ECN Forum Forums General Topics of the Trade General Discussion Area computer circuits??

If it's high-frequency enough, it's RF and that can do all sort of strange things. What was the frequency of the transient? I wonder if a good choke on the power supply would have stopped it.

I should point out that the better computer power supplies usually include such chokes and filtering since they're usually required for the computer to pass FCC Class A or B testing.

The cheap power supplies, which also aren't UL or CSA listed, do not. Of course what do you expect when a 300 watt power supply costs $15--that should be a clue as to what you're getting, which is a piece of crap.

The RF filter is on the line inputs but the EGC is direct connected to the frame of the supply, which is connected to the baseplate and the system board grounds to the baseplate so any transients on the EGC will get reflected on all the signals. As long as all the EGCs are going up and down together you will be OK but if you span 2 services these transients get reconciled across the signal cables. There are a number of things you can do to minimize this, like the ferrites you see on parallel printer cables but when you get too far away or on different grounding systems the only answer is bonding around the signal cable. Start with bonding all the services and if that doesn't fix it bond the machines.
To avoid NEC questions we call that a "drain" wire and make it anything but green. (Black THHN was the usual choice)
The "drain" wire we had at Holiday Inn had some very ugly stuff on it on a sunny day. In a thunder storm it was really seeing a lot of nasty stuff. This was observed with a current probe connected to a scope. Try that sometime to see what is on your grounds.

I think I've seen power supply filters that pass the grounding conductor through a choke.

That would make the problem worse. The ground should shunt out the transient, not provide impedance that would allow it to propagate. Look again, I bet that line filter only uses the EGC for a reference

NJI ---

My practice is as follows:

Office computers draw 100-200 watts. Supply power with a 1500va UPS.

LCD monitors draw 20-30 watts.

There is no need for any other consideration.

Much computer equipment these days no longer even has a ground connection, which may solve the problem at least as far as that equipment is concerned.


[This message has been edited by brianl703 (edited 10-27-2006).]

Incidentally, it would seem to me that connecting all of the computer chassis together would cause a ground loop. But apparently it fixed the problem anyway?

It certainly creates a ground loop but that doesn't really seem to be as bad as coupling inter ground transients through the signal lines.

I wonder if ScTP (screened twisted pair) would've helped too.

As I recall the shield/screen on ScTP is connected to ground at both the NIC and the hub/switch end (if indeed the hub/switch is grounded--many are not).