No, you'll get 12V @ 1.0A output with 8 watts of heat dissipated in the 7812.If I put 20VDC 1.0A into an LM7812, do I get 12VDC 1.6A out?
I have a pump which requires 12VDC, 1.4A.
I want to power the pump with a wall adapter (what some folks call a "wall wart") that gives 20VDC 1.0A.
Might burn up both the pump and the adaptor
Now, there is a regulator called an LM7812, that will give a regulated 12VDC output. The headline on the datasheet (from Fairchild) describes this component as a "1A regulator". But later, the datasheet claims a maximum 2.2A peak current.
If I put 20VDC 1.0A into an LM7812, do I get 12VDC 1.6A out?
Not continuously.
What does it say on the adaptor name plate?I want to power the pump with a wall adapter (what some folks call a "wall wart") that gives 20VDC 1.0A.
Willbe, I measured the no-load voltage of the 12VDC wall adapters. It's wanders about 14VDC. Does that meet your assumption "Assuming each +12v @ 800 mA supply is actually a +14v"?
Yes; see below
eblc1388, you must be correct that the initial start-up of the pump will require more than 1.4A current. Does that mean the proposed solution -- two 800mA wall adapters with their positive leads tied together -- will not work?
see below
Or is there some compensating "wall adapter magic" for the start-up load? Could I use three wall adapters, tied together as Willbe suggests? Could I add a capacitor for the start-up event?
see below
I'll take it that the 2-amp fuse in the pump manufacturer's spec sheet means the two-adapter solution will survive the start-up current. Can someone please confirm?
Two adaptors will handle 1.6 A steady state current which is >1.4 A. I doubt that the short-duration startup current will hurt the adaptors.
Is it a slow blow or fast blow fuse? If you can find trip curves for the fuses you can approximate the startup current. Generally a 2 A fast blow fuse can handle 4 A for one second.
If you all have the strength to go back to the point where there was
- an "elegant" solution from which I diverted you, and
- a "not elegant" solution which we pursued....
At my local Radio Shack I see, at hosfelt price's, a 12.6VAC 3A power transformer. After the discussion about start-up current for the pump motor, is that elegant power transformer still a viable solution?
The transformer has 3 (black, black, and yellow) wires facing the 115VAC mains, and 2 (black and yellow) wires facing the 12.6VAC. Also for sale was a 4A 400V full-wave bridge rectifier for about $2; however, it only has 4 connection points. Do I connect the yellow mains wire to earth/ground, and there's no polarity in the others?
The + - markings are the DC output of the bridge; the ~ marking is the AC input.
I wouldn't give Radio Shack a single penny.
At http://www.kpsec.freeuk.com/powersup.htm, they mention a smoothing capacitor.
Using their calculation, I get 2uF. Does that sound right?
I don't think you need one for a motor. It will raise the DC voltage across the motor somewhat.
This solution will cost. The two-adapter solution will cost me nothing (bless the inventor of yard sales).
Is there an upside to this power transformer solution?
The internal resistance of the transformer might be 1/2 that of the two adaptors.
Thanks again.
There would be (1.26/0.8)^2 = 2.5x the normal heat dissipation in the adaptor, so the adaptor wouldn't last very long.
Willbe, I am very sorry for being partly the agent that required your purchase of a shiny new engineering calculator. At the same time, I am happy to hear its calculations. If two 800mA 12VDC wall adapters can start the pump, I'll have a few years to find replacements. The magic for me, at all, was that you could place the output of two adapters in parallel.
Thanks, everybody.
https://en.wikipedia.org/wiki/Pulse-width_modulationHero, thank you for the effort of creating a circuit. I guess this is the implementation of your earlier thought, "You could PWM the motor at 60% duty cycle." I'll build the circuit during the coming (North American) winter; earlier if I must. I see the famous timer chip, but I do not yet understand PWM. Is there a good website or book that you could recommend?
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