LTspice 555 timer duty cycle calculation help

[satiz]

Hi there,

I doing 555 astable simulations in LT spice and I got the output. my problem is expected Ton and Toff time is not matching with my simulation results.
Below is my schematic.

The formula I used to calculate Ton and Toff times are given below with results:
Ton=0.693(R1+R2 x C)=5.04Sec
Toff=0.693(R4 x C)=10.06

Simulation results:
Ton = 5.96Sec
Toff = 11.88 sec

I am thinking this difference is because of the D2, D3 diodes. If yes, how to include this in the formula? Please help me.

I am attaching the .asc code for reference.

Version 4
SHEET 1 1584 680
WIRE -16 -384 -320 -384
WIRE 48 -384 -16 -384
WIRE 208 -384 48 -384
WIRE -320 -352 -320 -384
WIRE 48 -352 48 -384
WIRE 208 -352 208 -384
WIRE 48 -272 48 -288
WIRE 208 -240 208 -272
WIRE -320 -208 -320 -272
WIRE -224 -208 -320 -208
WIRE -160 -208 -224 -208
WIRE -320 -144 -320 -208
WIRE -160 -144 -160 -208
WIRE -320 -16 -320 -80
WIRE -160 -16 -160 -80
WIRE 128 96 80 96
WIRE 512 96 352 96
WIRE 80 112 80 96
WIRE -320 160 -320 64
WIRE -224 160 -320 160
WIRE -160 160 -160 64
WIRE -160 160 -224 160
WIRE 128 160 -160 160
WIRE 528 160 352 160
WIRE 576 160 528 160
WIRE -320 208 -320 160
WIRE 128 224 -112 224
WIRE 544 224 352 224
WIRE 576 224 544 224
WIRE -112 256 -112 224
WIRE 128 288 64 288
WIRE 448 288 352 288
WIRE -320 304 -320 272
WIRE 448 304 448 288
WIRE -112 368 -112 336
WIRE 448 400 448 368
WIRE -112 480 -112 432
FLAG 80 112 0
FLAG 208 -240 0
FLAG 544 224 TRIG
FLAG -320 304 0
FLAG -16 -384 VCC
FLAG -224 160 TRIG
FLAG 64 288 VCC
FLAG 448 400 0
FLAG 528 160 Discharge
FLAG 48 -272 0
FLAG -112 480 0
FLAG 512 96 VCC
FLAG -224 -208 Discharge
SYMBOL Misc\\NE555 240 192 R0
SYMATTR InstName U1
SYMBOL voltage 208 -368 R0
WINDOW 0 42 37 Left 2
WINDOW 3 45 65 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL res -336 -368 R0
WINDOW 0 37 54 Left 2
SYMATTR InstName R1
SYMATTR Value 1K
SYMATTR SpiceLine tol=0.1
SYMBOL diode -336 -144 R0
WINDOW 0 36 34 Left 2
WINDOW 3 31 61 Left 2
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL polcap -336 208 R0
WINDOW 0 40 22 Left 2
WINDOW 3 39 44 Left 2
SYMATTR InstName C1
SYMATTR Value 22µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=2 Irms=3 Rser=0.015 Lser=0
SYMBOL res -336 -32 R0
WINDOW 0 36 55 Left 2
SYMATTR InstName R2
SYMATTR Value 330K
SYMATTR SpiceLine tol=0.1
SYMBOL res -176 -32 R0
WINDOW 0 36 54 Left 2
SYMATTR InstName R4
SYMATTR Value 660k
SYMATTR SpiceLine tol=0.1
SYMBOL diode -144 -80 R180
WINDOW 0 -45 37 Left 2
WINDOW 3 -96 15 Left 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL cap 432 304 R0
WINDOW 0 41 21 Left 2
SYMATTR InstName C2
SYMATTR Value 0.01µf
SYMBOL polcap 32 -352 R0
WINDOW 0 39 25 Left 2
WINDOW 3 38 49 Left 2
SYMATTR InstName C3
SYMATTR Value 15µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=63 Irms=1.28323 Rser=0.15 Lser=0 mfg="KEMET" pn="T521X156M063ATE150" type="Tantalum"
SYMBOL res -128 240 R0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL LED -128 368 R0
WINDOW 0 84 37 Left 2
WINDOW 3 76 61 Left 2
SYMATTR InstName D1
SYMATTR Value NSPW500BS
TEXT 164 376 Left 2 !.tran 500sec
TEXT 736 -176 Left 2 ;Ton=0.693(R1+R2 x C)=5.04Sec
TEXT 736 -200 Left 2 ;Toff=0.693(R4 x C)=10.06

 

[Easy peasy]

you did not take into account the loss of volts across the diodes you are using - this accounts for the small-ish error ...

 

[Pjdd]

You guessed right in thinking that the discrepancy is caused by the presence of D1 and D2. With these diodes in place, the charge and discharge are no longer operating from Vcc but from Vcc minus a diode drop. On the other hand, the changing voltage level of the timing capacitor is still being compared to the internal reference voltage which is not affected by the diodes. Since the charge-discharge process is now operating from a lower voltage, it takes longer to reach the switching thresholds, hence the longer periods.

 

[Easy peasy]

Guessed...? - no guessing involved. - thanks for re-stating my answer, the switchpoints are still 1/3 & 2/3 Vcc correct, the current into/out of, the timing cap is reduced as the driving voltage is reduced by the Von of each diode ... but I already said all that ...

 

[Pjdd]

Guessed...? - no guessing involved. - thanks for re-stating my answer, the switchpoints are still 1/3 & 2/3 Vcc correct, the current into/out of, the timing cap is reduced as the driving voltage is reduced by the Von of each diode ... but I already said all that ...

That was directed at the OP, not you. You posted your reply while I was typing mine and I posted mine without seeing yours. It happens in forums all the time.

 

[BradtheRad]

By applying a voltage to control pin 5 is an easy way to vary duty cycle. It also changes frequency to some degree so that may be a disadvantage.

 

[Easy peasy]

@ pjdd - apologies -

 

[Akanimo]

Hi,

Yes, just like stated, it is the diode forward drop that's causing the discrepancy. I just simulated the same circuit except for the diode that I used the diode model (.model Mydiode D(Vfwd=0.001 Ron=1 Riff=100Meg) just to reduce the diode forward drop to a negligible value. The result came out almost as calculated.