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Diode In Reverse Parallel

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Suraj143

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In circuits I used to put a diode in series with the Vdd (5V) Line to protect the circuit.

But I see some are putting this diode in reverse parallel to the Vdd & Vss & not in series with the Vdd line.

Can anybody tell me why doing like this?Is that method good?
 
do you have an example circuit ?
Is it a zener diode ? Is it connected to some inductive component or circuit, ?
 
It could also be a TVS diode, designed to reverse breakdown and conduct at above a certain voltage, to clamp the line voltage during a surge or electrostatic discharge event.
 
Here its like this.

dknguyen I cant understand your comment it has lots of difficulty words :(
 

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All diodes conduct in reverse direction when voltage gets too high. Most diodes die when this happens. Some diodes designed to do this specially. When voltage gets too high they conduct and keep voltage line from getting too high for protection. Like ESD or voltage spike.

UNderstand?

But diode in your schematic is normal type. I don't know why it is there. WHat does block in schematic do?
 
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High voltage does not need to be high. For 3.3V circuit, 5V from voltage spike or ESD shock can be too high. ESD shock especially much much higher (1000s of volts).

But yeah, specified LED is normal and not used for this purpose. I don't know what it does. Maybe reverse polarity protection? Usually diode like this would burn out in that case, but may save circuit if problem was realized and circuit disconnected right away. Reverse polarity protection diode in series works better and protects for longer, but has voltage drop on output. Parallel diode doesn't protect for very long but has no voltage drop on output.
 
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High voltage does not need to be high. For 3.3V circuit, 5V from voltage spike or ESD shock can be too high. ESD shock especially much much higher (1000s of volts).

But yeah, specified LED is normal and not used for this purpose. I don't know what it does. Maybe reverse polarity protection? Usually diode like this would burn out in that case, but may save circuit if problem was realized and circuit disconnected right away. Reverse polarity protection diode in series works better and protects for longer, but has voltage drop on output. Parallel diode doesn't protect for very long but has no voltage drop on output.

I've seen this used in high current mobile radio installations. It's for reverse voltage protection without suffering the voltage drop of a series connected diode. However there must be a fuse or other current limiter between the voltage source and the diode for this to make any practical sense ;)
 
So Finally that diode is for reverse voltage protection without a voltage drop.

Here after is it ok if I add diodes in parallel instead of series to Vdd.
 
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So Finally that diode is for reverse voltage protection without a voltage drop.

Here after is it ok if I add diodes in parallel instead of series to Vdd.

hi Suraj,
The diode/s you are posting about are not in 'reverse parallel' connection, thats a different configuration.
Reverse parallel is where two diodes are connected in parallel with each other,
the anode on one diode being connected to the cathode of the other.
So they form an 'ac' type clamp, which 'clamps the 'ac' signal to about +/- 0.7V.

The diode you have posted is a reverse voltage protection diode and as already been pointed out you should have a series fuse in the supply line.
 
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hi Suraj,
The diode/s you are posting about are not in 'reverse parallel' connection, thats a different configuration.
Reverse parallel is where two diodes are connected in parallel with each other,
the anode on one diode being connected to the cathode of the other.
So they form an 'ac' type clamp, which 'clamps the 'ac' signal to about +/- 0.7V.

The diode you have posted is a reverse voltage protection diode and as already been pointed out you should have a series fuse in the supply line.

Hi ericgibbs thanks for correcting that mistake.

Oh I see its a reverse voltage protection diode.Can I use it without a fuse If I supply 5V.
 
Hi ericgibbs thanks for correcting that mistake.

Oh I see its a reverse voltage protection diode.Can I use it without a fuse If I supply 5V.

hi,
Assume you used a 1amp diode, say a 1N400x and your power supply could deliver 5amps..... result = smoke = burnt out diode [ possibly o/c failure] so you now have -5V applied to you logic/PICs.. result = dead logic/PICs..:eek:

If you havn't got space to have a fuse holder, use a short length of 1amp fuse wire solder it onto the pcb as you would a resistor, on the incoming +Vline.

OK.?:)
 
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