M7 Diode

Hi all,

I’ve been checking Arduino schematics and I found that M7 diode is not always in the same position. For Leonardo, the cathode is connected to ground…

I guess it’s a dumb question but I wonder know why…

M7uno.PNG

m7leo.PNG

canal: Hi all,

I've been checking Arduino schematics and I found that M7 diode is not always in the same position. For Leonardo, the cathode is connected to ground...

I guess it's a dumb question but I wonder know why...

Well they both protect the board from applying a reverse polarity voltage via the DC power connector but there are slight differences in effects and trade offs made.

  1. The Uno method (series diode) blocks current from flowing if reversed polarity is applied. So no damage. Down side is that the diode creates a .6vdc voltage drop reducing the DC voltage by that amount.

  2. The Leonardo method (diode wired from DC voltage to ground) short circuits the DC voltage to ground if polarity is wrong thus protecting the board from seeing a dangerous reverse polarity situation. However during the short circuit period either the diode will burn open eventually due to over current draw or the DC voltage source will be short circuited to ground which may or may not cause damage to the DC voltage source whatever it is. In short this is a pretty poor method and assumes that the diode won't blow open which would then end the protection to the board and then allow board damage to occur, or it assumes the voltage source will self limit to some value that won't damage the diode or the voltage source itself. As the arduino has no control over what form of external DC power supply will be applied this seems a pretty dangerous method to rely on. I have often seen diodes wired like this but they always relied on the DC voltage source having a upstream fuse designed to blow open before the diode would self destruct.

Other's comments?

Lefty

retrolefty: I have often seen diodes wired like this but they always relied on the DC voltage source having a upstream fuse designed to blow open before the diode would self destruct.

In which case it becomes a one time use LED.

[quote author=James C4S link=topic=151990.msg1141152#msg1141152 date=1362333498]

retrolefty: I have often seen diodes wired like this but they always relied on the DC voltage source having a upstream fuse designed to blow open before the diode would self destruct.

In which case it becomes a one time use LED. [/quote]

Explain please? Mobil ham radios some being pretty expensive have had for decades used the diode to ground method to blow a fuse if polarity was reversed with the only damage being the fuse blowing. The difference was of course the fuse was built into the radio.

Lefty

Without the fuse, it becomes a one time LED...

[quote author=James C4S link=topic=151990.msg1141165#msg1141165 date=1362333927] Without the fuse, it becomes a one time LED... [/quote]

Of course. Or unless the voltage source has some kind of built in current limiting at less then the diodes max current rating.

Lefty

So, what would you recommend for a new board?
May be a simple PTC instead of the diode? (as Vinciduino does)

PTC Vinciduino.PNG

A PTC and the reverse bias diode would be ideal. The power is applied backwards, the diode creates a short circuit, the current rises above the "blow" threshold of the PTC fuse, which goes into meltdown, shutting off the power.

All is well with the world.

The series protection diode should ideally be a schottky diode, then the voltage drop would be more like 0.3 to 0.4V

In fact there are few circuits where a silicon diode is preferred over a schottky diode - schottkys' main weakness is higher reverse leakage at high temperature, but is generally superior for most uses below 60V or so.

Bottom line is the Leonardo protection design is a pretty flawed design compared to prior protection methods used on most boards. This coupled with the Rev 3 Uno and Mega boards using a op-amp to drive the on-board pin 13 led/resistor network but not providing a pull-down resistor for the op-amp input (granted not a serious problem) makes one wonder how well they vet their design revisions. Granted they have been releasing a ton of new products over the last year or two, lots of opportunities for hiccups.

Lefty

majenko:
A PTC and the reverse bias diode would be ideal. The power is applied backwards, the diode creates a short circuit, the current rises above the “blow” threshold of the PTC fuse, which goes into meltdown, shutting off the power.

Hi makenjo,

Something like the attached picture?

PTC+M7.png

Yes. As long as the PTC blows fast enough, when reverse current is applied and is conducted by the diode, the circuit will be safe.

If the PTC is too slow at blowing then the diode may go into meltdown instead of the PTC. Ordinarily I would use a proper fuse, of the "fast blow" type, but it should be fine with a PTC. Check it's data sheet to see how long it takes to blow, and check that your diode can handle large currents for that period of time.

retrolefty:
Explain please? Mobil ham radios some being pretty expensive have had for decades used the diode to ground method to blow a fuse if polarity was reversed with the only damage being the fuse blowing. The difference was of course the fuse was built into the radio.

That ‘difference’ is quite a big, important one…

fungus:

retrolefty: Explain please? Mobil ham radios some being pretty expensive have had for decades used the diode to ground method to blow a fuse if polarity was reversed with the only damage being the fuse blowing. The difference was of course the fuse was built into the radio.

That 'difference' is quite a big, important one...

The differences as I see it are twofold:

1) The "proper" way (as in the ham radios) provides all the circuit protection and doesn't rely on some unknown external power supply to be able to gracefully cope with a short circuit, and

2) This is yet another example of Arduino not really knowing quite what they're doing.

Arduino boards ought to have a small glass fuse on them. It makes sense for their target market, ie. educating people in electronics.

fungus: Arduino boards ought to have a small glass fuse on them. It makes sense for their target market, ie. educating people in electronics.

There's many things the Arduino boards should have, but due to budget limits, or shear incompetence, they don't. But as the first main low-cost development board they have defined the market, and while there are much better options available people still flock to Arduino.