[Asterisk-Users] Qs about FXO/FXS cards

[Andrew Kohlsmith]

On January 3, 2005 11:52 am, Steven Critchfield wrote:
> While this has stemmed from my semi-educated guess, I would be most
> concerned with current instead of voltage. When I previously abused a PC
> PSU it was driving a high torque 12v electric motor. The really high
> torque motors would peak out at 5 amps during spool up. That would cause
> a PC PSU to pulse between something like .5 amps and 4 amps. If the
> spool up was controlled, I could keep the motor under 4 amps and the PSU
> could drive the motor. I had no interest in the voltage because it was
> the current that ran the motor.

I've done something similar -- I used to drive a small he-ne laser tube with a 
car ignition coil connected up to a transistor on an LPT port, with the PSU 
+12V supplying the coil.  At certain switching frequencies I generated enough 
EMI to actually reboot the PC, but a snubber circuit cleaned that up.  :-)

Current spikes will cause voltage dips/swells on a power supply -- how much 
this dips and swells depends on how good the regulation is.  Watching the 
current can be indirectly measured by watching the voltage.  (Indeed, one of 
the most basic methods to measure current is to use a shunt with a known 
resistance and measure the voltage drop across the shunt)

> Okay, link this to my rambling above and you would see that by thrashing
> the disk, you are actually keeping the spindle spooled up and not
> measuring the spool up draw. My guess is a spooled down machine getting
> a random incoming call that then must generate ring and spool up the
> HD(s) to start writing logs at the same time on a questionable PSU.

Why would the spindle be starting up when the system is already running?  If 
you've got power management turned on, turn it off!  :-)

> As far as I know, with the exception of a couple of video cards that
> resorted to an external PSU, all video cards have to draw from the PCI
> or AGP bus. This means the TDM card is likely to draw more than the
> graphics card.  Heavy HDD thrashing uses less power than the spooling up
> of an idle drive.

However Digium added the drive power supply connector to mitigate this exact 
problem.  As I said, I don't believe that this is the issue at hand unless 
you don't have that connected.  In my particular case the drive(s) are always 
running, so drawing locked rotor current as the drive first spins up is not 
the issue in my case, and I feel I've already proven that it's not a +12/+5V 
line sag/surge with my equipment.

> I'm also starting to wonder about specific phones now as well. While
> this next bit of rambling is not asterisk related, it is phone related
> and something to consider. While out at my mothers house, she had a
> phone next to her PC that wouldn't ring and when used would cause the
> DSL to drop out. I happened to buy a new cordless phone for her and
> replaced a phone that was working elsewhere in her home. After putting
> the new cordless phone in, the phone by her PC started ringing properly
> and was able to be used without dropping the DSL.

My Panasonic 900MHz cordless phone plays silly bugger with the TDM400P card 
all the time.  For whatever reason it either draws far too much power or just 
plain does not like the TDM430P.  My Aastra 390 and a couple other "regular" 
phones seem to work just fine, but that cordless phone will crackle and 
sputter for the first 10s or so of the call, at which point it quiets down 
and behaves itself.

So yes, the actual PHONE connected to the card may be an issue.  I haven't 
actually 'scoped out what my Panasonic is doing to the system, it'd be 
interesting to see.

-A.