[GreenKeys] Western Union Ticker-5A Stock Quotation Machine

[Brooke Clarke]

Hi Peter:

Did you try using a current source instead of a voltage source?  Did you use a 
protection diode across the coil?

I've been studying this for most of today and I've found two key factors.

1) the loop time constant is L / R.  It may take 3 to 5 time constants for a 
signal to settle so the time constant needs to be a very small part of a bit 
time or you're going to have problems.  But using a current source brings the 
time constant near zero, i.e. R is near infinity.

2) the coil in the electromagnet does not like to change currents.  If there is 
already current in the coil and you turn it off the coil tries to generate a 
voltage that's high enough to maintain the old current.  This is how the spark 
coil in a car works.  This voltage pulse can easily destroy the driver 
transistor.  The common way of fixing that is to put a diode across the coil 
with it's polarity such that when the coil is forward biased the diode is back 
biased, but (forgetting about the bi polar drive for a minute) that will not 
work and at the same time allow the coil to operate quickly, instead the diode 
slows the coil way down since now there's a current circulating in the diode 
and coil with it's own long L/R time constant.

I think the fix for (2) is to use a bi polar Transil.  It's sort of like back 
to back zener diodes (which probably would also work) with a breakdown voltage 
greater than the DC drop across the coil when saturated but much much faster 
and rated to absorb spikes.

I'd like to hear from someone who has done this type of experiment.  My gut 
feeling is that a properly designed driver can work with a low voltage supply. 
  My brain is telling me that there's a lot of people using high voltage that 
would not be if there was an easy way to avoid it.

Have Fun,

Brooke Clarke
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com



Peter Gottlieb wrote:
> Well obviously a power op-amp (Burr-Brown) or homebrew (complimentary 
> pair, see National app notes) will take RS-232 and make bipolar.
> 
> I am not too familiar with the dynamics of polar relays but TTY selector 
> magnets need a high voltage to get current into the high inductance 
> rapidly.  I couldn't get my 2B to run at 28 volts even though I needed a 
> series resistor to drop the current, but go to 100 and it ran like a champ.
> 
> Peter
> 
> 
> Brooke Clarke wrote:
> 
>> Hi:
>>
>> I'm trying to bring a Western Union Ticker-5A Stock Quotation Machine 
>> back to life.  This is the machine that replaced the older stock 
>> ticker machines that were under glass domes to keep down the noise.  
>> I'd say that it's father was the Edison Universal stock ticker that 
>> changed Edison from someone who was broke to a multi millionaire.  
>> It's mother was the model 26 Teletype machine since they both use a 
>> similar, but not interchangeable, selector cage.
>>
>> It's my understanding that the 26 was good for 100 CPM yet the 5A was 
>> good for 500 CPM.
>>
>> The 5A uses a permutation code that consists of 5 bits but instead of 
>> using a character for shift to numbers and another character for shift 
>> to letters the 5A uses another code bit for this so the total number 
>> of bits in the code is 6.  This solves the stuck in figures problem.
>>
>> But unlike teletype machines the 5A does NOT use start and stop bits.  
>> This is one of the things that makes it fast.
>>
>> The thing I'm working on now is how to drive it.  The input goes to a 
>> polar relay that expects to see current polarity reversals, not the on 
>> off keying that a normal teletype machine uses.  Testing has revealed 
>> that it wants to see 50 to 100 ma and being a 20 ohm coil a little 
>> over 1 volt is all that's required.
>>
>> Are that circuits already in existence that will take in RS-232 and 
>> supply the bi polar drive current?  I think that might be more common 
>> in the UK or Australia than in the US.
>>
>> Why do teletype machines typically have loop voltages in the 100 to 
>> 200 volt area?  Could these be replaced by current sources that have a 
>> voltage compliance a litter higher than the voltage drop across the 
>> input relay coil?
>>
>> For more on the 5A see my web page:
>>
>> http://www.pacificsites.com/~brooke/WU5A.shtml
>>
>> Have Fun,
>>
>> Brooke Clarke, N6GCE
> 
>