[GreenKeys] Teletype power supply design

Sat Apr 18 15:27:23 EDT 2009

I've been playing with Nixie tubes a little when I am bored with my TTY...
and there are a lot of designs for HV supplies (170v typ) floating around
for running those.  I've wondered if one of those designs wouldn't be
suitable for a 60mA loop.

Here's one that uses a MAX1771 and near the end of the discussion, he
talks about higher current capabilities.  He's getting over 120mA at
180v with the proper selection of the FET.  There is an inductor in the
circuit but not a transformer.

http://www.desmith.net/NMdS/Electronics/NixiePSU.html

I have seen other designs using TL494 also.

Chris N0JCF

On Saturday (04/18/2009 at 10:58AM -0700), John Nagle wrote:
>     As a design exercise, I've been building SPICE models of possible
> switching power supplies for Model 15 Teletypes.  The idea is to get
> rid of the bulky and hard to find 120VDC supply and inefficient load resistor.
> 
>     The usual setup for a Model 15 Teletype is a 120VDC 60mA supply with
> a 2K 10W resistor in series.  120VDC is needed to load up the selector
> magnet (4H inductance, 55 ohm DC resistance) in under 1ms.
> 
>     But 120V is only needed for the first 1ms after a SPACE to MARK
> transition to force 60mA through that huge inductance.  In about 1ms,
> a steady state is achieved, and only 3.3V is needed to maintain 60mA.
> 
>     So I'm looking at a design where, during SPACE periods, a capacitor
> is charged up to 120V.  On a SPACE to MARK transition, the capacitor is
> discharged into the selector magnet.  A 3.3V source is also fed in,
> so that once the capacitor is discharged, there's a sustaining source
> to keep 60mA flowing.
> 
>     At 45.45 baud, we have a minimum of 22ms dwell time in SPACE for
> charging.  Only 17.5ms at 56.9 baud (75WPM), which is the usual top speed
> for a Model 15, and at 75 baud (100 WPM), only 13ms.  Beyond 75 baud,
> 60mA current loops aren't usually used, I think.  So we need to be
> able to develop 120VDC in 13ms.
> 
>     How big a capacitor needs to be charged up?  About 1uF.  It takes
> about 0.0072J to load up a selector magnet (I=0.060 amps, V=120 volts,
> L=4 henries, W = 1/2 L*I*I = W = 1/2 C*V*V), and a 1uF cap at 120V
> holds that much energy.
> 
>     Charging is done with an IC intended for photoflash capacitor charging,
> the LT3484-1.  This is a specialized switching power supply in a single IC.
> It takes few external components, the main one being a tiny transformer.
> 
>     The whole thing could be powered from a 6VDC 250mA wall-wart type power
> supply.  No more need for a 120VDC supply and a big, hot current limiting
> resistor.  There's also the nice property that the stored energy is well below
> the 1-Joule safety limit for hazardous electric shock.  So the electronics
> doesn't need to be protected as a high voltage device.
> 
>     Below is a preliminary SPICE model of the charging circuit, just
> to show that this is possible.  This will work with the free version of
> LTSpice IV (http://www.linear.com/designtools/software/).  If you
> want to run this, download LTSpice IV, paste the text below into
> a file and name the file something like "ttytest.asc", and run it
> with LTspice.  Then you can look at the voltages and currents.
> The transformer isn't a stock part; I have to find a suitable one and
> redesign to it.  Nor is this a full design; it's just a proof of concept
> circuit simulation.
> 
>      Is this something people are interested in, or are users content
> with power supplies oversized by modern standards?
> 
> Version 4
> SHEET 1 1028 680
> WIRE -224 -112 -224 -160
> WIRE 0 -112 -224 -112
> WIRE 96 -112 0 -112
> WIRE 224 -112 96 -112
> WIRE 288 -112 224 -112
> WIRE 384 -112 288 -112
> WIRE -224 -48 -224 -112
> WIRE 288 -32 288 -48
> WIRE 0 16 0 -112
> WIRE 96 48 96 -112
> WIRE 224 48 224 -112
> WIRE -224 64 -224 32
> WIRE 384 80 384 -112
> WIRE 512 80 432 80
> WIRE 624 80 624 64
> WIRE 624 80 576 80
> WIRE 0 112 0 96
> WIRE 48 112 0 112
> WIRE 384 160 272 160
> WIRE 624 160 624 144
> WIRE 624 160 432 160
> WIRE 48 208 -224 208
> WIRE 624 208 624 160
> WIRE 160 304 160 272
> FLAG 160 304 0
> FLAG -224 64 0
> FLAG 624 208 0
> FLAG 624 64 OUT
> FLAG 288 -32 0
> FLAG -224 288 0
> FLAG -224 -160 Vcc
> SYMBOL ind2 368 64 R0
> WINDOW 0 -49 34 Left 0
> WINDOW 3 -52 75 Left 0
> SYMATTR InstName L1
> SYMATTR Value 10µ
> SYMATTR Type ind
> SYMATTR SpiceLine Rser=2
> SYMBOL ind2 448 176 R180
> WINDOW 0 -47 76 Left 0
> WINDOW 3 -64 43 Left 0
> SYMATTR InstName L2
> SYMATTR Value 140µ
> SYMATTR Type ind
> SYMBOL voltage -224 -64 R0
> WINDOW 123 0 0 Left 0
> WINDOW 39 24 132 Left 0
> SYMATTR SpiceLine Rser=10
> SYMATTR InstName V1
> SYMATTR Value 6v
> SYMBOL diode 512 96 R270
> WINDOW 0 32 32 VTop 0
> WINDOW 3 0 32 VBottom 0
> SYMATTR InstName D1
> SYMBOL cap 608 80 R0
> WINDOW 0 43 31 Left 0
> SYMATTR InstName C2
> SYMATTR Value 1µF
> SYMBOL res -16 0 R0
> SYMATTR InstName R1
> SYMATTR Value 100K
> SYMBOL cap 272 -112 R0
> SYMATTR InstName C1
> SYMATTR Value 10µf
> SYMBOL voltage -224 192 R0
> WINDOW 123 0 0 Left 0
> WINDOW 39 0 0 Left 0
> SYMATTR InstName V2
> SYMATTR Value PULSE(0 5 5ms)
> SYMBOL PowerProducts\\LT3484-1 160 160 R0
> SYMATTR InstName U1
> TEXT 344 184 Left 0 !K1 L1 L2 0.98
> TEXT -128 336 Left 0 !.tran 25m startup
> TEXT 288 328 Left 0 ;60mA Teletype power supply design test.\nPRELIMINARY\nJ. 
> Nagle (nagle at animats.com)\nApril 2009
> 
> 
> 				John Nagle
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-- 
Chris Elmquist