[SOLVED] Need to understand Protection diodes

Hi guys,
I have a circuit I am trying to understand. In the power input section, it has two protection diodes D1 and D2 (see attached diagram).
Now what i understand is : D1 is for reverse polarity protection. I am not sure what D2 is doing in the circuit. Diodes in this config (D2) are used to protect circuit from negative voltages but i am confused.
Please explain it to me guys.
thanks

nigh said:

Hi guys,
I have a circuit I am trying to understand. In the power input section, it has two protection diodes D1 and D2 (see attached diagram).
Now what i understand is : D1 is for reverse polarity protection. I am not sure what D2 is doing in the circuit. Diodes in this config (D2) are used to protect circuit from negative voltages but i am confused.
Please explain it to me guys.
thanks

Click to expand...

---------- Post added at 00:40 ---------- Previous post was at 00:29 ----------

D1 is for reverse polarity protection. It simply blocks the supply voltage if applied in the wrong direction. D2 is for the same purpose but will instead clamp (shortcircuit) the main supply in case, for example, D1 is defective (for example acts as a shortcircuit). D3 is a light-emitting diode (LED) which light if the circuit is powered. The next diode-like semiconductor (exactly this component is not completely standardized regarding it's drawing to my best knowledge), but it is working like two zener diodes connected back to back. It is in other words a very simple over-voltage protector. This kind of components are sometimes called Transils. They could be unidirectional (leading as a regular diode in one direction but seems as a break in the other direction...unless the voltage is high enough and it then clamp the voltage to a decent level. Transil diodes (or transient protection diodes, as they are also sometimes called) do their job very well because they are very, very fast. On the other hand they can't endure overload (meaing clamping a voltage to its nominal level and at the same time let an appreciable amount of current pass through it). It will very quickly be overheated and the normal defective mechanism (if not the component just simply explode) is that it will go into shortcircuit. So when designing such a circuit as this make sure that under normal use the transil doesn't lead any current at all. Design it so it will for a very short amount of time (in the order of microseconds) clamp the voltage over it to a level that the rest of the circuit can withstand. Such transil transient protection diodes have their use for example to not let a flash striking nearby totally ruine most of your circuitry. Hope it helped a little.

/Pim

nigh said:

Hi guys,
I have a circuit I am trying to understand. In the power input section, it has two protection diodes D1 and D2 (see attached diagram).
Now what i understand is : D1 is for reverse polarity protection. I am not sure what D2 is doing in the circuit. Diodes in this config (D2) are used to protect circuit from negative voltages but i am confused.
Please explain it to me guys.
thanks

Click to expand...

as Pim replied BUT D3 is the light emitting diode...

D2 is another reverse polarity protection diode. To me seems a bit redundant if you have D1.....
When polarity supplied is correct, D2 is reverse biased - current does not flow.
When polarity supplied is reversed, D1 is reversed biased and current does not flow nor even reach D2.

nigh said:

I am not sure what D2 is doing in the circuit. Diodes in this config (D2) are used to protect circuit from negative voltages

Click to expand...

This is a schottky diode, it is faster than silicon diode and has a voltage drop of 0.3V when forward biased.

You are right, when used with ground to its anode it protects from negative voltages. Because if cathode's voltage drops below -0.3V with respect to GND (anode), the diode would conduct. The current would then flow from ground to cathode.

In this circuit though, even you didn't provide information about DC power supply and regulator type, I think it is not needed. This is implemented where frequencies are involved, for example at applications like motor control with PWM and DC-DC converters.

Same stands for transil, propably it is a waste of money connected there. Please provide more information about power supply and regulator.

I didn't check what type of diode D1 and D2 are, but that both are the same. So I don't think it sensible to think D2 is there because D1 may be slow.
So, unless the regulator side has a ability to produce a negative voltage, I still say D2 is redundant.
The transil on the other hand may not necessarily be a waste of money when spurious energies are expected on the line. eg automotive circuits.
There is plenty of information related to selecting the correct one at Circuit Protection, Fuses, Thyristors, Automotive Circuit Protection - Littelfuse.com

Hi xaccto!

None said that D2 is there because D1 is slow, I just explained the difference between silicon diode and schottky. As for transil i used the word "propably" and said that we need more information about supply source and regulator, otherwise we can only assume. But I think we agree that in a general purpose regulator circuit all this protection is not needed.

Cheers!

Thanx guys

A little more info :
The circuit needs to be installed in car so voltage source is car battery.
The regulator is switch mode voltage regulator :: xaccto , I have no idea if it can produce any negative voltages ... plz guide.

As cars have very noisy electrical environment specially during engine start, I also think the presence of TVS is justified.
so far my understanding is : D2 is either negative voltage protection or is redundant. Either way it has no negative effect on the circuit.
and yes diodes are schottkeys

Another small question (maybe a silly one ... but plz)
The clamping voltage Vc of TVS should be less than the maximum input voltage of the regulator .... right?