Over-current PTT Board Melt Down

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Over-current PTT Board Melt Down

Postby Brent » Sun Mar 23, 2003 8:11 am

Help! On Kevins board had the 680uf cap explode and the etch went open between the +24 volt ground reference.....The .1ohm resistor providing the current reference from the power supply seems to be the reason. Anyone have any more detail surrounding the correct connection of the .1 ohm resistor to the power supply......I cant put the smoke back in the cap :x

thanks Brent
Brent
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OverCurrent board...

Postby Kevin » Sun Mar 23, 2003 8:32 am

Brent-

:!: That portion of the OverLoad circuit is 'Exactly'
like Steve's. It *IS* Steve's circuit, for that
matter.

Remember that the 0.1 ohm current sense resistor
must have a solid ground OUTSIDE the board, as the
full current of the system is flowing through it.
The board simply 'taps off' this resistor to detect
current. You cannot depend on the board to supply
a high current ground connection.
I believe I have note on the schematic to wire the
0.1 ohm resistor as depicted on Steve's schematic.

Also, are you bringing up the power slowly with
a variac during these tests?

Let us know how it goes...

Kevin
Kevin / KD1J
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.1 ohm resistor connection

Postby steve_qix » Sun Mar 23, 2003 12:48 pm

Hi Brent,

The .1 ohm resistor should more or less be connected directly from the negative side of the power supply to the ground reference. Mine is actually part of the power supply proper. Then, you can run a couple of wires from the .1 ohm resistor back to the overcurrent board. This keeps the high current away from the board etch runs.

If the .1 ohm resistor opens up, or if any of the connections to the .1 ohm resistor fail, you can get high voltage across the 680uF capacitor when a load is applied to the power supply.

Let us know :-)

Regards,

Steve
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http://www.classeradio.com
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Stupid problem found

Postby Brent » Thu Apr 03, 2003 8:26 pm

I know this is a late post, but I wanted to confirm the boards were not the problem. I had a bleeder resistor go to ground on the power supply caps and dumped negative 62 volts to ground....this little accident did damage to both the modulator high current rectifier and the power supply board. They were easily fixed.

As this was my first project....it was a learning curve that was pretty sharp. As of this post the rig has been super and on the air every night!

73 Brent W1IA
Brent
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Location: Derry, NH


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