LTspice question: modelling a solenoid

I need to model the "selector magnet" of a Model 15 Teletype. This is an electromagnet with an iron core. DC resistance is 55 ohms. Inductance is 4 Henries. Normal operating current is 60mA continuous, but because of the huge inductance, the normal power source is 120VDC fed through a 2K resistor.

How do I divide up the resistance between the SPICE "series resistance" and "parallel resistance"? Any ideas?

John Nagle

tor.

ance"

The 55 ohms is most likely just the series resistance of the coil; pure series resistance.

The parallel resistance is the resistance of the core to induced current circulating in the core. Working out how big it is presumably involves measuring the complex impedance of the coil over a range of frequencies.

You will also have to take into account the parallel capacitance of the coil, so it is a complicated task.

-- Bill Sloman, Nijmegen

Unless it's resonant frequency is near the operating conditions, "lump-lump" should be adequate.

Beware that the inductance of a solenoid can vary quite a bit with core position.

Is this related to the problem posed a while back about a driver that was more efficient than a brute force power supply?

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

             Buy a Fiarysler with your tax credit... 
          Ignore that grinding noise, that is normal :-)

Series resistance only plus the inductance. I've only found a need for a parallel component, when near electrical resonance, to model the "Q".

Measure the inductance as the "flat plate" moves. You can calculate force from that measurement ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

Go on.

Here the core is stationary, and attracts a flat plate to the ends of the coils. It's really an electromagnet, not a solenoid with a moving slug.

Yes. I have a design now. It looks like it's possible to generate the needed 120VDC 60mA from 5V at 250mA. (The high voltage is only needed to get a fast rise time despite that big inductance. After the first 2ms or so, only 3.3V is needed to push 60mA through the 55 ohm coil.) The design uses a Linear LT3484-1 photoflash charger IC (a little switching power supply) and a pulse transformer to charge up a 1uF cap to 120VDC during "off" periods. When the input signal turns on, the cap is discharged into the coil to pull it in. Then the 3.3V regulator provides holding current for the remainder of the "on" cycle.

LTspice model below. The key nets to plot are "serialin", "Selector" (the selector magnet voltage) and current through L3.

The circuit seems to better snubbing at the selector magnet, to dump the energy when the magnet is turned off. But I'm holding off on that until the model is realistic. For a real magnet, the R/C snubber shown is sufficient.

