[GreenKeys] Model 28 signal lines

Wed Oct 16 23:00:09 EDT 2019

Here's a link to a much more detailed version of the old electronic
selector magnet driver.

    http://www.navy-radio.com/manuals/tty/sel_mag-15-19-20-28.pdf

Paul, ad7i

On Wed, Oct 16, 2019 at 10:48 PM ad7i <ad7i at ad7i.net> wrote:

> Greetings --
>
> I've been looking at selector magnet current as a function of time the
> last few days, mostly inspired by this article (
> http://www.navy-radio.com/manuals/tty/570-224-400.pdf ).  And Micro-Cap
> circuit simulation software is now free (
> http://www.spectrum-soft.com/download/download.shtm ) which is a pretty
> piece of serious software.
>
> I've been intrigued by this topic because when I first started with the
> Model 15 in about 1982 I used my own constant current driver fed from about
> 24 volts to run the TTY from a computer.  It seemed to work well (meaning
> only it printed well).  I didn't do what I should have done and that was
> put a scope on the loop to measure magnet current as a function of digital
> signal input, to see how much of a delay I was introducing in getting the
> magnets up to 60mA.
>
> When I later tried that electronic constant current driver on a friend's
> 28, it didn't work at all.  When I mentioned that to Bill Henry of HAL he
> told me that the selector magnets on the 28 had much more inductance than
> the 15, the 28 magnets in standard configuration (parallel) being about 2H
> total and that it really did need a high voltage loop in order to use a
> large current limiting resistor, so that the circuit could have a large R
> to L ratio (for fast rise time in the presence of a large inductance).  As
> Jim pointed out, the 15 may have been more tolerant of timing issues for
> other reasons than just magnet inductance.
>
> I recently measured the inductance and resistance of my 28RO magnets (with
> the armature in an unknown state - the position of the armature may affect
> the inductance value a bit) as being 1.73 Henrys and 64 ohms.  Consider
> those to have an accuracy of about 5% (just reading things off the digital
> scope).
>
> I'm not up to speed on Micro-Cap yet, so I'm stuck with hand
> calculations.  Tonight I came up with the following numbers.
>
> For a conventional loop of 170VDC and 2800 ohms with selector magnets of
> 1.73 H and 64 ohms resistance (selector magnets in parallel) that gives a
> long-term stead state current of about 60 mA.  After closing the loop
> switch to allow current to flow, the current in the magnets starts at zero
> of course and then rises with exponential decay and reaches 55 mA after
> 1.5ms (and is now very slowly rising, but it's almost at 60mA).
> Theoretically, it never reaches the final value of 60 mA, but that's in the
> realm of how many angels can dance on the head of a pin.  When the current
> is within 10% of the final value I call it done.
>
> Consider now an electronic driver loop circuit that consists of a 32 VDC
> source, a 125 ma fast blow fuse (5 ohm), the resistance of the magnets (64
> ohms), the resistance of a MOSFET power switch in the ON state (less than 1
> ohm) and a 10 ohm current sense resistor.  The final long-term steady state
> current would be no less than 32/(5+64+1+10) = 400mA.  Which is not good
> for selector magnets, hence the old-school fast blow fuse if something goes
> wrong.  But the plan is to use an electronic circuit to disable that MOSFET
> when the current gets to 60mA and then a secondary MOSFET with a 470 ohm
> resistor holds the loop at 60 mA long-term.  So, in considering when MOSFET
> 1 is first turns on, the current in the magnets is at 0 mA and starts to
> rise with an exponential decay proportional to R/L, with R being only 80
> ohms in this case.  Running the numbers using the values above shows that
> the magnet current hits 60mA in 3.5 mS (again within 5%) and would keep
> rising if MOSFET one is not disabled at that point.  So the low voltage low
> resistance loop does introduce some additional delay compared with the HV
> loop, 2 ms more, which isn't a whole lot.  I'm inclined to think that's
> acceptable, but I'd need to test that to confirm.
>
> The reason I like the 32VDC source is the availability of $15 HVAC
> transformers that convert 120 to an isolated 24 VAC at about 40VA (meaning
> about 1.5 A is available on the secondary).  Put that 24 VAC into a full
> wave rectifier and one gets about 32VDC across the filter cap (with the
> amount of ripple depending on current draw).
>
> Another thing to consider is that once the armature is pulled in (I don't
> know how long that takes after the mag current hits 60 mA) it may be
> possible to reduce the magnet current to something like 36 mA (60% of 60)
> because the holding current of most relay-like structures is much less than
> the pull in current.  The only reason to look at this is to reduce heat
> dissipation from the loop circuitry (meaning the 470 ohm resistor which in
> the Marking state would normally dissipate a little less than 2 watts).
> It's probably not worth the extra circuitry to go down that path to achieve
> a slight reduction in heat disipation.
>
> If anyone would like a copy of my calculations or a PDF of the circuit I
> hope to start testing next month (will be placing a Mouser order this
> weekend for parts) just let me know.  I'd be happy to provide them.
>
> Please note that I could be all wet on this low voltage electronic magnet
> driver, but it's the type of stuff I like to play with.
>
> 73, Paul Newland, ad7i
>
>
>
>
>
>
>
>
>
> On Wed, Oct 16, 2019 at 9:32 PM Jim Haynes <jhhaynes at earthlink.net> wrote:
>
>> On Wed, 16 Oct 2019, Paul Heller wrote:
>> > Barry,
>> > My only point, and others may disagree, is that 48v is not enough.
>>
>> Oh, I was going to say that too.  Although with perfect signals like
>> you get off itty it might be OK.  Also there was a constant-current
>> circuit in QST a long time ago and I believe the article said it was
>> good to use with 48V.  I keep wishing someone who knows how would
>> simulate that circuit and see what kind of rise and fall times can
>> be achieved versus the simple power supply and resistor
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