[GreenKeys] Current loops

Tue Jun 25 09:43:00 EDT 2019

I understand the magnet in the US is on a spring and so the current works against the spring. So for example here:-

http://www.k7tty.com/development/tu/st-5/st-5_basic.htm

we have an MJE340 switched by the keyboard tongue.

In the UK (Creed) system there is no spring, although many creed devices can be set up for single current US style working. 

However normally current flows in one direction for MARK and the other for Space, so originally a 120-0-120 v supply . 

One end of the magnet connects to the ground, the other to the tongue on the TTY keyboard switch. The keyboard Mark and Space contacts then connect to the +120 or -120v.

The keyboard switch then sets current in one direction for mark. The other for Space.

I normally use a single rail  80-100v supply.  So looking at a UK version of the ST5

http://www.samhallas.co.uk/repository/telegraph/RTTY_the_easy_way.pdf

on Page 11 we can see that we have two MJE340 transistors keying each end of the magnet …..

Hope this clarifies things

Dave

From: Mattis Lind <mattislind at gmail.com> 
Sent: 25 June 2019 11:51
To: Dave Wade <dave.g4ugm at gmail.com>
Subject: Re: [GreenKeys] Current loops

Den tis 25 juni 2019 kl 11:17 skrev Dave Wade <dave.g4ugm at gmail.com <mailto:dave.g4ugm at gmail.com> >:

Sorry for Top Post. Outlook won’t let me bottom post HTML messages, and Thunderbird seems to have lost my google calendar…..

… whilst most US and German systems use a single current system, standard UK practice is to use double current. Could this type of approach be made to work for double current keying?

I think it could be possible since the current is controlled by the micro controller. Currently it can be set from 1mA to 60 mA in my design. The micro controller outputs a PWM signal that is RC filtered to create the reference voltage for the comparator. The RC filter of course has a time constant that would affect the step response.

But I am not aware of how the two current system in the UK works and in that case how the micro controller can deduce which current to use and what requirement the system has on current change responsiveness. Is it something like mark is using one current and space is using another current?

/Mattis

Dave

G4UGM

From: greenkeys-bounces at mailman.qth.net <mailto:greenkeys-bounces at mailman.qth.net>  <greenkeys-bounces at mailman.qth.net <mailto:greenkeys-bounces at mailman.qth.net> > On Behalf Of Mattis Lind
Sent: 25 June 2019 09:33
To: harold at w6iwi.org <mailto:harold at w6iwi.org> 
Cc: greenkeys at mailman.qth.net <mailto:greenkeys at mailman.qth.net> ; Jim Haynes <jhhaynes at earthlink.net <mailto:jhhaynes at earthlink.net> >
Subject: Re: [GreenKeys] Current loops

Den tis 25 juni 2019 kl 08:45 skrev Harold Hallikainen <harold at w6iwi.org <mailto:harold at w6iwi.org> >:

>
> Now we all know the canonical circuit is 120V power supply in series with
> 2K ohms to produce a 60 ma loop current with acceptable rise time.
>
> I've seen circuits published using the constant-current properties of a
> transistor to reduce the rise time and allow a lower voltage power supply.
> What I haven't seen is any simulation or measurement of these circuits,
> and I'm not young and smart enough to be up on things like SPICE.  Wish
> someone who knows how would do this and show us how well it works.
>

I don't know how it can be done with a low voltage. E=L*di/dt. If you need
a certain current slew rate (di/dt), and you have a certain L, you need a
certain E (voltage). In our loop, that full voltage is across the magnet
when the keyboard closes, then tapers off the voltage determined by the
resistance (I*R). The full voltage is needed only for a short time, but it
is still needed.

Back when I built my neighborhood Teletype loop in high school, I found I
could get 60 mA with a low voltage supply (I think around 12V), but the
text was garbage. Going up to higher voltage with series resistance fixed
it.

That's what I think, though I could be wrong!

I agree with this. You need the high voltage so that the electro magnet in the teletype react as fast as possible. But after this all the line voltage will be dissipated in the resistor. Using a linear current source will not make any difference. What I did was to make a switch mode current source. It will require considerably less power. There are some losses of course due to the switching process.

Some info is here, work in progress though: 

https://github.com/MattisLind/teleprinterHat

/Mattis

Harold

-- 
FCC Rules Updated Daily at http://www.hallikainen.com
Not sent from an iPhone.
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