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Diode application

07/29/2008 5:11 AM

Hi!

I need to ask very elementry question about diode application.

What is the purpose of general purpose diode connected in reversed bias across the DC Supply terminal in any electronic circuit?

e.g. 1N5402 is connected accross the +12V and GND terminal in reverse bias.

I think such diode gets short upon reverse voltage application to the circuit causing 0 voltage to remaining circuit eventually protects the circuit against reverse polarity .

Pls correct me if I am wrong with that notion.

If that is right , then how the ampere rating of that diode can be calculated to be used in the design?

Regards!

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Guru
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#1

Re: Diode application

07/29/2008 5:21 AM

<...diode gets short upon reverse voltage application to the circuit causing 0 voltage to remaining circuit eventually protects the circuit against reverse polarity...>

Correct. When it conducts, the supply fuse will blow.

<...how the ampere rating of that diode can be calculated to be used in the design...>

Will it withstand the rupture current of the supply fuse without itself being damaged?

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Associate

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#2
In reply to #1

Re: Diode application

07/29/2008 5:54 AM

Thanks a lot for being so quick to respond !

Actually , I have seen such reference circuit design during internet surfing.

There is no provision of any fuse in that design.

But 1N5402 is rated for 3 amp, if someone wish to use supply fuse then , should fuse rating be greater or lesser than 3 Amp (than diode rating)?

Let me be more specific.

I wish to use such protection technique in my project design which has 12 volt, 5 amp SMPS (and with 5 amp supply fuse) as power supply and application control circuit would have max. 3.5 amp load .

How can be the proper diode choosen according to design?

Regards!

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#3
In reply to #2

Re: Diode application

07/29/2008 6:40 AM

Hello fireball

The designs you have seen generally will not actually show a fuse, because a fuse is an 'implied' part, needed for safety, but not actually required for the circuit to work.

The reverse connected diode should have a rating at least twice that of the fuse rating.

So in your case, the fuse would be 5 Amp, and the Reverse connected Diode a rating of at least 10 Amp.

That way, should there be a major problem on the circuit, the fuse will blow, and if the supply polarity is reversed, the diode will conduct, ensuring once again, the fuse will blow.

In some DC circuitry, where a supplied DC may not always have the correct polarity, makers often use a 4-diode assembly called a "Bridge Rectifier" unit.

The Supply DC connects to the "Bridge Rectifier" terminals marked AC, whilst the "Bridge Rectifier" output + and - terminals connect to + and - of the assembled circuit respectively.

With a "Bridge Rectifier" (4-way diode assembly) input, no matter which way the input is connected, the output is always correct.

There is the small voltage loss through the diodes, but often circuit designers design the circuitry to work on that reduced voltage, and the small extra cost of the modified design, added to the small extra cost of "Bridge Rectifier" unit, allow peace of mind to the user against incorrect supply polarity problems.

Trust that assists you.

Kind Regards....

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#4
In reply to #3

Re: Diode application

07/29/2008 8:39 AM

You can not rely on that figure Sparky...

A fuse can be of many different types but the time it takes to open circuit will be much longer than the time to blow the diode, even at twice the fuse rating.

Usually the diode is there because the supply has a built in current limit of less than the diode rating - the diode is there to prevent damage to the output of the regulator by connecting a voltage (battery?) round the wrong way, or by using the supply to switch inductors which can give a hefty back emf into the supply.

John.

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#7
In reply to #4

Re: Diode application

07/29/2008 1:32 PM

I always said that going to the basics is the most sure way to give a good answer:

Murphy's rule #3: A fuse set to protect a semiconductor device by blowing up, will be protected by the faster blow up of the semiconductor!

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#19
In reply to #3

Re: Diode application

07/30/2008 10:19 AM

Excellent explanation.

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#5

Re: Diode application

07/29/2008 8:43 AM

Oh and reverse biasing a diode forms a very useful function in varying the capacitance of the diode.

Which is commonly used in tuning applications.

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#6

Re: Diode application

07/29/2008 10:50 AM

Just out of interest...using a diode to protect against reverse voltage (or a zener to protect against over voltage) is called 'crow bar' protection...
I think this derives from Electric railways and similar where a crowbar could be literally dropped across the tracks to cut the power shorting it out and tripping a breaker.

I'm sure someone will correct me if I've got that wrong....

Del

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#8
In reply to #6

Re: Diode application

07/29/2008 1:50 PM

I've not heard of that derivation of the word crowbar before Del...

Sounds good, but a tad dangerous when using the tens of thousands of amps used on a train line!!!

I was told it was to represent the same effect as dropping a crowbar across the power supply terminals, never heard of the train reference...

PS I think in the USA they call a crowbar a wrecking bar???

John

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#10
In reply to #8

Re: Diode application

07/29/2008 11:55 PM

In this part of the US, I'm equally familiar with both terms, but I believe I personally use 'crowbar'. I'm not sure if that is related to my electronic background, to my father having rebuilt several houses while I was a child, or perhaps just because its faster than saying 'wrecking bar'.

It would have to be a company crowbar - I'm sure not going to toss mine across the railroad rails!

