I'm powering a Model 15 Teletype current loop. The Teletype has a
selector
magnet resistance of 55 ohms and needs 60mA, so it only needs 3.3V. But
because of the huge inductance of the selector electromagnet (4H), a huge
voltage is needed to pull in the electromagnet fast enough to follow the
incoming data stream.
I'm using the traditional solution, a 120VDC supply through a 2K
10W resistor. It's really dissipating 10W, continuously. A 10W resistor
gets hot enough (250C!) that you can't mount one on a PC board. I'm
designing an interface board, and would like a better solution.
Basically, I need a constant-current regulator circuit that
takes in 120VDC and limits the output current to 60mA. Preferably one
that doesn't dissipate too much heat. Suggestions?
The crux of the issue is current rise time, which is equal to L/R, hence
the higher series resistance, the faster the rise of current. If you use a
resistor (in your case 2K), you're going to need 120 volts to push enough
current through it. That's the reason for the "traditional" teletype power
supply, which goes back to the early 20th century.
An electronic current source mimics a high resistance, while allowing a
lower compliance (ie. open-circuit) voltage. The following LTSpice
simulations show firstly a "traditional" teletype circuit, and secondly a
suggestion for a simple current source using only a 24 volt supply.
Dissipation is less than 2 watts total.
The pulse source represents a TTY signal of 1:1 mark/space (RYRYRYRY)at
50 baud (AFAIK, the Model 15 did 45, 50, or sometimes 56). Compare the
rise times of current in L1 in the two examples, whilst not quite as fast
in the suggested circuit, it should prove adequate.
Traditional circuit:
Version 4
SHEET 1 880 680
WIRE 64 176 -64 176
WIRE 272 176 144 176
WIRE -64 224 -64 176
WIRE 272 224 272 176
WIRE -64 368 -64 304
WIRE 96 368 -64 368
WIRE 272 368 272 304
WIRE 272 368 96 368
WIRE 96 400 96 368
FLAG 96 400 0
SYMBOL voltage -64 208 R0
WINDOW 3 -429 54 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 120 0 100n 100n 40m 80m)
SYMATTR InstName V1
SYMBOL res 48 192 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R1
SYMATTR Value 2k
SYMBOL ind 256 208 R0
SYMATTR InstName L1
SYMATTR Value 4
SYMATTR SpiceLine Rser=55
TEXT -494 506 Left 0 !.tran 100m
TEXT 320 264 Left 0 ;R (series) = 55 ohm
Suggested circuit:
Version 4
SHEET 1 880 832
WIRE -96 96 -128 96
WIRE 16 96 -32 96
WIRE 144 96 80 96
WIRE 240 96 144 96
WIRE 352 96 320 96
WIRE 144 128 144 96
WIRE -128 192 -128 96
WIRE -128 192 -256 192
WIRE -64 192 -128 192
WIRE 96 192 16 192
WIRE 256 192 192 192
WIRE 400 192 320 192
WIRE 464 192 400 192
WIRE 400 224 400 192
WIRE 464 224 464 192
WIRE -256 240 -256 192
WIRE 400 304 400 288
WIRE -256 400 -256 320
WIRE 80 400 -256 400
WIRE 352 400 352 96
WIRE 352 400 80 400
WIRE 400 400 400 384
WIRE 400 400 352 400
WIRE 464 400 464 304
WIRE 464 400 400 400
WIRE 80 448 80 400
FLAG 80 448 0
SYMBOL ind 448 208 R0
SYMATTR InstName L1
SYMATTR Value 4
SYMATTR SpiceLine Rser=55
SYMBOL pnp 192 128 R90
SYMATTR InstName Q1
SYMATTR Value TIP32C
SYMBOL diode 16 112 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode -96 112 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL res 224 112 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res 32 176 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 10
SYMBOL voltage -256 224 R0
WINDOW 3 -429 54 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 24 0 100n 100n 40e-3 80e-3)
SYMATTR InstName V1
SYMBOL diode 256 208 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL res 384 288 R0
SYMATTR InstName R3
SYMATTR Value 2k
SYMBOL diode 416 288 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D4
SYMATTR Value 1N4148
TEXT -688 504 Left 0 !.tran 100m uic
TEXT 512 272 Left 0 ;R (series) = 55 ohm
TEXT -680 536 Left 0 !.model TIP32C pnp\n+IS=1e-09 BF=134.366 NF=1.29961 VAF=10\n+IKF=0.742988 ISE=1e-16 NE=1.40014 BR=0.1\n+NR=1.46599 VAR=100 IKR=3.21978 ISC=1e-16\n+NC=2.71657 RB=7.44433 IRB=2.41268 RBM=0.218936\n+RE=0.0152284 RC=0.0761421 XTB=0.1 XTI=1\n+EG=1.05 CJE=3.26474e-10 VJE=0.446178 MJE=0.464223\n+TF=1e-08 XTF=3.50642 VTF=8.2848 ITF=0.0305862\n+CJC=3.07595e-10 VJC=0.77548 MJC=0.476497 XCJC=0.799334\n+FC=0.8 CJS=0 VJS=0.75 MJS=0.5\n+TR=1e-07 PTF=0 KF=0 AF=1
LTSpice is free, if you didn't already know that, from
www.linear.com.
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is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)