# Voltage conversion

Discussion in 'The Projects Forum' started by toxicburn, Mar 23, 2011.

1. ### toxicburn Thread Starter New Member

Mar 22, 2011
1
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Hello brains!!!
Writing to you in hope to get an answer to my question. P L E A S E

I have a 18v 3Amph battery.
I heed to power up a 12V 2Watt circuit. How do I do that and how long will it last (approximately). If someone can provide me a way to calculate all these, I'd have a virtual drink you

Pardon my electrical stupidity
Thank you in advance!

2. ### rwstowe New Member

Mar 23, 2011
4
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Dear Toxicburn,

In all applications, energy is always conserved. This means that the useful energy stored in the battery must be transferred to your load minus a small amount lost due to waste heat in the power supply and wires.

The energy stored in the battery (assuming that the amp-hrs rating that you gave is a reasonable method for determining the useful life of the battery) is 18 volts * 3 amp-hr * 3600 sec/hr = 194 kiloJoules. (Energy = voltage*current*time)

Assume that a non-isolated step-down switching converter has 90% efficiency. Please see my tutorial on power supply efficiency at http://www.truepowerresearch.com/2011/03/tutorial-power-supply-efficiency/.

The estimated run time for your load is 194 kiloJoules/2*watt*90%*1hr/3600sec = 24.3 hours. (Run time = Energy/power*efficiency and Power = voltage*current}

Digi-Key stocks a nice switching power supply that might be suitable for your use. It has a similar package as a TO-220 linear regulator and the same pinout. The part data sheet is attached to this response. The Digi-Key Part number is 811-1117-ND. The link is http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=811-1117-ND.

Be sure to carefully study the application guidelines. Switching power supplies are often not trivial to apply. This answer gets you started in the right direction, but you have more thoughtful application to do.

Do not use a linear regulator because the efficiency in this application will be poor and will result in a much shorter run time and you may have to deal with heat sinking issues.

For your benefit and the benefit of others, I am developing a power supply tutorial located at http://www.truepowerresearch.com/2011/02/power-supply-tutorial/.

Robert W. Stowe
Consultant
True Power Research, LLC

Last edited by a moderator: Mar 28, 2011