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Author Topic: From 24V to 5V DC voltage regulator  (Read 6900 times)
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Hello,

I have a power supply of 24V, I use this to power my lights. I would like to add a breadboard Arduino UNO so I can switch the lights on or off using a light sensor, PIR sensor,...etc

I know that feeding the 24V into a voltage regulator of 5V will blow the thing up because I need a proper head sink, also to much power will be dissipated.

Now because the 5V will only be used to for powering the UNO and some sensors, can't I use a voltage divider with resistors and feed the resulting (for example) 9V in the voltage regulator?

Thanks!
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In a word NO.  Voltage dividers are totally useless as power supply systems - they are only suitable for providing minimal current into sense ports.  You need a regulator device, linear chip such as a 7805 if you don't care about lack of efficiency and can make allowances for heat dissipation or one of the switching regulators that appear in most posts , cost very little and are extremely efficient (circa 94% or so)
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probably best to get a 24v capable UBEC for efficient high current voltage reduction.

http://www.ebay.com/sch/i.html?_sacat=0&_nkw=ubec&_sop=15
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Another one...

http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781
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i'm using a 7805 in one of my circuits and im getting interference issues i havent exactly been able to put my finger on it yet but could it be an advantage changing my regulator for something like this?
http://uk.farnell.com/xp-power/sr05s05/switching-regulator-5v-0-5a-o-p/dp/1861084

not cheap tho
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You should use something like this
http://www.mouser.com/ProductDetail/RECOM-Power/R-78E50-05/?qs=sGAEpiMZZMtwaiKVUtQsNdyQYa4auFR1JPviMI71Coc%3d

Have read here that current Uno regulators don't like being reverse biased. No first hand knowledge of that myself.
Perhaps get a 7.5V output, 9V, somewhere in there, and supply the barrel jack or the Vin header pin.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Reading the Uno's regulator datasheet, page 10:

http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF

It would seem to me that a 1N4001 diode from 5V to Vin would prevent any damage should Vin be unpowered and 5V driven directly from an external source:

"Protection Diodes
The NCP1117 family has two internal low impedance
diode paths that normally do not require protection when
used in the typical regulator applications. The first path
connects between Vout and Vin, and it can withstand a peak
surge current of about 15 A. Normal cycling of Vin cannot
generate a current surge of this magnitude. Only when Vin
is shorted or crowbarred to ground and Cout is greater than
50 F, it becomes possible for device damage to occur.
Under these conditions, diode D1 is required to protect the
device. The second path connects between Cadj and Vout, and
it can withstand a peak surge current of about 150 mA.
Protection diode D2 is required if the output is shorted or
crowbarred to ground and Cadj is greater than 1.0 F.

A combination of protection diodes D1 and D2 may be
required in the event that Vin is shorted to ground and Cadj
is greater than 50 F. The peak current capability stated for
the internal diodes are for a time of 100 s with a junction
temperature of 25°C. These values may vary and are to be
used as a general guide."
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... Use a 1N5822 from 5V to ground anode to ground and cathode to 5V . There is already an input protector in the DCin jack, A series diode pointing to the regulator.
Here is My $0.02 worth on Switchers @ $1.70 Ea...
http://www.electrodragon.com/?product=better-than-lm2596-dc-dc-step-down-adjustable-power-supply-module

Always better Prices.

Doc
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... Use a 1N5822 from 5V to ground anode to ground and cathode to 5V . There is already an input protector in the DCin jack, A series diode pointing to the regulator.
Here is My $0.02 worth on Switchers @ $1.70 Ea...
http://www.electrodragon.com/?product=better-than-lm2596-dc-dc-step-down-adjustable-power-supply-module

Always better Prices.

Doc

Yes, great price. But they do need to work on their usage of engineering units:

Quote
Output Current: 2V continuously, and peak value at 4V

Lefty
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Here is My $0.02 worth on Switchers @ $1.70 Ea...
http://www.electrodragon.com/?product=better-than-lm2596-dc-dc-step-down-adjustable-power-supply-module
Always better Prices.
That $1.70 has magically turned into $3
I'm using an LM2596 module.
Only $0.99 on eBay.
« Last Edit: May 20, 2013, 05:50:54 pm by Nisei » Logged

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