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Old 12th May 2012, 04:46 AM   #1
Teti is offline Teti  United Kingdom
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Default 9VDC battery powered power supply

Hi everyone,
I would like to build battery power supply for my new M2Tech HiFace Evo USB -> S/PDIF converter.

Input from 12VDC battery, output 9V. Converter accept 5-11V.
I've done some research and this is what I draw.

Now question:
- I the schematic correct?
- Do I need C1 and C2 caps if DC is on input? I guess that caps are for additional filtration of AC, some switch mode, judging by 100uF.
Please hep to beginner.

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Old 12th May 2012, 07:54 AM   #2
RJM1 is offline RJM1  United States
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According to the data sheet
http://www.st.com/internet/com/TECHN...CD00000449.pdf
scroll down to page 10 for the 78s09; the minimum input voltage for 9V output is 12V. If you eliminate D1 (not necessary if it is powered from a DC source like your 12V battery) and change C1 to .33uF and C2 to .1uF as in the examples given in the data sheet (page 8 figure 5) it should work until your battery voltage drops below 12V. Perhaps you should look for a low drop out regulator.

Last edited by RJM1; 12th May 2012 at 08:15 AM.
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Old 12th May 2012, 08:48 AM   #3
RJM1 is offline RJM1  United States
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Or you could do something like this. This will give you 9.3V if you are drawing 2A of current and 10.2V with no load. This fits within your voltage range for your S/PDIF converter. 5-11V. The capacitor smothes any load variation caused by your S/PDIF converter.
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Old 12th May 2012, 09:20 AM   #4
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Your schematic is correct. The 0.5W rating for the LED ballast resistor is overkill - the dissipation there will be minimal (~50mW) so any 1k resistor will work.
RJM1 is right about the capacitors and that the Operating input voltage spec for the 78S09 is stated as 12V, but if you look above that their Output voltage test conditions for Vo are shown as Io = 1 A, Vi = 11 V. I think their 12V spec is derived from the graph on page 22, "Dropout voltage vs. junction temperature." In that graph you can see that at a lower current (and I'm going by a Evo current of ~150mA), the dropout voltage is less, more like 1.5V.
I'm mentioning all this mainly with an idea that you have the parts in hand. If you don't, a LDO may be a better idea. You want to insure that your battery can drop to 10.5-11 volts and still provide enough for a regulated output.

edit: I see that ~150mA is idle current. TBC...
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Last edited by sofaspud; 12th May 2012 at 09:22 AM.
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Old 12th May 2012, 11:11 AM   #5
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If the application will run as low as 5V then why not use one of the lower output regulators so that you maintain the drop out voltage.

ie. 7807. or even the 7805.
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Old 12th May 2012, 11:20 AM   #6
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Default The humble TL497

If you want to go a little bit more advanced you can always use the humble TL497.

This will operate from between 4.5 and 15V volts and give you a stable output voltage.

You simply adjust VR1 and R2 and R3 to give you 1.2V at pin 1.

L1 doesn't need to be anything special just as many turns as you can get on a salvaged ferrite ring or pot core. (Unless you are after higher current output - in this guise it will output up to 500mA).

Although the article states that using this circuit above 6V is pointless, it is assuming that the input voltage never drops below the drop-out voltage of a series regulator.

This circuit has been used in many guises to extend the battery life of some toys and other applications.
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Last edited by KatieandDad; 12th May 2012 at 11:33 AM.
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Old 12th May 2012, 11:55 AM   #7
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The TL497 is pretty commonly available for next to nothing.

TL497ACN VOLTAGE CONVERTER 14 PIN DUAL IN LINE | eBay
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Old 12th May 2012, 01:46 PM   #8
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The TL497 doesn't use RF type frequencies so you don't have to be mega-careful with the layout either.
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Old 12th May 2012, 07:30 PM   #9
Teti is offline Teti  United Kingdom
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Thank you very much guys, actually I made mistake, converter accept 7-11VDC (15VDC max.), Supply current: 140mA (idle, no output connected)
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Old 12th May 2012, 07:58 PM   #10
Teti is offline Teti  United Kingdom
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What bout this variable one?

Click the image to open in full size.

I've even designed my very 1st PCT for that one. What you reckon?

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