[GreenKeys] Current loops

[Peter Gottlieb]

120 volts at 60 mA is only 7.2 Watts. I agree a switching regulator is more efficient but one has to watch for RFI. 


Peter

> On Jun 25, 2019, at 4:33 AM, Mattis Lind <mattislind at gmail.com> wrote:
> 
> 
> 
> Den tis 25 juni 2019 kl 08:45 skrev Harold Hallikainen <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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