I am unable to simulate the followingg PUT-based relaxation oscillator in the most recent LTspice (with BC547B and BC557B):
Nothing wants to oscillate.
Looking for a bug I have also tried the simplest possible neon lamp oscillator (with Vstrike=100V, Vhold=50V, R=220k, C=1u), but there are no scillations either: the voltage on the capacitor saturates at 50.25V.
What magic should I apply to make such circuits work as expected?
Best regards, Piotr
Works fine for me. Frequency of 62Hz.
It is probably having a "hard start" issue where the oscillation will not begin.
Two possible issues...
(1) Model parameter BF of transistors may be so high that current thru
220K exceeds holding current.Try increasing 220K OR add a resistor, NPN base to ground, to raise holding current above what 220K can support.
(2) Oscillators, in any form of Spice, _often_ will not start on their own.
Try .IC of capacitor as zero volts. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
You need to start the SIM at 0 volts.
Jamie
The 220k timing resistor is too small, keeping the PUT continuosly on. The following thing oscillates.
--- clip clip ---
Version 4 SHEET 1 908 704 WIRE 576 80 48 80 WIRE 848 80 576 80 WIRE 48 144 48 80 WIRE 576 144 576 80 WIRE 48 272 48 224 WIRE 240 272 48 272 WIRE 384 320 304 320 WIRE 576 320 576 224 WIRE 576 320 384 320 WIRE 48 384 48 272 WIRE 848 384 848 80 WIRE 384 416 384 320 WIRE 576 416 576 320 WIRE 240 464 240 368 WIRE 320 464 240 464 WIRE 384 576 384 512 WIRE 48 688 48 448 WIRE 384 688 384 640 WIRE 576 688 576 496 WIRE 848 688 848 464 FLAG 384 688 0 FLAG 576 688 0 FLAG 48 688 0 FLAG 848 688 0 SYMBOL pnp 304 368 R180 SYMATTR InstName Q1 SYMATTR Value BC557B SYMBOL npn 320 416 R0 SYMATTR InstName Q2 SYMATTR Value BC547B SYMBOL res 560 128 R0 SYMATTR InstName R1 SYMATTR Value 3.9k SYMBOL res 560 400 R0 SYMATTR InstName R2 SYMATTR Value 3.9k SYMBOL res 32 128 R0 SYMATTR InstName R3 SYMATTR Value 1Meg SYMBOL cap 32 384 R0 SYMATTR InstName C1 SYMATTR Value 33n SYMBOL voltage 848 368 R0 SYMATTR InstName V1 SYMATTR Value 9V SYMBOL LED 368 576 R0 SYMATTR InstName D1 TEXT -80 640 Left 2 !.tran 0 1 0
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-- -TV
This neon thing oscillates. The current has to be kept low.
Version 4 SHEET 1 880 680 WIRE 32 48 -32 48 WIRE 144 48 112 48 WIRE 256 48 144 48 WIRE 144 112 144 48 WIRE -32 128 -32 48 WIRE 256 128 256 48 WIRE -32 240 -32 208 WIRE 144 240 144 208 WIRE 256 240 256 192 FLAG -32 240 0 FLAG 144 240 0 FLAG 256 240 0 SYMBOL Misc\\neonbulb 144 160 R0 SYMATTR InstName U1 SYMATTR SpiceLine Zon=500 Ihold=200u SYMBOL cap 240 128 R0 WINDOW 0 73 12 Left 2 WINDOW 3 74 47 Left 2 SYMATTR InstName C1
SYMBOL res 128 32 R90 WINDOW 0 -7 54 VBottom 2 WINDOW 3 41 58 VTop 2 SYMATTR InstName R1 SYMATTR Value 1E6 SYMBOL voltage -32 112 R0 WINDOW 0 -87 31 Left 2 WINDOW 3 -94 68 Left 2 SYMATTR InstName V1 SYMATTR Value 200 TEXT 312 64 Left 2 !.tran 4 uic
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Try checking "skip initial operating point" in the sim/edit/trans menu.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
To add to that...
