[sdiy] Diode reverse polarity protection revisited

[Harry Bissell]

I'd call these series and shunt diode...

The series diode always has a forward voltage drop, sometimes you cannot stand the voltage loss. Usually not a 
problem with +/-12 or 15V but a killer at lets say 3.3V or below. The series diode must pass the normal supply
current at all times, so it must always dissipate the power (.7V drop x supply current = watts). If the circuit
has a variable load (flashing lights, whatever...) the change in voltage drop across the diode affects the circuit
voltage regulation (its worse..)

The shunt diode MUST be combined with some series element designed to limit current flow, a fuse or a polyswitch fuse.
The diode clamps the voltage reversal to one diode drop, and conducts so much current that the fuse blows in a very
short time or the polyswitch goes high impedance, limiting the power dissipation in the diode. If not, the diode will
probably overheat and die (but likely as a short circuit, continuing to protect the circuit even in death...).

A shottky diode will ahve about a .3V forward drop, enough to protect almost any semiconductor (most need greater than .7V
to conduct current and get damaged...

If you are really serious about protection, you can monitor the supply and combine current limiting and active clamping
(like an SCR crowbar circuit), but that adds a lot of complexity.

Personally, I don't use diode protection either way. If I hook the supply up backwards I spank myself silly...

H^) harry





----- Original Message -----
From: Justin Owen <juzowen at googlemail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 11 Jan 2011 06:07:24 -0500 (EST)
Subject: [sdiy] Diode reverse polarity protection revisited

Following the recent discussions about circuit protection I spent some time using the 1N4001 diode in anti-parallel (is that what it's called?) mode i.e. Anode to GND and Cathode to Positive Rail...

Running the diode in series I get a 0.3V drop and when it's reversed I get about -0.07V at the Cathode. Fine.

In anti-parallel I get no voltage drop (advantage) but when the input voltage is reversed I get a) -1V at the Cathode and b) within seconds the diode is too hot to touch.

...can someone explain the benefit to this method over running it in Series? Right now I'm not seeing it.

Thanks again,

J



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-- 
Harry Bissell & Nora Abdullah 4eva