TVs diode

[macd@polo]

Hi all,

in my RF-ID i notice that TVs diode 1.5ke24ca burned on side of terminal, and circuit has no power. what can i used i don't have that specific diode . will 1,5ke18ca diode not work? or what type rating diode should i used to replace 1.5ke24ca diode , zener or any please help?

[Vovk_Z]

If you want to device just work then just unsolder the broken TVS diode, and it 'll work (unprotected by that diode).
1.5ke24ca is two-sided (symmetrical) 24 VDC and 1500 W (peak) rated diode.  This isn't something rare.
I wouldn't go to lower voltage. It can be more safe to use higher voltage rated TVS.

[Psi]

Assuming the diode is just protection across a supply rail then yeah, you can remove it and the equipment will still work, just in a unprotected state. 
I wouldn't do anything with the equipment in this state other than a quick check that it works to prove the TVS diode was the only thing wrong.  Then power off until you can source a new 24V 1500W TVS diode.

It can be a higher wattage if you want, but i would stick with 24V, and it needs to have CA on the end (symmetrical).  Maybe you could get away with 22V or 26V but why bother, a 24V TVS should be easy to find/buy

[macd@polo]

thanks for advice,  will shop around for 24v diode.. and i am using it in RFID reader

[macd@polo]

black in-circle is the diode , so u mean if i removed it from circuit will still work ,,,

[CaptDon]

A 24 volt TVS diode is a poor choice across a 24VDC supply! I would go with perhaps a 26 volt TVS. No wonder the old diode eventually failed.

[TimFox]

Note that the TVS diode is a Zener-like device that has been characterized for transient suppression (including how much energy it can absorb safely) instead of voltage-regulation service.
Often, the symmetrical version looks like two Zeners in series to work with AC voltage.
If the nominal voltage on the supply rail is 24 V, then look carefully at the TVS data sheet for specs and tolerances on working voltage and breakdown voltage.
A manufacturer's viewpoint:  https://blog.semtech.com/tvs-its-just-a-diode-right-part-one
(Their equivalent circuit shows a slightly complex circuit inside a full-wave bridge network of four diodes to make a symmetrical device.)

[sparkydog]

First, a TVS isn't a component I'd expect to "just fail" due to normal wear. More likely, either the circuit was exposed to a power event that cause a failure, or is misdesigned resulting in a failure. In either case, if you can, I'd check that the rest of the circuit is working as intended. (That said, the hope is that a TVS will fail short and so turn into something like an inverse fuse if it's overloaded, shunting power away from the rest of the circuit. Did your circuit also blow a fuse? Does it have a fuse?)

I wouldn't go to lower voltage.

Likewise, but in particular, do not go lower voltage than the nominal circuit voltage. If we're talking about a 12V circuit, an 18V rated TVS is probably okay, but if it's there to "eat" the voltage spike from an inductor, changing the voltage is going to change the magnetic field decay time, which may or may not be a problem.

Note that some TVSs are there for "paranoia's" sake, i.e. to protect the circuit in case of electrostatic discharge or idiots plugging in the wrong power supply. These events may never happen, and so the circuit might work just fine with the TVS removed. Others are there to "eat" spikes from inductors that occur in normal operation; removing them can result in other components seeing extremely high voltage spikes (thousands of volts) that would not otherwise occur.

It can be more safe to use higher voltage rated TVS.

Uh... no? Using a higher voltage means exposing other components to higher voltage. That's sort of the opposite of "more safe". I wouldn't recommend that unless you're confident you understand the circuit, know what components the TVS is protecting, and know that the TVS you're considering will still clamp any voltage spikes comfortably within the overvoltage tolerances of other components.

A 24 volt TVS diode is a poor choice across a 24VDC supply! I would go with perhaps a 26 volt TVS. No wonder the old diode eventually failed.

A TVS rated "24V" generally doesn't start to conduct significantly until ~25-26V. It isn't necessarily "bad", especially if the components it is protecting are sensitive to even small overvoltages.

[TimFox]

A typical Vishay bi-directional TVS (SMA5J24A), datasheet values:
stand-off voltage:  24 V
max current at that stand-off voltage: 1 uA
breakdown voltage:  26.7 to 29.5 V

The actual meaning of those values is that the current is measured at 24 V, and must be below the maximum.
The breakdown voltage spec is a range where all parts of that part number must have a value in that range.

[macd@polo]

The power is shut off from whole circuit.

[Psi]

TVS diodes voltages given are their working voltage, (usually).
Working voltage meaning the highest safe voltage you can have across the TVS where it absolutely will never conduct.

So normally a 24V TVS is ok across a regulated 24V rail.  But it usually makes people feel better to add some safety margin. Although its arguable if you need it

A 24V TVS will almost definitely do jack shit until around ~28V and wont clamp hard until around 34V.
Perhaps if you work at extreme temps you might see it start to conduct closer to it's rated working voltage

[sparkydog]

Normally a 24V TVS is ok across a regulated 24V rail.  But it usually makes people feel better to add some safety margin.

Depends what it's doing. For example, I have circuit designs with 18V TVSs across 12V relay coils where the upstream stuff can handle 48V. In such cases, higher breakdown voltage is better for the coil, so I'm picking values based on how much overvoltage I can tolerate (after applying a generous safety margin).

That said, I've been guilty of eyeing TVSs a step or two higher than operational voltage just for paranoia's sake. Maybe I'll stop doing that. 🙂

[TimFox]

Again, note that "24 V holdoff" means that the manufacturer guarantees < 2 uA (in my example above) at the quoted voltage.
The other spec "breakdown voltage" means that the manufacturer guarantees a range of voltage at a specified current, just as Zener diodes are specified.

[CaptDon]

Could be plausable that the 24 volt TVS had developed some leakage beyond the specs do to age or current spike and began to heat which would lead to an uncontrolled race condition, more heat = more leakage and it finally just shorted out. If the original lasted for many years then replace it in kind.