this work

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lew247

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Joined: 23/12/2015
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Posted: 11:12pm 13 Mar 2016
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I want to power the unit from where it says power which will be 5V
but
#I want to use 4.5V battery backup which will only supply power when the 5v supply is missing
 
isochronic
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Joined: 21/01/2012
Location: Australia
Posts: 689
Posted: 11:30pm 13 Mar 2016
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I would put D2 so it feeds to C4 otherwise the battery source will have two voltage drops, also use schottky diodes and a low drop out regulator if they are not already
 
lew247

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Posted: 11:42pm 13 Mar 2016
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How about this one?

The diodes are schottky 0.3VF and the battery backup can be 2 X AA 1.5V cells in series (3V)

Hopefully the way I have it the battery backup won't do a thing until the main power input gets disconnected then the 3v should takeover immediately without powering down the pic chip

 
Downwind

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Joined: 09/09/2009
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Posted: 01:08am 14 Mar 2016
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The last circuit works much better for me than the first circuit.
Sometimes it just works
 
lew247

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Posted: 05:03am 14 Mar 2016
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Thanks Downwind
 
Pete Locke
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Joined: 26/06/2013
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Posted: 08:12pm 14 Mar 2016
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The issue you will have with the second circuit is back feeding through the regulator. Simple test. Put your multimeter on diode test and measure from the output of the regulator to ground and see if you get any numbers. It should read voltage drop (depending on the meter) so would be another path for the backup battery to go. Don't know what reg' chip you are using (most have protection diodes to take back feed away from the internal clever bits), but if it's adjustable, put the diode in the output side and tweak the volts up a bit to cover the drop across the diode, and the back up battery diode past that. So the two cathodes are joined at the output of the regulator. Hope that makes sense.
Cheers
Pete'.
 
Pete Locke
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Posted: 08:30pm 14 Mar 2016
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Also, there is a wee trick from the old days to allow for the forward series drop on the output. Hope this gives you the idea....


 
isochronic
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Posted: 08:59pm 14 Mar 2016
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I thought the circuit was to supply near 4 volts. Otherwise the two cell battery
will work but the dependable volts out even with working cells will be more like two and a bit , maybe that is ok.Edited by chronic 2016-03-16
 
lew247

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Posted: 11:49pm 14 Mar 2016
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Thanks Everyone

2nd question - should this work? or would I need something like a TIP120

Base of the transistor is being driven by a pic chip

Edited by lew247 2016-03-16
 
Pete Locke
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Posted: 11:52pm 14 Mar 2016
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....You would do better to use a PNP for the supply switch.
 
lew247

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Posted: 12:44am 15 Mar 2016
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Why?
Can you show or give me an example circuit please?
 
Pete Locke
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Posted: 06:43pm 15 Mar 2016
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So you can get full saturation of the transistor. It's used as a switch here, not a linear device so 'full on' is asked for.


 
larny
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Joined: 31/10/2011
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Posted: 07:55pm 15 Mar 2016
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This how I would do it.

I assume (I have not checked the data sheet)
that U2 is a low drop out Reg.




 
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