Re: Series Diode in Power Supply Line #ubitx

Jerry Gaffke

An LM2940T-12 is similar to an LM7812 except that it is low dropout, offers polarity protection, and is not quite so prehistoric.
I'd recommend a 3A fuse between power supply and the rig, use an LM2940T-12 inside the rig to power the main board.
Have the power supply with 3A fuse drive the IRF510's directly to avoid overloading the LM2940T-12.
If the supply is reversed, the intrinsic diodes in the IRF510 will conduct and blow the fuse.
With that arrangement, you could supply 20vdc to the rig if you want more power, the main board will still only get 12v.

For a really cheap solution on a high volume project where we can assume the power supply voltage
does not exceed the maximum that the rig can handle, replace the LM2940T-12 with a small shottky diode.
Offers polarity protection, and a short on the board will blow the diode.

A search for "reverse polarity" in this forum will find long discussions with dozens of ways to do this.
One of those things that everybody seems to have an opinion on.
Including me.

Jerry, KE7ER

On Wed, Aug 7, 2019 at 08:21 AM, Ashhar Farhan wrote:
I would second that. A voltage regulator is best. However, the generic ones like 7812 will need at least 15v (3v more than the output) as their inpit voltage. A small 1uf and a couple of 0.1u caps across the output makes for a nice, low noise supply.

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