John Nagle

Version 4 SHEET 1 1784 680 WIRE -64 -336 -128 -336 WIRE 528 -336 0 -336 WIRE 768 -336 608 -336 WIRE 160 -304 0 -304 WIRE 720 -304 640 -304 WIRE 768 -304 768 -336 WIRE 160 -288 160 -304 WIRE 640 -256 640 -304 WIRE 832 -256 784 -256 WIRE -80 -176 -240 -176 WIRE 0 -176 0 -304 WIRE 0 -176 -80 -176 WIRE 32 -176 0 -176 WIRE 352 -176 288 -176 WIRE 416 -176 416 -208 WIRE 416 -176 352 -176 WIRE 640 -176 416 -176 WIRE 704 -176 704 -256 WIRE -80 -80 -80 -112 WIRE 352 -80 352 -112 WIRE 832 -64 832 -256 WIRE 896 -64 832 -64 WIRE 1008 -64 896 -64 WIRE 896 -48 896 -64 WIRE 160 -32 160 -64 WIRE -240 0 -240 -176 WIRE -64 0 -240 0 WIRE 0 0 -64 0 WIRE 96 0 0 0 WIRE 224 0 96 0 WIRE 384 0 224 0 WIRE 1008 0 1008 -64 WIRE -240 16 -240 0 WIRE 0 16 0 0 WIRE 496 16 432 16 WIRE 592 16 592 -32 WIRE 592 16 560 16 WIRE 640 16 592 16 WIRE 704 16 704 -176 WIRE -240 32 -240 16 WIRE 96 48 96 0 WIRE 224 48 224 0 WIRE 896 48 896 16 WIRE 384 80 384 0 WIRE 432 80 432 16 WIRE 592 80 592 16 WIRE -64 96 -64 64 WIRE 0 112 0 96 WIRE 48 112 0 112 WIRE -240 160 -240 96 WIRE 384 160 272 160 WIRE 592 160 592 144 WIRE 592 160 432 160 WIRE 896 160 896 128 WIRE 896 160 592 160 WIRE 1008 160 1008 80 WIRE 1008 160 896 160 WIRE -128 208 -128 -336 WIRE -128 208 -304 208 WIRE 48 208 -128 208 WIRE 592 208 592 160 WIRE 160 304 160 272 WIRE -304 336 -304 288 FLAG 160 304 0 FLAG -240 160 0 FLAG 592 208 0 FLAG -64 96 0 FLAG -304 336 0 FLAG -240 -176 Vcc FLAG 640 -256 0 FLAG 1008 -64 Selector FLAG 592 -32 Pulse120V FLAG -304 208 serialin FLAG 352 -80 0 FLAG -80 -80 0 FLAG 240 -32 0 FLAG 416 -208 Continuous3.3V SYMBOL ind2 368 64 R0 WINDOW 0 -49 34 Left 0 WINDOW 3 -52 75 Left 0 SYMATTR InstName L1 SYMATTR Value 17µ SYMATTR Type ind SYMATTR SpiceLine Ipk=0.5 Rser=2.1 SYMBOL ind2 448 176 R180 WINDOW 0 -47 76 Left 0 WINDOW 3 -64 43 Left 0 SYMATTR InstName L2 SYMATTR Value 247µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.7 SYMBOL voltage -240 0 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=5 SYMATTR InstName V1 SYMATTR Value 4.75 SYMBOL diode 496 32 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMBOL cap 576 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 -80 0 R0 SYMATTR InstName C1 SYMATTR Value 22µf SYMBOL voltage -304 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 -166 114 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 5 5ms 100us 100us 13.2ms 26.6ms 99) SYMBOL PowerProducts\\\\LT3484-1 160 160 R0 SYMATTR InstName U1 SYMBOL res 512 -320 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R2 SYMATTR Value 500 SYMBOL Digital\\\\inv -64 -400 R0 WINDOW 0 -47 67 Left 0 WINDOW 39 -47 95 Left 0 SYMATTR InstName A1 SYMATTR SpiceLine Vhigh=5 SYMBOL ind 992 -16 R0 SYMATTR InstName L3 SYMATTR Value 4H SYMATTR SpiceLine Rser=55 Rpar=10000 SYMBOL sw 688 -256 M90 WINDOW 0 -23 94 VRight 0 WINDOW 3 -53 175 VLeft 0 SYMATTR InstName S1 SYMATTR Value hvopto SYMBOL diode 640 32 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMBOL diode 640 -160 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D4 SYMBOL cap 880 -48 R0 SYMATTR InstName C3 SYMATTR Value 1000pf SYMBOL res 880 32 R0 SYMATTR InstName R3 SYMATTR Value 100 SYMBOL PowerProducts\\\\LT3080 160 -176 R0 WINDOW 0 -59 -46 Center 0 SYMATTR InstName U2 SYMBOL cap -96 -176 R0 SYMATTR InstName C4 SYMATTR Value 1µf SYMBOL cap 336 -176 R0 SYMATTR InstName C5 SYMATTR Value 2.2µf SYMBOL res 144 -16 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R4 SYMATTR Value 330K TEXT 344 184 Left 0 !K1 L1 L2 0.98 TEXT -168 360 Left 0 !.tran 60m startup TEXT 288 328 Left 0 ;60mA Teletype power supply design test.\\nPRELIMINARY\\nJ. Nagle ( snipped-for-privacy@animats.com)\\nMay 2009 TEXT 336 208 Left 0 ;Pulse PA1546NL TEXT 640 -368 Left 0 ;Simulated ideal optoisolator TEXT 536 -400 Left 0 !.model hvopto SW(Ron=4.25 Roff=1Meg Vt=2.0)

LTspice doesn't like that "\\n", though it proceeds anyway.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

[snip]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Unwrap this line

What it's balking at is a wrapped line that occurred during posting.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

What does the flyback condition look like with the _real_ solenoid? At

4H I would expect some pretty low frequency "squirrelies" ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

The traditional driving circuit is a 120VDC supply fed through a 2K resistor and keyboard contacts, with no snubbing at all. The contacts arc visibly and produce noticeable ozone. That's the 1930 technology in its original form.

