SMT Soldering

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I don't think it matters, but if you connect the

battery backwards, the electrons are trying to

go into Vcc.

 

--Mike

 

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PolyZen for all though it's a SMD; USB version has a 5.9V breakdown that meets the USB suspend current requirement.

PolyZen Devices for Overvoltage-Overcurrent Protection - Littelfuse

 

"Dare to be naïve." - Buckminster Fuller

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I believe you are discussing the location of a reverse polarity protection diode that is in series with the power supply and the target device.

 

I always put the diode in the V+ power rail, usually just after the manual power on/off switch for the projects that have a switch.

Electrically it will work, obviously, in either lead.

I was just operating under the principle of always interrupting the Ground Lead / Plane as little as possible, and I don't usually want my PCB's Ground Level sitting a diode's Vf voltage drop ABOVE Ground.

 

JC

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MarkThomas wrote:

I am notorious for putting chips in backwards and plugging in power reversed.  I have been lucky in most cases that no damage was done to the circuit.  I keep thinking as a general rule I should put in a Schottky diode to solve the reversed power problem, and using a 5.6V Zener and single transistor for over voltage protection, although I dont over volt very often.  I have a big bag of 5.6 V Zeners.

 

I'm not sure if I should put the Schottky on the + or - terminal on power in, or if it makes a difference.  The electrons come into the - terminal, so maybe it is better to put it there.  Is there a general rule on how to do this?

 

I use a mosfet. Put a P channel mosfet in the + lead, DRAIN to the +, SOURCE to the load. Connect the gate to the - lead. If the voltage is too high for the gate voltage, put a resistor and zener, or even a resistor divider to hold the gate voltage within spec. Remember to connect the source and drain BACKWARDS so the diode does not conduct if polarity is reversed. No diode drop because the correct polarity turns the fet on. (Use an N channel on the - lead if you like.)

 

"We trained hard... but it seemed that every time we were beginning to form up into a team, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing. And a wonderful method it can be of creating the illusion of progress while producing confusion, inefficiency and demoralization." Petronius Arbiter, approx. 2000 years ago.

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Torby wrote:
I use a mosfet. Put a P channel mosfet in the + lead, DRAIN to the +, SOURCE to the load. Connect the gate to the - lead. If the voltage is too high for the gate voltage, put a resistor and zener, or even a resistor divider to hold the gate voltage within spec. Remember to connect the source and drain BACKWARDS so the diode does not conduct if polarity is reversed. No diode drop because the correct polarity turns the fet on. (Use an N channel on the - lead if you like.)

Thanks Torby.  I am going to have to try that out.  Sounds like an elegant solution, and no voltage drop.  So cool.

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I've used that very same arrangement.

MarkThomas wrote:
Thanks Torby. I am going to have to try that out. Sounds like an elegant solution, and no voltage drop. So cool.
Non-zero voltage drop, but yes generally quite small.  Much smaller than a diode.  It rises linearly with current, since the MOSFET has fairly flat Rds(on) for a given gate voltage.

 

https://www.google.com/search?q=mosfet+reverse+polarity+protection&tbm=isch

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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I build the power supply first.  If the voltage looks good, I add the AVR.

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steve17 wrote:

I build the power supply first.  If the voltage looks good, I add the AVR.

 

Good strategy.

 

"We trained hard... but it seemed that every time we were beginning to form up into a team, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing. And a wonderful method it can be of creating the illusion of progress while producing confusion, inefficiency and demoralization." Petronius Arbiter, approx. 2000 years ago.

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That's how I used to assemble through hole PCB's.

These days I place all the surface mounted components and heat the board on a frying pan, so the ability to easily do the power supply first is lost.

 

JC 

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DocJC wrote:

That's how I used to assemble through hole PCB's.

These days I place all the surface mounted components and heat the board on a frying pan, so the ability to easily do the power supply first is lost.

 

JC 

 

That's how I do them.

 

As long as your power supply does not need anything on the bottom, like through-hole connectors, you can still build the power supply, test it, then add the rest of the circuit. Yes, once there is something on the bottom of the board, I can't fry it again. I have hand-soldered the TQFP type avr.

 

"We trained hard... but it seemed that every time we were beginning to form up into a team, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing. And a wonderful method it can be of creating the illusion of progress while producing confusion, inefficiency and demoralization." Petronius Arbiter, approx. 2000 years ago.

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