Hello,

I need to feed an IC ATMEGA 328 alarm system in an unattended farm.
The power available is 48 vdc from a solar array plus batteries. LV Disconnect is 47.4 volt and max voltage is 57.4
So normal pos regulators that have 40 volt max input differential are not enough.

I tried to cascade two: one 7818 and one 7805.
I am sure of the wiring and the capacitors.
Load varies from 36 mA of the IC and its signaling leds to 98 mA when the alarm relay triggers.

I am surprised it did not work.
Even at min current the 7818 drops to 6 volt and the 7805 gives 2.8 volt.

If I disconnect the 7805, the 7818 comes up to 18 volt, but then when I reconnect it drops again.
Strange behavior, I am missing something.
Of course I dont have the possibility of a DC DC converter, I am far away from eBay !!!
I have the alternative of reducing from 48 to 18 with a transistor plus zener, but I am curious about my mistake.
Or take an additional wiring from the half of the battery bank, awork with 24 to 5. Not elegant !

Thanks for your help,
Horacio

 

Hello,

Have a look at the XL7056.
That is a switching regulator that will even work upto 80 Volts.

Bertus

 

I tried to cascade two: one 7818 and one 7805.
I am sure of the wiring and the capacitors.

[snip]

If I disconnect the 7805, the 7818 comes up to 18 volt, but then when I reconnect it drops again.
Strange behavior, I am missing something.

So there is no misunderstanding, post a schematic of your circuit. Be sure to include the loads the regulators are driving.

 

If everything else is correct, you may be running up against the preset internal wattage limit.

 

I use these....
Recom R-78HB5.0-0.5L
72V max input. 5V 500mA out.
https://au.rs-online.com/web/p/switching-regulators/6727095/
Just a bypass cap on the input and output. Easy.

 

Hello,

First thanks for all the responses.
My circuit is attached, is very simple.
I changed regulators for new ones. I feed with four 12 volt batteries that provide 53 volt.
Capacitors are tested.

With reference to the switching solutions, I prefer the linear one with this very low energy, and avoid noise.

Thank you.
Horacio

Circuit used:

 

My circuit is attached, is very simple.

The problem still remains, the max input volts of the LM7818 is 35V. You are asking for smoke! And then probably kill your load device as well. Watch out!
Also, the switching regulator will be more efficient than a linear one.
At rest, (36mA) the linear will dissipate around 1.5Watts, and at max load (98mA) about 4 watts.
If you are really worried about noise, use a 12V or 9V switching reg in front off the linear one. But noise will most likely not be a problem with good bypassing.
Using a linear regulator for this application is not the best way to go.

 

Just connect the 5 volt regulator to only one of the batteries.
SG

 

Thanks for all the answers !
Kind regards,
Horacio