Ron R:
I do have some 7812 lying around. I do not have diodes and am going to skip on that part. Have some 0.22uF caps am going to use that. It is a nice 'trick' and the same idea once hit me. I have some questions:
a) Have you checked the draw of this circuit? You know that this is also a linear regulator.
Yes I know it's a linear. But for this application it's an advantage. If you want you can also use a integrated switching regulator, the one I mentioned in my other posting.
Disadvantage of the switching regulator compared to the linear is that (at 24V) the swtiching can handle a max load of 0.5 A whereis the linear can handle at least 1.0 A.
Because this regulator only has to deliver current for the alarm and memory, 1.0 A is more than sufficient, at least in my setup. It's now in use for about 8 years.
b) 7812 is going to get pretty hot, have you mounted a heatsink?
Hm, not normally. The current for memory and alarm/anti-theft will be something like 40-50 mA. Taken into account that the efficiency is near the 50% mark, the draw from the battery will be in the 0.1 A range. Less than half the draw you have now.
The 7812 can even do without heatsink, but -as I wrote in my other posting - I've put the little circuit in a alumimum box. The 7812 is bolted to one of the walls of this little box. That box now acts as a heatsink.
The 'body' of the 7812 is mass, so you can bolt it directly to the alu-box and fix it onto the car-body.
c) LM7812 is made to withstand load of 0.5 ampere. Now the CD player memory and car alarm... I know just the loud speaker (thingy that make noise) is 50watts. Besides, anti theft system would also consume some amps. You really think just 0.5amp would do the trick?
The 7812 is available in various capacities. Even 3 A-versions are available.
At rest -when there is no alarm - a 1A version is more than sufficient. When the alarm is active the draw probably will be more than 1 A but only at short bursts. Keep in mind that although the speaker is rated at 50 Watts, that's only peak, I doubt it will really draw 4 A.
The 7812 has an internal protection so, even if overloaded, it won't get damaged.
d) You should not use the LM7812 and the 'big' converter on same rail. You must introduce a relay such that when your 'big' converter kicks in, the homemade one kicks-out. Else, since LM7812 is not load capable and produces say 12V (which is lower than your 'big' converter as usually they produce 13.8V or 14.4V, hence LM7812 at lower potential will result in current flowing towards LM7812 from the big converter. And I doubt the diodes would help much (suppose when there is no load i.e. CD player off).
You are right in saying that the two (the regulator and the switching convertor) should not be on the same rail. Never connect two powersources to each other.
But that's why I've used the diodes. They effectively 'disconnect' the two powersources because they work like a one-way valve. Current can travel from the source (the 7812) but not towards it.
Because the 7812 will effectively deliver ~11.3V (12V minus loss over the diode) the switching convertor will override it when it kicks in at ~14V. And because of the diode, no current will flow towards the 7812.
This 11 and a bit as voltage may seem to be low, but that is no problem at all to maintain memory and keeping the alarmsystem alive.
I found that, at a battery level where I was unable to start the engine, the memory was still alive and alarm was working properly. Of course, if the batteries get fully drained (i.e. several start-attempts) then story ends.
