ECU - Tach Circuit Simulation

Thread Starter

Garry48

Joined Jul 18, 2017
10
I'm trying to understand this circuit (see attachment), so I can build it. I have ordered a Speeduino kit and so I can study the Engine Control Unit (ECU) signals. I believe this circuit is supposed to be able to provide an RPM (tach) signal that can be recognized by an ECU. I have not been able to determine the voltage rating of Zener Diode D4. I'd also like to know why the Zener Diode is in parallel with polarized and non-polarized capacitors.

Can someone explain the Zener usage in the circuit?

TIA
 

Attachments

DickCappels

Joined Aug 21, 2008
5,897
The zener is being used as a shunt regulator to provide a constant voltage for the circuit. The polarized capacitor is there to smooth out low frequency noise across the Zener. The non-polarized capacitor is there to assure that the voltage does not dip during the high current spike when the NE555 output (pin 3) switches between high and low states.

More about shunt regulators:
http://www.radio-electronics.com/info/power-management/linear-power-supply-psu/shunt-voltage-regulator-theory-circuit.php
 

Thread Starter

Garry48

Joined Jul 18, 2017
10
The zener is being used as a shunt regulator to provide a constant voltage for the circuit. The polarized capacitor is there to smooth out low frequency noise across the Zener. The non-polarized capacitor is there to assure that the voltage does not dip during the high current spike when the NE555 output (pin 3) switches between high and low states.

More about shunt regulators:
http://www.radio-electronics.com/info/power-management/linear-power-supply-psu/shunt-voltage-regulator-theory-circuit.php
Sir, thanks for the explanation and the link. Is there a way to determine what the Zeners voltage rating should be?
Would a regular 1N400x diode perform the same functionality?

TIA
 

Alec_t

Joined Sep 17, 2013
10,381
The zener is being used as a shunt regulator to provide a constant voltage for the circuit.
Perhaps it's there as a fall-back regulator in case the 7805 fails? I can't see that you'd want to regulate to anything less than 5V, since the 555 would struggle with a lower supply voltage.
 

Thread Starter

Garry48

Joined Jul 18, 2017
10
Sir, thanks for the explanation and the link. Is there a way to determine what the Zeners voltage rating should be?
Would a regular 1N400x diode perform the same functionality?

TIA
Sir, thanks for the explanation and the link.
Since the 555 voltage range is 4.5 to 16V, can I pick any voltage I want to be supplied to pins 4 & 8?
That voltage would also be supplied to the frequency determination part of the circuit.

I hope to put the circuit in Spice or EveryCircuit and test various voltages.

I need more electronics education!

Thanks
 

Thread Starter

Garry48

Joined Jul 18, 2017
10
Sir, thanks for the explanation and the link.
Since the 555 voltage range is 4.5 to 16V, can I pick any voltage I want to be supplied to pins 4 & 8?
That voltage would also be supplied to the frequency determination part of the circuit.

I hope to put the circuit in Spice or EveryCircuit and test various voltages.

I need more electronics education!

Thanks
Gents,
I think I answered my own questions. I found Chapter 3 on this site:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/zener-diodes/

Outstanding information in an easy to understand presentation.
Time to 'pull up' my circuit simulator and start running with the circuit.

Thanks to all!
 

DickCappels

Joined Aug 21, 2008
5,897
Sir, thanks for the explanation and the link. Is there a way to determine what the Zeners voltage rating should be?
Would a regular 1N400x diode perform the same functionality?

TIA
Those diodes marked at zener diodes will regulate according to their specifications. 1N400X rectifiers can operate in a similar manner but since they are not designed to do so they are not reliable. It is much better to use a diode designed to operate in that region.
 
Top