[GreenKeys] Re: Voltage and Current

[gil smith]

Hi Bob:

I was able to get a 60-mA machine (M31) running with only 18VDC in the 
loop, but lower loop voltage results in less selector range margin, hence 
more character errors.  It will depend on the inductance of the selector in 
the specific model (I don't know whether they are all in the same ballpark 
or not) -- the higher the inductance, the higher voltage (actually, higher 
resistance) needed for decent selector range.  Someday I should measure the 
inductance of various models to find out.

I have not played with constant-current design, so can't add anything on those.

Let me paste in an except from the tty-connect manual:

"There is a practical reason for loops to use high-voltage with certain 
machine models, such as M14/15/19/20/26/28/31. First, the higher voltage 
will keep some of the dust and oil burned off of the keyboard and TD 
contacts to help keep them clean. Second, the high DC loop supply voltage 
is needed to overcome the effect of the selector magnet inductance, which 
impedes the rise in current when going from SPACE to MARK. Using a high 
voltage in series with a large resistor (to obtain 60- or 20-mils) 
minimizes the effect of the inductance, permitting the current to rise 
rapidly, thus preventing deterioration of the receiving selector margin. 
The circuit will act faster and give less distortion if a higher voltage is 
used. You could compare the usable range finder settings using different 
loop supply voltages -- you would expect to find a much greater range with 
a higher-voltage loop.

The math: the inductance of the selector coils is significant, and coil 
voltage is proportional to L*di/dt. But it's technically the loop 
resistance, not the loop supply voltage, that sets the current waveform in 
the coil. Of course, to use a larger R, you need to use a larger V, to get 
the 60 or 20 mA needed.

For a series circuit with a voltage source V, resistor R, and inductor L, 
when initial current (t=0) is zero, the current for t>0 is:

i(t) = (V/R) - (V/R)  * e (-t/T)            note:  e (-t/T)  is exponential 
e raised to (-t/T)

where the time constant T = L/R. The first term (V/R) is the loop operating 
current, which will be a constant 60 mA (or 20 mA), by design. The second 
exponential term affects the leading edge of the waveform, but note that 
the V/R scaling magnitude is again a constant (0.06 or 0.02) and not 
actually dependent on V. It is the T= L/R in the exponent that sets the 
rise time of the waveform, larger R resulting in faster rise times."


gil


At 09:36 PM 1/23/2007, WA5CAB at cs.com wrote:
>EE Mode On.
>
>12 volt constant current supply is a contradiction in terms, at least in the
>context of this group where most machines run on loops @ either 20 or 60 mA.
>Or to put it another way, a 12 volt supply with electronically controlled
>series resistance (i.e., a current regulator) will not push a square wave 
>current
>wave form through either M15 or M28 holding magnets.  Consequently, the error
>rate will go sky high and the range will go to zilch.
>
>EE Mode off.
>Greasy fingered tech mode on.
>It won't work.
>
>In a message dated 1/23/2007 9:47:21 PM Central Standard Time,
>drhouse at nadcomm.com writes:
> > Bob,
> >
> > 48 volts should be the minimum used to operate selector magnets.
> > Personally I prefer -130vdc 62.5 ma but you know that I suppose.
> >
> > Don
> > K9TTY
> >
> >
> > On 23 Jan 2007, at 8:42 PM, Bob Camp wrote:
> >
> > Hi
> >
> > A typical constant current loop control transistor puts a fixed bias
> > on the base (or gate) and resistance from the emitter (or source) to
> > the other side of the bias. As the current in the resistor drops the
> > current in the transistor increases.
> >
> > These days you can do some interesting things with a switching
> > regulator. Twelve volt supply constant current loops anybody?
> >
> >
>
>Robert & Susan Downs - Houston
><http://www.wa5cab.com> (Web Store)
>MVPA 9480
><wa5cab at cs.com> (Primary email)
><wa5cab at houston.rr.com> (Backup email)
>_______________________________________________
>GreenKeys mailing list
>GreenKeys at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/greenkeys



Vaux Electronics, Inc.
480-354-5556
(fax: 480-354-5558)
www.vauxelectronics.com