AVR-Chat

Thanks, Jim.  I had chosen Transguards with a working voltage of 3.3 V
because I use 3.3 V signals (most DC, but a few up to 10-50 KHz).  It sounds
like you saying you would be a bit leery of doing that, and it would it be
better to go with ones with some headroom.  The next available working
voltage seems to be 5.6 V.  Would that be a better choice?

I looked at the AVX staticguard series, which is labeled as "for CMOS", but
in 0603 or larger package sizes the working voltage is "<= 18 V".  Same with
all the USB series.  Is it ok to use these for lower signal levels like 3.3
V?  I assume it must be because USB is 5 V.

Steve

| -----Original Message-----
| From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
| Behalf Of Jim Wagner
| Sent: Wednesday, January 05, 2011 8:14 PM
| To: AVR-Chat@yahoogroups.com
| Subject: Re: [AVR-Chat] AVX Transguard
| 
| 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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