Declan, This is my problem. My client already uses regulated 5V wall-warts for his USB hub rated at 2.1A. He wants to use the wall-wart for the device I just designed. Actually I just got PCBs back today, no protection. Anyway, it uses all 5V and is not tolerant of anything less than 5V. I can't get away away with 4.5V or 4.6V using a LDO regulator rated at about 1A or 1.5A (most LDOs of this size have 400mV-600mV dropouts at rated load). Scare tactics don't work here, I will just lose my work if I start arguing. I came too close a year ago arguing a digital fact I knew a lot about. I need the work, not much else out there right now.
At 09:45 AM 2/13/03 -0800, Declan. Moriarty wrote:
I had backed out of this one, but now feel like getting back in. A triac is
an A.C. device, with two scrs back to back. Why is it suggested as a
crowbar? Probably in case you had an A.C wall wart. Personally, I think
it's not the way to go at all. The idea with a triac is that you control
A.C. I would use an SCR if you have to have a crowbar. I would never try
to fire one with simply a zener. Gates get sensitive in the heat ...
You are going to fart around and lose a week with the triac unless you
find an application and copy it.
Have you considered a low drop regulator? I recently used a LP 2981
which is guaranteed max 0.1V drop at full load. Put a schottky in
series, use a 6V supply and you're smiling. You may just need a little
more bottle with whoever is telling you to use a 5V regulated external
supply, which leaves your circuit exposed. Tell him it cannot be done
safely. He must hold his face up if/when the solids hit the ventilation
as his part of the design goes down the tubes. Write a memo warning them
- that is a great scare tactic, and your backside is covered.
With best Regards,
Declan Moriarty.
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