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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.
 

Ashhar Farhan
 

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.

- f

On Wed 7 Aug, 2019, 8:44 PM Evan Hand, <elhandjr@...> wrote:
Yes, I agree for the reverse polarity power protection.  Still need a way to drop the voltage to the board separated from the finals.  The most accurate solution is a voltage regulator with the relay for the reverse protection.

The Diodes and fuses do work, as I did connect an old HP power brick that had the center pin negative, and sleeve positive.  Popped the fuse nicely.

FWIW
73
Evan
AC9TU

Evan Hand
 

Yes, I agree for the reverse polarity power protection.  Still need a way to drop the voltage to the board separated from the finals.  The most accurate solution is a voltage regulator with the relay for the reverse protection.

The Diodes and fuses do work, as I did connect an old HP power brick that had the center pin negative, and sleeve positive.  Popped the fuse nicely.

FWIW
73
Evan
AC9TU

MadRadioModder
 

This is better.  No shorting diodes to replace, etc.

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of MVS Sarma
Sent: Tuesday, August 6, 2019 10:49 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Series Diode in Power Supply Line #ubitx

 

There would be power loss as we need around 3 amps.  Addex to that the 12  or 13.8 will actually suffer 2v drop even though we say that sili on dioďe drop is 0.7v 

 

The best optiòn appears to have shunt diode, that too àfter à fuse. The fuse protec ts the rig from  it getting reverse voltage eternally.

 

On Wed, 7 Aug 2019, 9:04 am Wayne Leake, <wayneleake@...> wrote:

 While the two diodes in series would be adequate, I would still prefer to keep the diode across the positive to negative line, so it will let you know by blowing the fuse.

 A bit redundant, but still a good idea.

 Hey,the diodes are cheap enough.

 I have far more than I'll be using for some good while.

 

 Oh yes, another idea is to use a full wave bridge, so you can connect either way;

  I had a printer buffer many years ago, that could have the power connected either way.


 Wayne WA2YNE

 

 


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…_. _._

_Dave_ K0MBT
 

I put two series diodes on the radio side only of my V3 uBITX. I think it cleans up the audio a little. The power side I am not worried about that is RV1's job.

Evan Hand
 

Here is how I have connected power to my uBitx.

Uses 2 fuses and 3 1n4000 series diodes.

FWIW
73
Evan
AC9TU

MVS Sarma
 

There would be power loss as we need around 3 amps.  Addex to that the 12  or 13.8 will actually suffer 2v drop even though we say that sili on dioďe drop is 0.7v 

The best optiòn appears to have shunt diode, that too àfter à fuse. The fuse protec ts the rig from  it getting reverse voltage eternally.

On Wed, 7 Aug 2019, 9:04 am Wayne Leake, <wayneleake@...> wrote:
 While the two diodes in series would be adequate, I would still prefer to keep the diode across the positive to negative line, so it will let you know by blowing the fuse.
 A bit redundant, but still a good idea.
 Hey,the diodes are cheap enough.
 I have far more than I'll be using for some good while.

 Oh yes, another idea is to use a full wave bridge, so you can connect either way;
  I had a printer buffer many years ago, that could have the power connected either way.

 Wayne WA2YNE


Wayne Leake
 

 While the two diodes in series would be adequate, I would still prefer to keep the diode across the positive to negative line, so it will let you know by blowing the fuse.
 A bit redundant, but still a good idea.
 Hey,the diodes are cheap enough.
 I have far more than I'll be using for some good while.

 Oh yes, another idea is to use a full wave bridge, so you can connect either way;
  I had a printer buffer many years ago, that could have the power connected either way.

 Wayne WA2YNE


Rob French (KC4UPR)
 

Hey all, I'm planning on putting a couple of diodes in series with the positive line from the power connector, to bring down the voltage a tad.  Just wanting to make sure I'm not missing something... If I do that, do I still need the reverse diode from the negative to positive terminals?  I would think that the series diode in the positive supply line would be sufficient.  

Thanks!
-Rob