TVS diode placement and rating

Hi All,

i’ve used this TVS diode at the output of the voltage regulator.

So all works well but the TVS diode get a little hot. Hot enough burn the finger when place for over 10 seconds.

Is that normal ? or is it under rated ?

TVS.png

Are you getting 5V? If you're getting less than 5V the diode may be defective or backwards, and if you're getting more than 5V the regulator isn't working.

The diode probably isn't necessary anyway. I've made lots of LM7805 regulator circuits and I've never used a diode on the output. (Some kind of protection on the input might appropriate if there is the possibility of over-voltage.)

Your link only takes me digikey's home page so I've no idea what component you have.

Why fit a TVS on the output of the 7805? If you're worried about transients on the input supply it should be on that . And should be rated rather above the normal input voltage, but lower than the maximum input the 7805 can tolerate. Power rating according to the expected wprst-case transient...

A series fuse would give you further safety.

Note :a TVS is there to do what it says - catch transients. Don't use it as a regulator like a zener.

regards

Allan

The part number is 1N5908GOS-ND.

The reason was for transient voltage protection and the supply is for a stand alone arduino project. I’ve also got a PPTC (part#RUEF090HF-ND). Just being on the safe side hence these components.

@DVDdoug
I get 6 volts across the TVS diode and 5v at the out of the regualtor when tested out of the board.

Show us your wiring.

.

There's something wrong... how can your TVS have a higher voltage across it than the regulator feeding it??

But see my previous - I think it's the wrong part in the wrong place....

regards

Allan

Your link took me to a page with 5.176.067 results.

What voltage is your TVS?

The PK4EXX shown on your schematic looks like a generic part number, where the XX is replaced with a number to indicate the voltage.

The voltage of a TVS is normally chosen so that it doesn't conduct during normal operation, but only conducts (briefly) when a transient occurs.

You would normally place a TVS at the input to your circuit, not after the voltage regulator.

Your schematic is a bit hard to read, there is a convention that the more positive voltages are shown at the top of the diagram, and that inputs are shown on the left, and outputs on the right.

The TVS after the out from the regulator was placed if there would be any transients such it would protect the underlying components, mainly the atmel IC. Though it would be ideal to have it there.

I repeat my post#5…

plus

If you put your TVS on the regulator input it would protect that too…

regards

Allan.

makes sense.

so would the rating of the TVS 1N5908GOS-ND be ideal ?

Measure the output of your bridge rectifier under normal conditions. Then fit a TVS with a voltage 10% higher than that across the input of the 7805.

Note you're using a 3.2VA transformer - voltage not given. Why not parallel the two secondary windings to give it an easier life..?

And 3.2va at say 8 volts is only about 400mA - so you won't need a huge TVS.

And it would have to be a ginormous transient to blow that 1.5A fuse - I'd fit a 500mA

egards

Allan

The transformer

Then fit a TVS with a voltage 10% higher than that across the input of the 7805

wouldn't it be after the cap ?

So the voltage at the rectifier is 9.89 and if am not wrong, the ideal TVS Diode would be

Unidirectional Stand-Off voltage: 9v Break dow voltage: 10v Clamping voltage: 15.4v Power - Peak Pulse: 5kW

The part# from digikey is 5KP9.0A-TPCT-ND, still the wattage is quite high.

and if am not wrong,

Sadly you are wrong. You should not have started a new thread about this circuit.

well it was two different questions i had. Could you please suggest me the ideal rating if i need to place it after the cap and before the regulator ?

Guess this would be nearest,

Unidirectional Stand-Off voltage: 10 Break dow voltage: 11.1v Clamping voltage: 17v Power - Peak Pulse: 500W

Should be OK

regards

Allan.

anishkgt: well it was two different questions i had. Could you please suggest me the ideal rating if i need to place it after the cap and before the regulator ?

That makes no sense. It is a parallel circuit so before the cap and after the cap are exactly the same thing.

Grumpy_Mike: That makes no sense. It is a parallel circuit so before the cap and after the cap are exactly the same thing.

TVSes are meant to absorb huge, brief surges of power. That's the kind of thing where I believe placement does matter, and it should probably be as close to the output of the bridge rectifier as possible to minimize inductance in the traces and to keep the physical size of the surge current loop as small as possible.

Guess this would be nearest,

Unidirectional Stand-Off voltage: 10 Break dow voltage: 11.1v Clamping voltage: 17v Power - Peak Pulse: 500W

You have not yet given the output voltage of the transformer, so we can't say if it'll work or not. If it's a 12V transformer, you're going to destroy that diode.

Jiggy-Ninja - see posts 11 and 13

regards

Allan