Help needed with 24vdc to 12vdc

If you have to ask, then it's a bad idea to hit a 12 volt device with

24 volts. You're going to have to tell us what the device is. No definitive answer is possible with the information you've supplied. Kludge: If the device doesn't require tight regulation, you may be able to use a string of about 17 1N00X diodes, each dropping about 0.7 volt, to reduce the voltage from 24 to 12. Very inefficient, to be sure; the diodes will use up just as much power as your load. But it's quick and dirty, easy and cheap, if you have the power to waste. Best to use a converter, of course...

You may be able to, if the unit specifies that it can tolerate 24V and regulate it as it needs internally... also assuming the 24V supply can provide the necessary current. But probably not.

Does your unit require a regulated 12V supply? This is more a question for you to answer than anyone else. How could we know if your unit requires a regulated or unregulated supply??

If it doesn't require a regulated supply??? Perhaps you are using terms you don't entirely understand?

This is .basics.

I think you need to provide more information. Your unit requires

12VDC. And you have the figure of "500 mA," as well. Without you saying otherwise, normal assumptions would be that the 12VDC must be regulated, so that it doesn't vary by more than a few tenths of a volt over various loads up to something a bit over 500mA, but allowed to be much less as well. In other words, it holds the voltage regardless of the current required, up to some comfortable margin above the 500mA requirement. Also, I think usual assumptions might suggest that the power supply can take some reasonable time to come up to 12V DC on the regulated line -- perhaps on the order of 50 milliseconds or more. This may or may not be allowed by your unit.

You don't say what your 24VDC supply is like. Is it also regulated, already? (If it varies a lot, by some volts let's say, this suggests something additional to watch, for the 12V DC regulator.) What current handling can the 24V supply support?

What about heat? Can you tolerate losing 12V*500mA or 6 watts of excess heating in your "space premium" situation?

How much space is allowed here? How much cost?

Jon

The advantages of a converter are probably isolated and regulated 12 volts at high efficiency (little heat and wasted 24 volt supply power.

Wasting excess 12 volts while supplying 500 mA means that a linear regulator will have to dump 12*.5=6 watts of waste heat. A cheap integrated regulator (like the LM7812) and a few capacitors could do this job for a few dollars, if you have a heat sink surface available, or can add a heat sink that keeps the regulator below about boiling water temperature at the highest ambient temperature while being heated with 6 watts. Here is the data sheet for this regulator, that shows the recommended additional capacitors and possibly diodes for most applications.

Here is the data sheet for an isolated 24 V in 12 V out DC to DC converter able to supply up to 1 amp, at 87% efficiency (12 watts out with about 1.5 watts waste heat), CC10-2412SF-E (about $23 from Digikey) on page 10 of:

Note that the junction-to-case thermal resistance is 4C/W for either the TO220 or TO3 packages (the other arrangements are hopeless, being at least 32C/W.) 150C is the maximum junction temp and assuming some reasonable maximum for ambient (45C?), we are talking about at most

100C rise over ambient. Less to be safe. Let's call it 80C rise. This figures to 80C/6W, or about 13C/W total. 4C/W is already used in the J-to-C, so the heat sink needs to achieve about 9C/W or less. These are in the 1.5" x 2" x .5" size range, I think.

Jon

For example the 7-345-1PP-BA has a rating of 9degrees C per watt:

It will fit in a 2"X.75"X.56" rectangular prism. Of course, it needs a lot more space around it and oriented so that the fins form little chimneys, to convect away the 6 watts.

get your self a LM7812 or any manufacture of a 7812 3 terminal 1 amp voltage regulator. Radio Shaft also sells them. mount the tab on a heat sink.. connect the ground/common (middle leg) to your - 24 volt supply. the #1 leg is the + input from your 24 volt DC supply and the #3 leg is the regulated 12 volt output ..

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Simplest/best/cheapest/smallest *might* be a resistor in series with your 12 volt unit. It depends on how much current your unspecified "unit" draws and whether it draws that current all the time.

Ed