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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
For a while now, I have been developing a 'Dead simple DRSSTC' and recently boxed the thing, and tidied it up. Normally, I have it computer controlled, but decided to build the ubiquitous 555 interrupter for it, for when I don't use the laptop with it.
I built a circuit using 3 555's, one for BPS, one for PW, and one for burst mode. The output is fed to a transistor switched LED, since I like to send my signals over fiber optic.
The circuit works fine, except, that when I happened to use a half dead 9v battery to run it, it ran for a little while, and as the battery became flat, the circuit stopped oscillating, and the output pin went high, and remained high!
If I had run this attached to a coil, and the battery had gone flat, I imagine the consequence would have been a smouldering mess!
The solution was to replace all of the 555's with their CMOS counterparts. In this scenario, when the voltage dips low enough, the LED goes out first, protecting my DRSSTC. Secondly when the voltage dips low enough for the 555's to stop functioning, the output doesn't go high, it goes low!
I wonder of anyone else has come across this, or destroyed a bridge because of it?
I had heard of this issue (555's putting out a HI signal when Vin is below nominal) back when I first started learning electronics, but I've never encountered it in person. Though, most of the 555's I use are CMOS based. A good reminder anyway!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My DRSSTCs all have a wired connection between the interrupter and the driver, using 9-pin serial extension cables.
The interrupter is powered off the driver's 15V supply, which is monitored by an undervoltage lockout circuit that inhibits gate drive if the voltage is too low.
Later I added an optical input, but I AC coupled it. A constant high level would only make a burst that lasted 10ms or so.
In industrial power electronics these things are really important. You don't want all your expensive IGBTs exploding because of a brief power dip.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Steve Conner wrote ...
In industrial power electronics these things are really important. You don't want all your expensive IGBTs exploding because of a brief power dip.
I can imagine sitting down, working out what can go wrong, and mitigating it, is a significant portion of the design process in industry. Especially in critical applications such as life support!
I always ask the vendors what are the default state of their unprogrammed parts (i.e. pins floating, high, low, pulled up, down or unknown) as we tend to program them in system if it is not documented in the datasheet or user manual. Knowing the conditions of how the pins react to active signals without power (aka "hot-socketing") is important if you design cards that can be hotswap.
One time there was a sales guy from Analog Device when they said they carried those RS232 parts that are the same as Maxim. He wisely STFU when I told him the part number of an Analog Device 232 part that did a latch-up when externally connected to an active signal un-powered where as a Maxim part doesn't. So do not expect the generic jelly bean part all have the same behavior from all vendors.
Checking for all of these conditions are just part of a good design.
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A curious bug when using battery powered 555 interrupters
