[GreenKeys] Re: Voltage and Current

[Bob Camp]

Hi

What the gizmo does is buffer the RTTY by about one bit or so. When  
the magnets need to be closed and are open it charges up a cap. When  
it's time to close the magnets the constant current transistor limits  
the current through the magnets. The cap discharges at about the  
right rate to keep the voltage on the transistor nice and low. When  
the magnets are closed the regulator goes into low voltage mode and  
the transistor keeps the current at the right level. Net result is  
not much power in the constant current transistor. Best guess is that  
it's running around 1/4 of a watt to 1/2 watt depending on the data  
stream.

The rate of change on the magnets is dependent on the voltage on the  
cap. More voltage before the current flows = faster change on the  
magnets. The sooner the magnets get to 60 ma the sooner you can start  
dropping the voltage. It's sort of a "power for free" sort of thing ....

Bob

On Jan 24, 2007, at 11:53 AM, gil smith wrote:

> 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
>