TVS placement question

Kerrigan,

Each TIU channel already has a TVS built-in.

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He also asked this...And how does one tell when the TVS has shunted it's last spike and is no longer functioning? I am curious as well.

you can tell because they short out and can be checked with  A CONTINUTY METER IN OHMS! they will show a short when ohm meter I across the tvs diode leads!

Alan

What is the reading when open?

What is the reading  and how would you test if the unit is good?

If they fail open, you won't know.  I wonder if tvs' s in my 15 year old tiu's are still functional.

A TVS can fail in a variety of ways, here's a paragraph from a Vishay Application Note.  If they fail sorted, it's easy to detect.  However, if they degrade or fail open, there's no in-circuit way of detecting it.

If you're concerned about the old TIU, replace the TVS diodes, they're big and easy to replace.

Transient Voltage Suppressors (TVS) will fail if they are
subjected to conditions beyond their designed limits. It is,
therefore, important to understand the types of failure
modes of TVS devices before designing them into a circuit
application. There are three basic types of failure modes:
shorts, open and degraded (outside of the specification
limits). Although the silicon avalanche junction Transient
Voltage Suppressor (SAJTVS) will first fail short in most
applications, there is always one transient event that will
cause it to open initially. In this case, the transient energy is
large and of short duration that the silicon chip itself
explodes.

To test our TVS devices, you would need a variable DC supply of about 50 volts, and the device would need to be isolated from the circuit it is protecting.

I have seen a couple of failures of the smaller versions, but not the 1.5 (KW) versions.

Dale, I've replaced a couple of the 1500W versions in the TIU's that were shorted, they do fail.  Of course, I have no idea if any of the others were open or degraded.

Putting them on the TIU as "backup" I guess is what people are recommending at the club.  So, going that route, do I need them just across the input (transformer) side of each of the four circuits ( 2 fixed, 2 var) or do I need to put them on BOTH the input and output sides of the TIU, as I saw at a friend's layout Sunday ... ?

Didn't know TIUs already had TVS in place .... so people are adding the external TVS devices as added insurance?

If they fail open you would not know.  I have seen several failed short in TIU, Transformers and those added to Trains, all 1.5kw Versions.

As far as testing it, wouldn't that be a partially destructive test as all documentation I have read states TVS degrade as they absorb spikes.  Hence an effect on longevity.  If you were ultra conservative you would replace the external versions you add periodically.  G

Are we saying that spike events are cumulative, and that after a certain number the TVS will fail? That does not seem likely to me somehow. I would guess the TVS will continue to operate as intended indefinitely, as long as its limits are not exceeded. If exceeded it would fail; likely shorted. 

Rod

I don't believe that's the case with a TVS.  However, MOV devices actually do degrade over time as they absorb transients, that's a totally different technology.  A TVS that never sees an overload should last for a very long time, just like any other semi-conductor that isn't overloaded.  Obviously, there are a few random failures that will still occur.

You would test a TVS just like you test a Zener diode - variable voltage source through a resistor to the device.  Ramp up the voltage until there is a voltage drop across the series resistor.  The specs give a test current and the voltage range for that current.  Our AC units would require this test twice, once with each polarity.  I would keep currents down below 50 mA, which would be less than 2 watts.  (If you have a good laboratory-grade adjustable power supply with adjustable current limiting, you can eliminate the resistor and just watch the output CURRENT as the voltage is ramped up.

A TVS is like any other semiconductor device.  If the spikes do not exceed its rating, the device will not be degraded, but large spikes can cause excessive heat, and that melts the semiconductor materials,   Our devices are rated at 1.5 Kilowatts, but only for a brief instant.  Steady state they are only good for a couple of watts.

DO NOT use the above test while the device is still in the protected circuit!  This would be applying excessive voltage to the device that is supposed to be protected.

All interesting information.

My question was do we need the TVS diodes on BOTH the input AND output sides of the TIU, or JUST the input (transformer) side.

Thanks!

The TIU has 1500W TVS diodes on the output side from the factory.

Instead of replacing the TVSs in a tiu (unless shorted), and you fear they may be broken open-circuit, why not just add some on the outside?

The "bottom line" for TVS installation is,  I gather from the discussions:

Put TVS on the input side of the TIU.

The TIU has TVSs on the output side of the TIU, so unless they are blown, no need to put TVS on the output side of the TIU.

I se no reason to put them on input side.  A reason to add them on output side is that the internal TVS may have failed open.

From my files on TVS use.  Sorry, I did not write down who wrote this.

You need one of these for every input and output of any of the transformers, TPC's and any other device that conducts electric to your layout. Fuses alone will not protect any of the devices from a power surge. A number of people have had the electronics in the TPC300's and 400's burn up because of power surges that is why it is recommended to install a TVS on both inputs and outputs.