You need to put real part numbers on the transistors and the NPN needs to have a pull down R at the base to common/grd so that it'll discharged the cap in the base of the NPN etc.. A 220K should work or something in that ball park.
Only going by your schematic, using an LED. I suppose a Neon maybe behave differently.
Jamie
Thank you all very much, in both cases the non-oscillation seems to be caused by too low resistor value. In the case of the PUT-based circuit I have not enough experience to judge whether Spice is right or wrong, but I remember well my experiments performed over a decade ago with a *real* neon oscillator. It used to work with pretty any reasonable resistor. The simulation provided by John does not oscillate with 470k and with 560k it oscillates *exactly twice*. With 680k it works well. I must confess that my excitement about Spice has just entered a colder period... :-/
Once again, thank you for your help!
Best regards, Piotr
And what do you do then?
Best regards, Piotr
You didn't read the last line of my post...
"Try .IC of capacitor as zero volts."
This _should_ be settable in the capacitor part attributes, if Spice conventions are followed by LTspice (likely).
If not, add this "Spice directive"...
.IC V(Node_Name_At_Top_Of_Cap)=0
Some forms of oscillators (LC is one) require a "kickstart", a current pulse into the tank.
(This is because, unlike in the real world, transient Spice simulations have no noise to start the oscillation.) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Spice is often very, very wrong. Oscillators, especially high-Q resonator oscillators, can be very tricky. I'm currently playing with a coaxial ceramic resonator oscillator, and can make LT Spice do all sorts of goofy stuff by playing with the time step. The default (automatic) time step results are absurd. 1 picosecond begins to behave maybe-sensibly, and anything smaller explodes run times.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Do the math first and you won't get cold >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
In LT Spice, just check "skip initial operating point solution" in the transient analysis setup window. I do that a lot, for power supplies and such where powerup matters.
The other thing to do is use a pulse generator for the power supply, and bring it up after some small time delay, optionally with some realistic rise time.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The LT Spice neon model isn't right.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Yes, the differential equation which describes many oscillators has the valid solution V(t)=constant. But IMHO in the case of a simulator it is a bug, not a feature. Why don't they set the initial voltage at the nodes (esp. capacitors) to some small, random values instead of 0?
Best regards, Piotr
Try this one (fix the wraps before using)...
****************************************************************** .SUBCKT NE-2H A1 A2 PARAMS: Vs=130 Ii=50u Ti=1m Vh=80 Rh=1k Ia=4m Varc A1 arc 0 ; current sense Garc arc A2 VALUE = {sgn(V(arc,A2))*LIMIT((abs(V(arc,A2))-V(ref))/(Rh*V(abn)),0,1)} Carc arc A2 5p ; stray terminal capacitance Gref 0 ref VALUE = {Vh+(Vs-Vh)/(1+V(ion)**2)} ; Rpar=1 ; voltage transition Rparref 0 ref 1 ; Cref ref 0 1n ; tiny capacitance here aids convergence Gion 0 ion VALUE = {abs(I(Varc))} ; Rpar={1/Ii} ; measure of free ions Rpar 0 ion {1/Ii} Cion ion 0 {Ti*Ii} ; gas ionization time constant Gabn 0 abn VALUE = {Ia**2}+I(Varc)**2 ; Rpar={1/Ia**2} ; abnormal glow Rparabn 0 abn {1/Ia**2} Cabn abn 0 1p ; tiny capacitance here aids convergence .ENDS NE-2H ******************************************************************This was originally posted on the LTspice list by snipped-for-privacy@ieee.org and I then modified some of the behavioral expressions to improve convergence. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Actually, I don't design much with neon bulb oscillators lately.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Then it would be nondeterministic. It's a simulator, not reality -- it just churns numbers. It's your duty to build a model that's representative of reality.
If reality means adding noise voltage/current sources for important components (or all of them), that's what you have to do!
Also keep in mind, .MODELs (diodes, BJTs, and bad MOSFET models) are an idealized approximation of the internal chip only. You must add package parasitics yourself. .SUBCKTs usually include this, but inspect the source to be sure it's reasonable.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
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