EMRFD Message Archive 791

Message Date From Subject
791 2007-05-24 07:01:43 patt896 Reverse Polarity Protection -- VK3EM Solution -- "Transorb"
The simple protection scheme I was talking about is one physical part.
Inside that part, which looks like a diode, is a zener
(for over voltage) in parallel with a reverse power diode. Providing
you have a fuse in the circuit before the Transorb, then if an
over-voltage or reverse voltage event occurs, the Transzorb goes dead
short and the fuse blows.

We use SMCJ15A (Unidirectional). You can get these in axial packages
too, so you can solder them into the back of plugs
(providing the cable has an in line fuse).

Cheers
Luke
VK3EM
796 2007-05-25 08:22:16 larry allen Re: Reverse Polarity Protection -- VK3EM Solution -- "Transorb"
What zener voltage do you use?
Larry ve3fxq

----- Original Message -----
800 2007-05-25 08:23:26 Shawn Upton Re: Reverse Polarity Protection -- VK3EM Solution -- "Transorb"
Recently did some work with TVS's (transorbs, or
transient voltage suppressors). They seem to be rated
at a "withstand" voltage--that is, if it is a 5V TVS,
then if you place 5V across it, it will leak say 1uA
(1mA, look at the datasheet, and it will depend upon
size and temp). But at say 10% above that voltage, it
will turn on hard.

For protection circuits, that may not be an issue--but
the rating isn't the same as if it was a zener!

One thing to watch out for: they have a high
capacitance. May not matter on power supply lines,
but on data lines the extra capacitance may cause
issues.

I'm not sure what a unidirectional one does in
reverse--if it acts as a diode or is open.

Shawn KB1CKT


Shawn Upton, KB1CKT


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803 2007-05-25 19:15:33 vk3em Re: Reverse Polarity Protection -- VK3EM Solution -- "Transorb"
Thanks Larry! :)

Firstly, the Transorb is far from an ideal protection circuit.

But, if you want one device that can easily retrofitted into an
existing DC supply, this is probably a good compromise (in conjunction
with a fuse of course).

Yes, they have lots of capacitance, so they are no good data protection.

The unidirectional ones, when reverse polarity is applied, act like a
diode. So a unidirectional transorb provides some form of over-voltage
and reverse voltage protection.

A 15v Transorb will typically breakdown at 18v, so you need to be
confident your equipment will cope with that.

When going short circuit, they do have a habit of staying short
circuit. That means, they will fail short circuited. If you don't use
an external fuse, the transorb will stay short circuited until it goes
up in smoke and falls apart.

You can get them in axial form too (as opposed to surface mount),
which means you can retro-fit them into the back of connectors, etc.
Just make sure you use the correct rated external fuse AND the power
source has the capacity to blow the fuse. ie: 2A switcher with 2A fuse
may not have the capacity to blow the fuse (due to current limiting in
the switcher). If you turn your back, the Transorb could also go up in
the smoke but your fuse may never blow.

Food for thought.

Cheers
Luke
VK3EM

>
> Recently did some work with TVS's (transorbs, or
> transient voltage suppressors). They seem to be rated
> at a "withstand" voltage--that is, if it is a 5V TVS,
> then if you place 5V across it, it will leak say 1uA
> (1mA, look at the datasheet, and it will depend upon
> size and temp). But at say 10% above that voltage, it
> will turn on hard.
>
> For protection circuits, that may not be an issue--but
> the rating isn't the same as if it was a zener!
>
> One thing to watch out for: they have a high
> capacitance. May not matter on power supply lines,
> but on data lines the extra capacitance may cause
> issues.
>
> I'm not sure what a unidirectional one does in
> reverse--if it acts as a diode or is open.
>
> Shawn KB1CKT
>
>
> Shawn Upton, KB1CKT
>
>
>
____________________________________________________________________________________
> Park yourself in front of a world of choices in alternative
vehicles. Visit the Yahoo! Auto Green Center.
> http://autos.yahoo.com/green_center/
>