My present driving circuit is a 120VDC open-frame supply fed through a 2K resistor and a high-voltage optoisolator, with a snubber consisting of a 1pf cap in series with a 100 ohm resistor. This works fine, but it's a bit bulky.

I'll have to hook up a scope and look at the voltage across the selector magnet. (After wiring up a voltage divider to get the voltage down to the scope's range.)

Trying to do this off a 5V supply is mostly an elegance thing. But I may need to build a USB interface. Most USB to serial converters won't go down to 45.45 baud. (They should, from the spec, but in practice the firmware in common use won't go below

110 baud.) I want to run the Teletype from a laptop with USB ports only, instead of a mini-tower PC with a classic serial port that will run at 45 baud. So I may have to use an Atmel CPU with USB software as an interface device. If I do that, it would be convenient to eliminate the need for the 120VDC supply and run the whole thing off the USB port.

John Nagle

Here's how I'd attack your problem (in the manner of how I used to make CD ignition systems)....

(Paste to make sure your reader gets the parentheses part).

[1] It is left as an exercise for the student to choose a PowerMOS replacement for S1... I no longer carry a catalog in my head like I did in the '80's :-) [2] Also what logic to choose for the driving signal path. [3] The coil is presently modeled simply as 4H + 55 Ohms. If it were my project I'd take a network analyzer to the coil. I'd also measure inductance versus current, and create an accurate Spice model... I suspect that the real world will be full of ring-a-ding-dings ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

Now at device level (no behavioral)....

(Paste to make sure your reader gets the parentheses part).

11mW dissipation in the MOSFET ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

            If I\'m talking, you should be taking notes.

There's a problem with this, Jim. Your circuit works fine with a continuous pulse train, but teletype isn't like that, it can have long periods of no activity (constant Mark signal), then a few characters, then more inactivity. The selector magnet must respond to the first drop to Space (beginning of start bit) of the first character.

The idle state is constant Mark (positive or 0V, depending on whether it's polar or neutral signaling).

John's approach does take account of this.

--
"Electricity is of two kinds, positive and negative. The difference
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)

Running a simulation here, I get a massive (400V plus) negative spike at the coil on turnoff. I'd suggest replacing the RC snubber with a diode and a 2k resistor, which will give the same falling time constant as the original 120V "brute force" circuit, and reduce the -ve spike to around

-80V.

BTW, is there any reason for running your simulation at 75.75 baud, rather than the 45.45 you said you would be running the machine at?

--
"Electricity is of two kinds, positive and negative. The difference
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)

If "Mark" is current in the solenoid, it's fine. (I'm obviously not at all familiar with Teletype... I'm not THAT old ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.

On a neutral (current or no current) system, mark is no current. Makes idling more economical.

-- "Electricity is of two kinds, positive and negative. The difference 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)

Could the first "space" be made long?

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | Stormy on the East Coast today... due to Bush's failed policies.

The first space is the start bit from which the sample timing originates. It's the usual start/stop protocol: One start bit, five NRZ data bits, 1.5 stop bits. The start bit trips the clutch that starts the reading mechanism, which samples the position of the selector magnet at fixed intervals. The mechanism resets after 5 bit intervals and the clutch disengages. You can regard the idle condition as one long continuous stop signal, it's actually only 1.5 bit intervals between successive characters in a string.

Most machines have an adjustment to set the sampling window relative to the start so as to get the sample on the flat part of the pulses.

I once had a teletype as printer for a TRS-80 (remember those?), using the cassette motor relay for signaling. I think I've still got the Z80 assembly code somewhere.

--
"Electricity is of two kinds, positive and negative. The difference
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)

originates.

1.5

stop

characters

Punched tapes could reliably achieve this tightness of character separation. Manual typists had much more jitter in start to start. The system is truly asynchronous between characters. Intra character timing was created by steady speed electromechanical means.

the

Most machines needed either CR-CR-LF or LF-CR-CR as end-of-line, adding a delay, to allow the carriage or basket to fully return, otherwise you got the first character of the next line printing whilst the carriage was still flying.

That's the reason for the still-widespread CNC tape code using LF-CR-CR. It was designed for ASR33s et al.

When all the "modern" magnetic and semiconductor media have lost data, punched paper tape will still be readable, by humans if necessary.

--
"Electricity is of two kinds, positive and negative. The difference
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)