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Even More Magic
14 years ago
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Many years ago, in the MIT tech department, there lived a PDP-10 computer.
Glued to this computer's case was a switch with two settings. One said MAGIC. The other said MORE MAGIC. Flipping this switch would cause the computer to instacrash. Examination of the switch revealed that it had but a single wire leading from it, which was attached to a ground on the motherboard. The switch itself was a standard, off the rack switch. Nothng special, and connected in a way which served no electrical purpose at all. Logically, it seemed, this switch should have had no effect on the computer.
And yet it would remain switched to MORE MAGIC, because invariably, flipping it the other way caused the machine to crash.
This story has been handed down from tech geek to tech geek over the years as an illustratration of the fact that sometimes, things happen for reasons you simply cannot comprehend, and this goes double when you're dealing with technology.
I find myself in a such situation at the moment. I have an APC battery backup unit, which has worked reasonably well for several years now. Recently it started having a problem. The battery indicator would flash and the unit would beep. I would later find out that these were the hallmarks of a worn-out battery. Battery life is 3-5 years, and I was due for a replacement.
However, I discovered something, and it still doesn't make any sense to me.
I have an electric space heater on the same circuit as the battery backup. And I discovered that when the heater was on, the battery backup suddenly started working normally again. It would "find" the battery, report a full charge, and go about it's business. It would also slowly use up the battery.
If I turned the heater off, suddenly we're right back to beeping and flashing again. No battery found.
Now, it could be that switching the heater on caused a drop in power flowing into the unit, making it think that the power was browning out, and the battery needed to come online. That seemed reasonable. What wasn't, was that if I unplugged the unit, it would simply shut down and report the battery as gone missing, again. Beep beep. Flash flash.
Somehow, the load from the space heater kept the unit operating normally. As long as that extra electrical drag was there, everything was hunky dory.
I plan on getting a new battery tomorrow. I guess I'll see if that fixes things. As for why the heater was making my aging battery work again...
...well, that may just be dipping into the realms of magic.
Glued to this computer's case was a switch with two settings. One said MAGIC. The other said MORE MAGIC. Flipping this switch would cause the computer to instacrash. Examination of the switch revealed that it had but a single wire leading from it, which was attached to a ground on the motherboard. The switch itself was a standard, off the rack switch. Nothng special, and connected in a way which served no electrical purpose at all. Logically, it seemed, this switch should have had no effect on the computer.
And yet it would remain switched to MORE MAGIC, because invariably, flipping it the other way caused the machine to crash.
This story has been handed down from tech geek to tech geek over the years as an illustratration of the fact that sometimes, things happen for reasons you simply cannot comprehend, and this goes double when you're dealing with technology.
I find myself in a such situation at the moment. I have an APC battery backup unit, which has worked reasonably well for several years now. Recently it started having a problem. The battery indicator would flash and the unit would beep. I would later find out that these were the hallmarks of a worn-out battery. Battery life is 3-5 years, and I was due for a replacement.
However, I discovered something, and it still doesn't make any sense to me.
I have an electric space heater on the same circuit as the battery backup. And I discovered that when the heater was on, the battery backup suddenly started working normally again. It would "find" the battery, report a full charge, and go about it's business. It would also slowly use up the battery.
If I turned the heater off, suddenly we're right back to beeping and flashing again. No battery found.
Now, it could be that switching the heater on caused a drop in power flowing into the unit, making it think that the power was browning out, and the battery needed to come online. That seemed reasonable. What wasn't, was that if I unplugged the unit, it would simply shut down and report the battery as gone missing, again. Beep beep. Flash flash.
Somehow, the load from the space heater kept the unit operating normally. As long as that extra electrical drag was there, everything was hunky dory.
I plan on getting a new battery tomorrow. I guess I'll see if that fixes things. As for why the heater was making my aging battery work again...
...well, that may just be dipping into the realms of magic.
Under normal circumstances, the voltage and current in an ac electric circuit should peak at the same time and pass through zero at the same time. This is called unity power factor. However, due to capacitive and inductive loading, one can shift out of phase with the other such that voltage peaks first, or current reaches its zenith before the voltage. This can play havoc with devices that sense the power line, like a UPS.
Electric space heaters tend to be either resistive (no power factor change) to inductive (a lagging power factor) depending on the design of the heating elements. Switching computer power supplies tend to be capacitive (a leading power factor.) Most modern computer supplies now have a PFC (Power Factor Correction) circuit to compensate for that capacitive load. If that has malfunctioned, you wouldn't notice any change in your computer operation, but the UPS connected to the computer might. Adding the heater into the circuit would cancel the capacitive power supply effect, bringing the power factor back towards normal.
Now a fresh battery might have a low-enough internal resistance that it will mask the faulty power factor. Only way to determine that is to put a new battery in place and see. If it does, you're good. If it doesn't, and it still reacts the same way, try a different computer (or load) on the UPS and see if that changes things, A 100W lightbulb works well here.
Soooo yeah, not unusual.
Anyway, the magic switch was a ground plane voltage level issue. It's actually a fairly common thing designers need to take into account.