AVR-Chat

Steve -

Even though you may consider it AC, what really matters is the extreme  
peaks in normal operation, relative to the "ground" that the transient  
absorber is connected to). Transguards, and their relatives work fine  
with digital signals.

If your signal is between 0V and some V+, you need to choose one with  
the "working voltage" no smaller than V+. That is the largest voltage  
that the device is guaranteed NOT to conduct. As a quick example,  
suppose that you have a signal that swings between 0 and 5V. You would  
probably need to choose one about 5.2V and a breakdown about 7.5V (the  
two limit voltages won't be any closer than about this at 5V).

This demonstrates the "problems" with these devices. First, they are  
not very sharp breakdown and there is a lot of variability in the  
actual breakdown. One that is spec'd at 5.2V and 7.5V is simply  
guaranteed NOT to break down below 5.2V but to break down (at some  
specified forward current) at 7.5V. You can't tell where, between  
these limits, any individual device will do it. This means that (in  
this example) a 5V receiver might have to withstand a short-term  
transient of up to 7.5V during an extreme event. Actually, its not  
quite that bad because you will never have the spec'd 5A (or what ever  
the spec happens to be) in a real circuit.

I tend to use a transient absorber with a PTC "fuse".

If this is a high speed signal, you also need to be very careful about  
device capacitance. Its quite large for these things (often 100s of  
pf). There are, for example, low capacitance ones made especially for  
USB.

This brings us to bipolar vs unipolar. If its a logic signal, you want  
to use a unipolar one. If it is genuinely AC (swinging above and below  
the "ground" that the transient absorber is connected to), then you  
want a bipolar one.

Hope this helps

Jim Wagner
Oregon Research Electronics

On Jan 5, 2011, at 9:14 AM, Steve Hodge wrote:

> A quick question on the AVX Transguard transient voltage  
> suppressors. The
> specs give a DC and AC working voltage. The AC value looks like 0.7  
> x DC
> value, so I assume it is an RMS value.
>
> If so, is it then ok to put a 3.3 V Transguard (DC working V = 3.3  
> V, AC
> working V = 2.3 V) on, say, a 3.3 V level serial stream, even though  
> the
> stream could be labeled "AC"?
>
> Thanks, Steve
>
>
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