MOTM

I've often wondered if we were taxing the short circuit power-p current of
these power supplies with the all the caps we put on these modules.  Every
module has at least a 10 uF to 22uF cap on each power leg.  That could
easily reach ~ 500uF on top of the caps in the supply.  A cap does indeed
look like a short circuit when power is first applied.  Some modules have
lots more.  And, some modules, like the Blacet Time Machine and others draw
a lot more normal current.  38 of those would way overload a supply like.
But, who would have 38 Time Machines. :)  Perhaps someone trying to impress
list-lurker Harry with his collection of BDDs. :)
Larry H

----- Original Message -----
From: charlesosthelder <osthelder@...>
To: <motm@yahoogroups.com>
Sent: Saturday, December 27, 2003 8:40 AM
Subject: [motm] power anomoly


Hey all!

Here's one for the troubleshooting crowd - my power supply is doing a
*funny* thing.  Back when module #38 was installed, I switched on the
power and only the red LED's lit and stayed on solid-no modulation
from the LFO's, etc.  Naturally, I switched the supply off!  When I
got the courage to power it up again, all was well.  Or so I
thought...

Yesterday I installed module #44.  A new distribution board was added
and the wiring was rearanged to tidy up.  I powered up and this time
the green LED's lit solid.  I turned it off and then back on.  Now
the red LED's lit solid!  I left it like this to confirm what I
thought was happening.  Checking the voltages at my DC terminals on
the supply, I found a solid -15vdc and only +.71vdc.  When the green
LED's lit solid, the opposite was true- solid positive voltage and
only -.71vdc from the supply.

When the supply powers up normally, all works well.  The voltages at
any module show no sign of brownout.  With no load, the supply always
powers up correctly.  I haven't pulled one supply at a time to find
an offending module, because the odds are against any one module
shorting one-or-the-other half of the supply.  That is next on the
agenda, however.  The supply is a Power One HBB-15 which according to
many, *should* handle 45-50 modules.  A star wiring pattern has been
maintained, and the three distribution boards are MOTM-960's. The DC
wiring is 12-guage stranded from the supply to the first terminal and
12-guage to each distribution board.

What do you think?  My guess is the power supply goes into short
protection during inrush.  The .71 vdc would be a normal forward
voltage drop from the pass transistor, showing up in parallel at the
shut-down side of the supply.  Anyone else, have something like this
happen as they neared the limits of power?  Am I just going to pop
for the next bigger (or greater!) supply?

I summon the vast power of knowledge that is the MOTM Group!

Chub - half-powered!

ps-Paul should really go into cartooning!




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