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 Author Topic: What size voltage drop resistor needed to power Uno  (Read 594 times) 0 Members and 1 Guest are viewing this topic.
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 « on: May 29, 2012, 03:11:33 pm » Bigger Smaller Reset

Hi,

I am trying to power my Uno (rev 3) from a Dialight MDU-18 driver (http://www.dialight.com/Assets/Brochures_And_Catalogs/Illumination/MDEXU18V25100.pdf). Problem is that the Dialight supplies 33vDC and 350-700mA (under my control).

So I need to drop the 33vDC to 7-12vDC to feed into the Arduino's power jack socket. But I am unsure of the correct value and type of resistor to use. I've tried various calculations but have no confidence in my results.

Can anyone recommend a resistor value and type?

Many thanks,
Mark
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 « Reply #1 on: May 29, 2012, 06:37:42 pm » Bigger Smaller Reset

The usual suggestion here is to use a voltage regulator. Since you're looking for a fairly large decrease in voltage, you will likely need to use a heatsink too. Or step the voltage down through more than one VR. Or both.
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 « Reply #2 on: May 29, 2012, 07:10:32 pm » Bigger Smaller Reset

A drop of 20 volts or so with 500mA would be a good 10W of power.  Using a resistor would get very hot very quick.  Also, in order to use a resistor, you have to have a known constant current.  The current drawn by an UNO changes depending on what it is doing.

A linear regulator will get pretty hot too, dropping that kind of voltage, and a heatsink will be required.

A third option would be to use a switching voltage regulator.

These are far far more efficient than a linear regulator - they don't have to get rid of the dropped voltage in the form of heat - they convert it into current.

The best source for one of these regulators is a car cigarette lighter USB phone charger.  They will usually be able to cope with quite large voltages on the input in order to cope with the spikes the ignition system causes.  The one I had with my phone uses the MC34063 chip, which has an input range of up to 40V.  One of these could plug direct into the USB socket on the UNO to run it.
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 « Reply #3 on: May 29, 2012, 07:17:43 pm » Bigger Smaller Reset

Yes you almost certainly need a switching regulator, people like TRACO make nice small devices that will do this and have the same pinout as a normal 3-pin regulator.

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Rob
 « Last Edit: May 30, 2012, 04:16:47 am by Graynomad » Logged

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 « Reply #4 on: May 30, 2012, 12:57:17 am » Bigger Smaller Reset

Hi, Basic question: why do you need/want to use this particular power supply.  It is really unsuitable for this, being a constant-current supply with relatively high voltage.

Universal 100-240V 5V 1 Amp supplies are about \$2.50
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 « Reply #5 on: May 30, 2012, 10:37:45 am » Bigger Smaller Reset

Hi guys,

Thanks for all your feedback. I guess a simple resistor isn't going to cut it

The reason I'm using this particular driver is that it is also powering a number of high-power LEDs. The Dialight is primarily designed for this task and I was hoping that I could also use it to power the Uno too.

I could add a separate power supply for the Arduino - the downside being extra cost and I'm a bit tight for space. And being in the UK we can't get one for \$25.0

So I'm looking at simple but expensive solution which is a 9vDC transformer - http://uk.farnell.com/xp-power/ecl05us09-t/psu-5w-open-frame-9vdc-0-55a/dp/1765276

or

The cheaper but trickier option of using a switching voltage regulator (as recommended) and trying to build a suitable circuit. But I have no experience in this area so I'd very very grateful for any advice, recommendations or circuit diagrams.

Many thanks guys,
Mark

P.S. I also like the idea of cannibalising a car-charger
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 « Reply #6 on: May 30, 2012, 10:44:26 am » Bigger Smaller Reset

Quote
switching voltage regulator (as recommended) and trying to build a suitable circuit. But I have no experience in this area
You don't need any experience if you use the TRACO or Murata modules, just plug them in like a linear regulator, job done.

Google for TRACO TSR-1 Series and Murata MTU1 Series.

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Rob
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 « Reply #7 on: May 30, 2012, 02:40:22 pm » Bigger Smaller Reset

Quote
So I'm looking at simple but expensive solution which is a 9vDC transformer

Ouch! 16 Pounds??

This is \$2.50 and probably \$4 to ship. http://goo.gl/qClvL  and you can probably get one on Ebay for \$5 free shipping (but it will take 25 days instead of 15)..

Isn't anyone in GB selling this stuff at a reasonable price??

DISCLAIMER: Mentioned stuff from my own shop...
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 « Reply #8 on: May 31, 2012, 06:13:43 am » Bigger Smaller Reset

Thanks for your help guys! I've ordered a Traco TSR 1-2490 to play with as it's small enough to fit. I'll let you know how I get on.

Thanks again,
Mark
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