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Help needed with 24vdc to 12vdc

C

cabsandy

Jan 1, 1970
0
Hi all,
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?-does it matter if the
input is regulated or unregulated? If not, I understand that I may
need a DC to DC converter.The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

Sandy
 
K

kell

Jan 1, 1970
0
Hi all,
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?-does it matter if the
input is regulated or unregulated? If not, I understand that I may
need a DC to DC converter.The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

Sandy

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...
 
J

Jonathan Kirwan

Jan 1, 1970
0
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?

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 it matter if the
input is regulated or unregulated?

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 not, I understand that I may
need a DC to DC converter.

If it doesn't require a regulated supply??? Perhaps you are using
terms you don't entirely understand?
The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

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
 
J

John Popelish

Jan 1, 1970
0
cabsandy said:
Hi all,
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?-does it matter if the
input is regulated or unregulated? If not, I understand that I may
need a DC to DC converter.The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

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.

http://www.fairchildsemi.com/ds/LM/LM7812.pdf

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:
http://www.tdk.co.jp/tefe02/ea335_cc_e.pdf
 
J

Jonathan Kirwan

Jan 1, 1970
0
<snip>
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.
<snip>

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
 
J

John Popelish

Jan 1, 1970
0
Jonathan said:
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.

For example the 7-345-1PP-BA has a rating of 9degrees C per
watt:
http://rocky.digikey.com/WebLib/IERC/Web Data/7-344,7-345.pdf

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.
 
J

Jamie

Jan 1, 1970
0
cabsandy said:
Hi all,
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?-does it matter if the
input is regulated or unregulated? If not, I understand that I may
need a DC to DC converter.The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

Sandy
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 ..
 
E

ehsjr

Jan 1, 1970
0
cabsandy said:
Hi all,
Sorry if this seems a bit noddy but a few simple questions..

I have a unit that expects 12vdc at about 500mA.I want to place it in
a remote location but that location only has a 24vdc supply
available.

Can I simply connect it with no consequences?-does it matter if the
input is regulated or unregulated? If not, I understand that I may
need a DC to DC converter.The problem is , space and price are at a
premium, and I want to keep this as simple as possible .I guess the
converter is the "proper" option so I apoligise if this question
offends anyone or causes them to shake their head ;-)

Any advise gratefully received

Sandy

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
 
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