[GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)

[drlegendre .]

Hi Alf,

A couple other members wrote me (off list, not sure why) and your comments
echo theirs. I already had the part about the requirements for high driving
voltages - the inductance of the selector coils appears initially as a
rather high resistance. And as with any relatively high resistance, it's
going to require a relatively higher voltage to push any amount of current
through it.

But I wasn't aware of the SS magnet driver circuit in the M33 and similar.
I was also puzzled over the 20mA mode on the M15, which with 4X inductance
seemed to me would seriously impede the cycle time if still being driven
with 120V. Apparently it does slow things down, but not so much that the
machine won't function. From the sound of it, it's a power-saving
compromise intended for use when multiple machines are installed in
somewhat close proximity.

Best,
Bill

On Tue, Sep 15, 2015 at 3:07 AM, Alf Fisher <alf_fisher at tiscali.co.uk>
wrote:

> drlegendre (name?)
>
> To a large extent, it depends on the machine.
> In most of the machines I have come across, it is required to drive a
> current through an electromagnet which, having a high inductance, will do
> its best to stop that current rising to the desired value hence the need
> for the high driving voltage to overcome the effect of the inductance.
> There is a good explanation of this on RTTY dot com.
> In the Teletype model 33 (and others),  it is only necessary to drive a
> small current into some resistors to turn on a transistor and the work of
> driving the electromagnet is done by a driver transistor in the machine so
> that case, low voltages like 24 volts can be used.
>
> 73
> Alf
> G3WSD
>
>
> *From:* drlegendre . <drlegendre at gmail.com>
> *Sent:* Tuesday, September 15, 2015 1:48 AM
> *To:* Green Keys <GreenKeys at mailman.qth.net>
> *Subject:* [GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)
>
> When I began my M15 project, I was under the impression that all of these
> machines required the 'standard' 120V DC / 60mA loop supply. In short (no
> pun..) 120VDC squeezed through a 1K8 - 2K resistor.
>
> Also was expecting to find a ~50R selector magnet coil. What I found was a
> dual-coil setup that - through the miracle of DPDT - allows the machine to
> run at 20mA or 60mA.
>
> So if 120VDC / 2K was the expected supply for the 60mA setting, what
> open-circuit voltage / resistance was expected for the 20mA setup?
>
> Finally, did I just waste a lot of time building my RS-232 -to- 120VDC /
> 60mA converter? Could I have just as well used one of the recently rebuilt
> Camiacs Mdl. V RS232 -to- 20mA converters already in inventory?
>
> IIRC, the Camiacs supply an o/c voltage of 24 VDC - seems low, in that the
> selector magnets are in series in the 20mA position, which means the
> inductances sum, and as such, should produce a lot more resistance to
> current rise across the windings?
>
> (In the real installation, the lines are very short, a matter of a few
> feet - so it's not like there are any long-line effects to worry about. But
> what's the theory?)
>
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