Paralleling inputs to 78xx regulators?

[NewUser]

Is there any reason why I couldn't use the one 24v tx/rectifier/filter cap to feed 2 or more 78xx series regulators?

I'm thinking of one 7812 and three 7824's sharing the one DC supply, the 24v ones using the their inbuilt overcurrent shutdown to prevent one 24v circuit fault taking out the other two 24v circuits (sort of an electronic fuse box).

 

[ericgibbs]

Is there any reason why I couldn't use the one 24v tx/rectifier/filter cap to feed 2 or more 78xx series regulators?

I'm thinking of one 7812 and three 7824's sharing the one DC supply, the 24v ones using the their inbuilt overcurrent shutdown to prevent one 24v circuit fault taking out the other two 24v circuits (sort of an electronic fuse box).

hi,
There is no problem with connecting multiple 7812 to a 24Vdc system providing the total load dosn't exceed the rating of the 24Vdc.

You cant use a 24V input to a 24V regulator to give a regulated 24Vdc, the 78XX series regulators require about 3V more input voltage than the output voltage. ie; 27Vdc input.

Eric

 

[NewUser]

You cant use a 24V input to a 24V regulator to give a regulated 24Vdc, the 78XX series regulators require about 3V more input voltage than the output voltage. ie; 27Vdc input.

Thanks, glad you mentioned that or I'd have bought the wrong TX.

 

[ericgibbs]

Thanks, glad you mentioned that or I'd have bought the wrong TX.

hi,
NOTE::::
If you buy a 24Vrms transformer and fullwave rectify and smooth the rectified dc with a capacitor you will/can get about 32Vdc.

Tell us more about the current outputs you require and what you are planning to use the psu for and we can advise you.

Eric

 

[Tomble]

Eric's points on drop-out voltage aside, I'd hope that the 7812 in your circuit would be only used to supply quite low current, or else be well heatsunk, as this arrangement will have it dropping at least 15V.

 

[NewUser]

I ended up buying a multi-secondary tapped TX that gives me 12/15/18/24/27V @2A, so I guess I can pick the taps that produce, at the filter caps, closest to the 78xx's output voltage plus 3V (I assume the less difference between the 78xx's in and output voltages = less heat generated?).

The p/s is for a combination alarm/security lighting panel - the idea being if a thief chops through an outdoor cable feeding one of the 24VDC reflective PE switches it won't take out the whole system. The theoretical maximum current draw of all the panel's 24V timers/relays/indicators is 900mA.

 

[ericgibbs]

I ended up buying a multi-secondary tapped TX that gives me 12/15/18/24/27V @2A, so I guess I can pick the taps that produce, at the filter caps, closest to the 78xx's output voltage plus 3V (I assume the less difference between the 78xx's in and output voltages = less heat generated?).

The p/s is for a combination alarm/security lighting panel - the idea being if a thief chops through an outdoor cable feeding one of the 24VDC reflective PE switches it won't take out the whole system. The theoretical maximum current draw of all the panel's 24V timers/relays/indicators is 900mA.

hi,
The multi-tap TXR is a good idea, as you say, less heat and more efficient.
The current rating is well within the spec of 900mA.

Remember that when/if the intruder cutting, shorts the wires that the 78XX will draw its max current for a while, after which time, it could go into thermal shutdown.

So it maybe advisable to fit a power resistor in the high side of the 78XX's so that if one of the 78XX outputs gets shorted it dosn't pull the rest of the system down. You can determine the value of the resistors, from the current drawn thru each 78XX line.

Eric