Sure I can.could you elaborate on what you mean by "6 time constants" this makes no sense to me, particularly in this context- in the CD ignition I designed, the capacitor is charged from a bridge from a power supply that can source a fixed amount of current without saturating the transformer (and the voltage on a transformer with constant current is just the current times time). It is discharged into an ignition coil through an SCR, and the back voltage at the end of the cycle turns off the SCR and is recovered by a diode, as I recall. These are very simple circuits, parts values are non-critical. I found that setting the operating voltage (e.g. the voltage you have on the cap) to a little above the back EMF across the points when they open is sufficient for a hot spark - that is generally around 200V. Maybe you are referring to the non-electronic time constants such as the burn front propagation in the mixture?
Nope it has nothing to do with propagation."Maybe you are referring to the non-electronic time constants such as the burn front propagation in the mixture?"
Since all capacitors and coils(inductors) have resistance either ESR or wire resistance you have RC and RL time constants. It is the time it takes to charge/discharge the capacitor or inductor. If you put a resistor in front of a capacitor or inductor you automatically slow down the charging. These charge times or time constants can be calculated and are well know in electronics. Sometimes it is there and we use it without knowing that we are, a good example is the 555 timer. Now don't get me wrong but, yes we can make a simple circuit work by bogging something together. The circuit may or may not work the way we want it but, it still works. So if one puts something together and it works that good, if it doesn't then we're left with why not. The time constants can be used to get something to work or fine tune a circuit.
I now use a charging circuit that starts with a 1,100 volts to the capacitor. Need to get as much charge as I can in that T1.
The time constants:
The time constants are a fixed percentage based on measured times.
The series resistance can be either or both the internal resistance or external.
RC Charging
The time constant, τ is found using the formula T = R x C in seconds.
For example - The time constant τ is given as: T = R x C = 47k x 1000uF = 47 Secs
Time | RC Value | Percentage of Maximum | Percentage of Maximum | |||
Constant | Voltage |
| ||||
0 time constant | 0 | 0.0% | 100% | |||
0.5 time constant | 0.5T = 0.5RC | 39.30% | 60.70% | |||
0.7 time constant | 0.7T = 0.7RC | 50.30% | 49.70% | |||
1.0 time constant | 1T = 1RC | 63.20% | 36.80% | |||
2.0 time constants | 2T = 2RC | 86.50% | 13.50% | |||
3.0 time constants | 3T = 3RC | 95.00% | 5.00% | |||
4.0 time constants | 4T = 4RC | 98.20% | 1.80% | |||
5.0 time constants | 5T = 5RC | 99.30% | 0.70% |
Remember current leads voltage by 90 degrees in a capacitive circuit.
Series resistance ESR | 0.5 | Input Voltage | 12 | Capacitor uF | 1000 |
Voltage | Current | Time | |||
T0 | 0.000 | 6.000 | 0.00500 | ||
T1 | 7.584 | 2.208 | 0.00316 | ||
T2 | 10.380 | 0.810 | 0.00433 | ||
T3 | 11.400 | 0.300 | 0.00475 | ||
T4 | 11.784 | 0.108 | 0.00491 | ||
T5 | 11.916 | 0.042 | 0.00497 |
RL coil charging:
The percentage of charge for an inductor are the same as capacitance but the charge formula is different. Voltage leads current by 90 degrees.
The time constant, τ is found using the formula T = L/ R in seconds. | ||||||
Therefore the time constant τ (0) is given as: T = L ÷ R = 47uh ÷ 2 ohms = 0.0000235 Secs | ||||||
RL Charging Table | ||||||
Time Constant | LR Value | Percentage of Maximum | ||||
Current | Voltage | |||||
T0 | L/R | 0 | 100 | |||
0.5 time constant | 0.5T = 0.5L/R | 39.30% | 60.70% | |||
0.7 time constant | 0.7T = 0.7L/R | 50.30% | 49.70% | |||
1.0 time constant | 1T = 1L/R | 63.20% | 36.80% | T1 | ||
2.0 time constants | 2T = 2L/R | 86.50% | 13.50% | T2 | ||
3.0 time constants | 3T = 3L/R | 95.00% | 5.00% | T3 | ||
4.0 time constants | 4T = 4L/R | 98.20% | 1.80% | T4 | ||
5.0 time constants | 5T = 5L/R | 99.30% | 0.70% | T5 | ||
Series resistance = 20 | | Input Voltage =12 | | Inductor = 100uh | | |
The internal Voltage Drop of the Inductor | Current in Amps | Time in seconds | ||||
T0 | 12.000 | 0.000 | 0.000005000 | |||
T1 | 4.416 | 0.379 | 0.000003160 | |||
T2 | 1.620 | 0.519 | 0.004325000 | |||
T3 | 0.600 | 0.570 | 0.004750000 | |||
T4 | 0.216 | 0.589 | 0.004910000 | |||
T5 | 0.084 | 0.596 | 0.004965000 | |||
|
I have attached my Excel file in zip format if anyone wants to check it out.
Normally people/hobbyists don't bother with these calculations and why should they, capacitors and inductors/coils are generally inexpensive and just keep plugging in something until it works. I've learned that for low speed engines <5,000 RPM a 2.2uf works good. For RPM =>10,000 or multi-cylinder engines a 0.47uf works better.
Also here are some links if one wants to learn more. Ok to best. Some include formulas for calculating the energy stored in a capacitor or coil/inductor.
-https://www.allaboutcircuits.com/tools/resistor-capacitor-time-constant-calculator/
-https://www.digikey.ca/en/resources/conversion-calculators/conversion-calculator-time-constant
-RL circuit - Wikipedia
-23.1: RL Circuits
Best
-https://www.electronics-tutorials.ws/inductor/lr-circuits.html
If there are anymore questions I'll try to answer them.
Cheers
Ray
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