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#16
In reply to #8

Re: Diode application

07/30/2008 9:27 AM

Yep. That's where it comes from.

The technique is still used in an emergency in 3rd-rail and 4th rail electrified lines. It takes a bit of bottle to actually do it, mind.

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#22
In reply to #8

Re: Diode application

07/30/2008 6:44 PM

The comment: Dropping a crowbar across the railroad track, is not used on electric train operarion. But on most other Diesel/steam Engine systems. the crowbar is simmulating a train stopped on this track, in that it sets the signals on each side of it to the Track In Use Mode, thus saving a possible crash until the reason for the crowbar is removed. (just another Retired Telegraph Guys version of your term.)

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#9

Re: Diode application

07/29/2008 11:07 PM

If you're driving the circuit with the reverse bias diode protection from a proper power supply (linear or switch mode) it will almost certainly have foldback protection. That is, when short circuited (e.g. by a protection diode inside a reverse-polarity-connected load), the supply will turn itself off when the current exceeds its maximum rating, and will only turn itself back on when the short is removed. That means the diode does not really have to be rated for the full supply current - it is only going to experience a brief pulse of current until the foldback protection of the supply kicks in. Even a low current diode like a 1 amp 1N4001 can provide protection for foldback limited supplies of say 10A without getting destroyed.

In the sad case where your supply does not have foldback limiting, most times it is a PCB trace that will get vapourised and do the job of a fuse. Again the diode can usually sink enough current till the trace burns open (diode rating is related to its thermal dissipation, peak rating far exceeds continuous rating). A burned trace is always cheaper to fix than a whole board full of blown semis.

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#11

Re: Diode application

07/30/2008 12:35 AM

I think you'll find in this case, "Crow Bar" refers to the thing you dig post holes with. A wrecking bar is also called a crow bar or "pinch bar" or "pry bar" in some circles, but is some what smwller than the traditional crow bar, which is usually about 5-6 ft long (1.5-1.8m) and about 1" dia (25mm).

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#12

Re: Diode application

07/30/2008 2:21 AM

I saw this diode connection in three cases:two of them were mentioned before 1)coil in the circuit;2)protection against wrong batteries conection;3)a special circuit,(two diodes and common ground),to multiply by two the voltage.So could you show the circuit?.-

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#13

Re: Diode application

07/30/2008 7:54 AM

We often use a reverse biased diode as a transient supression circuit in circuits containing coils...

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#14

Re: Diode application

07/30/2008 8:21 AM

I have yet to see the reason for the diode. Yes it is reverse voltage protection, but why do you need that? Inductance. If you have relays or contactors down stream from the DC supply, upon removal of the voltage from the coil in a relay, you get a nasty reverse voltage spike as the magnetic field around a coil relaxes. I have seen $1000 pressure sensors in a plant fried when the emergency stop was initiated, suddenly turning off a major motor contactor with a 24VDC coil, because someone who built the control panel did not put in a reverse diode anywhere in the 24VDC circuit. The spike found the lowest path of resistance in the 24VDC circuit - the sensors. Also a good idea to have it on every coil. That surge is going to manifest itself greatest on the lowest resistance. The diode hopefully will handle the spike, but if not, it is a cheaper unit to replace than an entire instrument.

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#15
In reply to #14

Re: Diode application

07/30/2008 8:36 AM

Phys, remember these diodes are usually fitted to bench power supplies where testing of circuits is the main use...

accidentally connecting a second reverse voltage output to the first output would damage the power supply, but this diode prevents that happening.

Also many power supplies can be shut down very fast, so if they are driving an inductive load and they are switched off the back emf could damage the power supply as well.

Other factors that make this diode indispensable is when the power supply is driving a motor, for instance, and the power is removed the motor still turns and once again back emf can damage the power supply...

Incidentally, together with this output diode there will always be another diode across the series pass transistor, so turning off the power supply with a large capacitive load will not damage the output transistor.

John.

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#17
In reply to #15

Re: Diode application

07/30/2008 9:38 AM

Ohhh - lab power supplies for bread borders. I was confused by the real world use of power supplies. You're talking milliwatts of power where I am talking about thousands of watts.

I know - without the lab guys we wouldn't have the industrial devices we use.

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#20
In reply to #17

Re: Diode application

07/30/2008 10:31 AM

The opening post did refer to a 3 amp diode Phys - hardly thousands of watts with a PIV of 100 volts.

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#18

Re: Diode application

07/30/2008 9:55 AM

Both reasons proposed could be that for the reverse biased diode, i.e. somebody connecting the unit wrongly or to damp inductive spikes.

More interestingly is the "crowbar" protection. This I was taught was due the curve resulting from the plot V x I which looks like a crowbar. This assumes a shutdown of supply which will revert to full supply when the overload is removed.

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#21

Re: Diode application

07/30/2008 1:20 PM

To make sure that the fuse will open before the diode, the fuse I2T must be much lower than that of the diode. Both values should be available on their respective data sheets. In general, a 3A diode will be able to take about a hundred amps for a few uS. This is what the melting I2T is trying to measure. It is the energy required to melt the fuse. It has to be lower than the I2T handling of the diode.

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