Override jumper for a 7812?

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Hi,

In a project I wanna have a PSU like this:

Batteries+(16V approx)-+-->7812--+->Motors and stuff
                       |         |         |
                       +--Jumper-+         |
                                           |
Batteries-(0V)-----------------------------|

I want the device to have two speed settings: one with a constant, 12V, the other one at full battery power, 16V. According to the internal schematics of a 7812 (datasheet), it seems I have nothing to fear if I simply put a wire between the IN and the OUT pin, and leave the GROUND pin grounded.

Are there any risks involved in this method? Is there any chance of problems arrising (say, current going INTO the OUT pin of the 7812 etc.)?

Thanks,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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The only issue I have had with these parts is when the output voltage exceeds the input voltage. All you need to do to solve this problem is to put a diode from the input to the output. A 1N4002 works fine. Anode of diode to output and cathode to input.

Try that and see what it does. (The motor takes less than one amp, worst case?)

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Quote:
The only issue I have had with these parts is when the output voltage exceeds the input voltage.
It would be, in my case equal.

The motors will be two, each draining approx. 0.7A when stopped mechanically. I'll be using a 78S12, which AFAIK can regulate 2Amps.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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According the the Texas Instruments data-sheet, there will be an impedance of 5.4K Ohm to GND, if you can live with that.

But why not use the GND leg of the LM12? Just use an NPN transistor (E to GND and C to LM7812 ref (GND) leg) and control the base of the transistor. A high on the base = 12V out. A low on the base = MAX out.

Just a thought...

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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If you phrase your question in a different way, it doesn't sound like such a good idea. "Would it be OK to connect +16VDC to the output of a 7812 regulator?"

How about something like this?

Tom

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Quote:
But why not use the GND leg of the LM12? Just use an NPN transistor (E to GND and C to LM7812 ref (GND) leg) and control the base of the transistor. A high on the base = 12V out. A low on the base = MAX out.
Wouldn't there be an some voltage drop anyway? 7812 AFAIK aren't low drop, so, I'd have to give away approx. 2V just to let it work.

Zoomcity:
Looking at the datasheet, the internal schematics are mostly like this:

IN---+----C->Transistor--E----------+---->OUT
     |           |B                 |
     +-----Control, reference,etc---+

so, it doesn't seem that scary.

Why is there a diode in the ground of your drawing? Doesn't that mess up the reference?

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Quote:
Why is there a diode in the ground of your drawing? Doesn't that mess up the reference?

The diode in the ground leg compensates for the diode in series with the output. The output, after the series diode is 12V.

Use Schottky diodes to minimize the voltage drop when operating directly off of the battery.

Tom

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Quote:
so, it doesn't seem that scary.

Breadboard it up and give it a try. Monitor the current at the output of the 7812 while the input is connected to the output. For a one-off project, if this works, I think it would be OK. If I have any reservations, it's because I usually take a black box approach to what's inside an IC.

I'd be interested in knowing what your results are with this.

Tom

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Why not use a changer instead of the jumper?

                 +------------------------+ Bat+ +
                 |                                \  
                 |                                 +----+ 12V or Bat+
                 |                               
Bat+ (16V) +-----+------78S12 ------------+ +12V +
                          |
                          |
       GND +--------------+-----------------------------+

Regards
Sebastian

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Quote:
Why not use a changer instead of the jumper?

I thought of this also. One reason for not doing this is that power is lost between switching from one position to the other. If this can be tolerated, it's a good idea.

Tom

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An other point...
The 78xx regulators need at least a 2.5V higher input voltage than the output voltage for proper operation. So 12V + 2.5V = 14.5V. When the battery is loaded by the motor, the battery voltage will drop. And the battery voltage drops during discharging. So there is only a very small voltage window available where the regulator works in its specified operation range.
I think you better use a low drop regulator.

Regards
Sebastian

EDIT:

Quote:
The motors will be two, each draining approx. 0.7A when stopped mechanically. I'll be using a 78S12, which AFAIK can regulate 2Amps.

A heat sink might be neccessary, too.
When you stop the motors, they'll produce overvoltage (inductance). So you need some freewheeling diodes.

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Thanks guys for the suggestions, warnings, etc.

I've decided to go the safe way: Use a two position jumper, like sebastian advised.

Oh, and considering these test results.... I'd guess that it's the SMART way also. :-D Sorry for doubting your experience guys. Assumption really is the mother of failiure:

12V-->7805--->Motor               == 16mA

12V-->7805--->Nothing             == 5mA

12V-+>7805+-->Nothing             == 56mA
    +-----+

12V-+>7805+-->Motor               == 100mA
    +-----+

The motor was just a test load. :-D There was one 100n cap on both IN and OUT pins of the 7805. Dunno how the 7812 would have reacted.

Thanks again,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Nice work, David. Thanks for the interesting test results!

Tom