[R-390] Fan Power Supply Voltage and RTTY Question

[Scott, Barry (Clyde B)]

Thanks, Bob and Bill.

Yeah, I figured the cap was too small.  Thinking about it, the fan is supposed to draw 150ma at 12VDC which makes it somewhat equivalent to an 80-ohm load.  This makes for a 1.6ms time constant -- not nearly enough.

I should've known better -- however, I'm not sure it's a bad thing.  This only causes the fan to run a bit slower and thus quieter.  Maybe I'll leave it alone.

Thanks for straightening me out, guys.

73,

Barry(III) - N4BUQ


Barry,

The problem is that the capacitor is too small.

The maximum no-load voltage you will get from a simple power supply is
1.414 times the rms secondary voltage (the peak value of the secondary
voltage). Since you said you were seeing 14 volts and the transformer is
supposed to be 18vct I assume you were using a full wave center tap
configuration. This should result in 9 * 1.414, or about 12.7 volts, so
the transformer is probably giving you a little more than 18 volts or
you meter is off a little.

Anyway, the problem is that with your 120ma load and 20uf capacitor, the
ripple voltage is very large. You can calculate what the ripple voltage
will be for a given case this way:

Vripple = Iload/(120*C)

Where:

Vripple = ripple voltage (volts)
Iload = load current (amps)
C = capacitance (Farads)

If you rearrange things, you can find the capacitor needed for a given
load and ripple voltage:

C = Iload/(120*Vripple)

so for example, if you want 1 volt ripple at 120ma:

C = .120/(120 * 1)
  = .001F (which is 1000uF)

This assumes a full wave rectifier at 60Hz. If your supply is half wave,
use 60 instead of 120.

You can see a description of why this works near the bottom of this
page:
http://courses.ece.uiuc.edu/ece343/zdesign.html

larry/wa8ulo

"Scott, Barry (Clyde B)" wrote:
> 
> List,
> 
> Power Supply question:
> I built a simple power supply to drive a cooling fan for my R390A.  I used an 18VCT in a "double half wave" rectifier configuration (the same way the R390A HV power supply is done).  I slapped a cap across the output (20mfd as I recall) and it was delivering a clean 14VDC no-load voltage on the scope.  Fine, I thought.  Good enough to drive some small fans.
> 
> Well, when I connect the fan, the voltage drops to about 9 volts (according to the DVM).  The fan is drawing about 120ma.  What causes the sudden voltage drop?  I need to look at the voltage on the scope, but I suspect the cap is discharged through the load between half cycles and the DVM "sees" 9VDC when, in fact, it is a rippled DC signal.  Does this sound plausible?
> 
> RTTY question:
> 
> When tuning for RTTY, I know tha most all of them are LSB and tuning to the wrong side of the signal will invert the mark and space signals.  When you tune through a RTTY signal -- say from a lower frequency, through zero, to a higher frequency, which "side" of the signal is the right "side": the one "below" the zero beat or the one "above" it?  I think this may part of my inability to copy RTTY correctly.  Last night I finally was able to get the "five number" coded patterns I've heard others talk about.  Sadly, I don't remember which "side" of the signal I was on...
> 
> Thanks,
> 
> Barry(III) - N4BUQ
> 
> _______________________________________________
> R-390 mailing list
> R-390@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390

-- 
-----------------------------------
Wm. L. Townsend
TES, Inc.
Cincinnati, OH    USA
(513)661-3200 fax:(513)661-3732
www.tesnet.com
-----------------------------------