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24VDC to 5VDC (250mA) - cheapest option?

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kittydog42

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I tried this using a 7805, but I can't install a heatsink so it will not work well. Right now, I am usind a C&D NME2405 DC-DC regulator, but these are $11 each and I am running at the maximum rating. The nice thing about these is that they are compact. Does anyone have a suggestion for resistor dividers or zener/resistor combinations that would work? If I could use resistors, what would I change the value to if I was to have a 12VDC input instead of 24VDC?
 
The voltage difference 19V with 250mA is almost 5W dissipation when You use analog regulator. (5W is 5W, equal what is the dissipator - IC and/or resistor)
I recommend to use a "simple switcher" buck regulator from National Semi or Motorola, e.g. LM2594 (0,5A) or LM2675 (1A)
 
Please give up the notion of using a resistive divider. Where does this pernicious internet myth come from? At those currents and disipations the resistors would be huge, massively inefficient, and provide absolutely no regulation. Given my impression of the state of your knowledge I think you will be happier engineering a heatsink, then a switching regulator. For those currents and voltage drops a switching regulator is the only technical solution I would consider for a product as opposed to a hobby application. The inductor however will probably rival the size of the heatsink.
 
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Step down regulator.

Hi Kittydog42,

Here's another possibility:

**broken link removed**

on1aag.
 
go get switcher cad from Linear tech. it's a free download and has a built in power supply design function (file -> switch selector guide). I plugged in 20 to 28V input, 5V output at 250 mA and it gave me 14 different chips to choose from. select one of those and it will give you a complete circuit including all the parts and their values. couldn't be easier. if you ask linear nicely, they may even send you the chips as samples, for free.

national has a similar thing in their web bench on-line site but I much prefer switcher cad.
 
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