Transient Suppressors

[guavatone]

I was thinking about using these 12V transient suppressors for DI's and and Mic input upstream from the phantom supply 6.8K resoistors. These are pretty much like the back-to-back zeners for transients right? I was also wondering if they would affect phantom supply to the mics.


here's the datasheet of the ones I was thinking about:
http://www.vishay.com/docs/88378/sa.pdf

 

[jdbakker]

[quote author="guavatone"]I was thinking about using these 12V transient suppressors for DI's and and Mic input upstream from the phantom supply 6.8K resoistors. These are pretty much like the back-to-back zeners for transients right? I was also wondering if they would affect phantom supply to the mics.[/url][/quote]
Those transient suppressors tend to have a higher capacitance (and more importantly a higher absolute capacitance variation wrt applied voltage) than back-to-back Zeners, which is Not Good from a distortion POV.

It's not quite clear to me what you mean by 'upstream from the phantom supply 6.8K resistors'. What kind of transients are you trying to suppress ?

JDB.

 

[Samuel Groner]

As JDB said--voltage-dependent capacity hurts here. It may work half-way OK for a mic input but no way for things with higher source impedances (DI etc.). They may be helpful in cooperation with standard clamping diodes (e.g. 1N914B or 1N4007GP) in cases where you want to limit at some volts below the power supply, e.g. for fast recovery.

Samuel

 

[guavatone]

Thank you. i gues I'll stick to the Zeners then. JDB, I meant between the 6.8K resistors and the preamp input.

 

[Svart]

look for RF TVS devices, extremely low capacitance..

 

[Ptownkid]

I find that the best way to suppress transients is to make sure you lock the garbage bins at night and close the doors on all box cars.

 

[mikep]

[quote author="Svart"]look for RF TVS devices, extremely low capacitance..[/quote]

what he said. there are TVS devices available that are better suited for this. hint: look for the smallest packages.

 

[guavatone]

[quote author="Ptownkid"]I find that the best way to suppress transients is to make sure you lock the garbage bins at night and close the doors on all box cars.[/quote]

I can totally picture your Avatar saying that Bryan.

 

[jdbakker]

[quote author="mikep"]hint: look for the smallest packages.[/quote]
That doesn't help you much if you need to deal with relatively high-energy events.

Those teeny RF TVSs may be fine to protect that GaAs LNA from HBM static discharges (~100pF @ several kV), but for audio work you'll need to deal with accidental phantom blocking cap shorts (~47uF @ 48V). Rise time may be much lower than for ESD, but the total pulse energy is more than an order of magnitude higher. People have seen 1N4148s fail through Phantom fault conditions, and those are quite a bit more resilient than most miniature RF TVSs I have seen (and designed in).

Transient voltage suppressors are always a trade-off between the damage they cause by being in-circuit and the damage you could suffer by not having them. Kind of like a protection racket, really.

JDB.
[and then there are the TVSs you need to protect in-car equipment from all the nasties that show up on the "12V" net...but I suspect that others here are more qualified to share war stories about those]

 

[CJ]

What are you tryong to protect?
How could it be damaged?
where is the transient gonna originate?
A snare hit from hercules?
I have never seen over voltage protection on a mic input, but maybe not a bad idea if you are going into a chip with a sensitive input.

Most devices are too slow save an oxide barrier, so you hope and pray.

 

[guavatone]

the thought started with a DI like fabio's 312 where plugging in a 1/4" TS plug triggers a relay to switch from mic to DI. Without Back to back zeners (also used on Bo's DI) the pop was HUGE. I think I then saw a preamp Diagram somewhere, maybe SG's or a Walt Jung design, where someone had a similar set up. Probably to limit the Phantom Pop from going forward to the preamp.

I just thought one component would be a nice way to save on PCB real estate.

 

[CJ]

ahh, i see.
those DI's, sometimes you get big ground loops, if the git is going into another system at the same time, so over voltage protection is a good idea.
need something that will clamp 120 ac in a hurry, low current.
must be a package deal out there in the thomas register, but who reads those?\
we use them to stack up the bench, since the web csame along.
but i do like hard copy.

 

[dale116dot7]

I have seen this on automotive communication lines. I use a TVS with a bias resistor up to +12 volts, and a normal diode from the input to the TVS. That isolates the capacitance of the TVS and the capacitance of the diode is quite a bit less than the TVS.