Using 7805 with 24V DC suply [Archive] - Do It Yourself Christmas Forums

Henrik Albinsson

07-30-2009, 02:08 AM

Hi,
Is there any one that know how the circuit around the 7805 shuld bee if using 24V DC as the suply to make shore that the 7805 is doing it job as well as possible ( and not getting to hot ).

Henrik

g2ktcf

07-30-2009, 04:38 AM

well the data sheet says it is rated for that input level. However, I would use the larger heatsink that attaches to the board similar to the one used on the FireGod modules.

RPM

07-30-2009, 10:12 PM

Hi,
Is there any one that know how the circuit around the 7805 shuld bee if using 24V DC as the suply to make shore that the 7805 is doing it job as well as possible ( and not getting to hot ).

Henrik

As long as the 24vdc is regulated then you could get by with using a resistor from the 24vdc to the input of the 7805.
You would need to know the maximum current drawn from the 5vdc side of the 7805 to calculate the resistance and wattage you would need to use.
If the 24vdc is unregulated it could possibly exceed the maximum input voltage the 7805 can handle even if a resistor is used. This could possibly happen if there was little to no load on the 5v side on the 7805 and the input exceeded 35vdc.

Robert

kostyun

07-31-2009, 12:33 AM

I used a 47ohm 10w resistor to drop the voltage on my Ren24LV with no big issues (aka heat issues)

http://gallery.kostyun.com/ssp_director/p.php?a=XF9VXiA8OCYqQFtFPzw4JzQ6Jis9OzouNyo%2BMS4j KyU%2BIg%3D%3D

Prior to using the resistor - the 7805 ran fine but was HOT! Now its warm, and the resistor is warm rather than the scalding that the 7805 was.

It is not the cleanest way to do it, and it wastes some wattage (about 5 in the resistor) but, its a cheap way to do it.

I am using a regulated 24vdc power supply.

I am also looking at an alternative which is a drop in replacement of the 7805 but is a 5v switchmode PS. More to come on that as soon as I get my chips.

RPM

07-31-2009, 02:35 AM

I am also looking at an alternative which is a drop in replacement of the 7805 but is a 5v switchmode PS. More to come on that as soon as I get my chips.

You could use one of these if you need 500mA or less output current... they are pricey though :(

http://www.mouser.com/Search/ProductDetail.aspx?qs=35WhQNrE6p3r8osawIMOlg%3d%3d

Robert

mrpackethead

07-31-2009, 07:00 AM

frank, what chip is a drop in?? I saw the murata ones that RPM is suggesting.. I'm using some LM2594's, they will run up to 60V, but its not a single drop in replacement..
( diode, two caps, inductor and regulator )

mrpackethead

07-31-2009, 07:01 AM

frank, what chip is a drop in?? I saw the murata ones that RPM is suggesting.. I'm using some LM2594's, they will run up to 60V, but its not a single drop in replacement..
( diode, two caps, inductor and regulator )