... could be wrong, but automotive arc suppresing capacitors placed across the points are rated at 450 volts. So it seems that a diode in that vicinity would see similar voltage levels.

 

You are wrong. The diode is only reverse biased when the solenoid is powered, and therefore sees only the supply voltage in reverse. When power is removed, it becomes forward biased, and the voltage across the solenoid is limited to the diode drop, which is the entire point of putting the diode there.

Without a diode, the voltage can rise quite high when power is removed, hence the arcing, and hence the high voltage rating of the capacitor.

Bob

 

... could be wrong, but automotive arc suppresing capacitors placed across the points are rated at 450 volts. So it seems that a diode in that vicinity would see similar voltage levels.

Those capacitors resonate with the coil inductance and so ring and produce multiple sparks at the plugs.
A diode will behave quite differently as post #22

 

... the 1N400x datasheet shows that the Maximum Recurrent Peak Reverse Voltage of that diode changes with the diode load current designation. You would have to get at least the 4 amp 1N4004 diode to get 400 volts reverse voltage protection, or better the 5 Amp 1N4005 to get 600 reverse volts as specified by the datasheet.

No... All 1N400x are rated for 1 Amp. Higher number (x) indicates higher rated voltage.

 

No... All 1N400x are rated for 1 Amp. Higher number (x) indicates higher rated voltage.

... you are correct ... the designation of the 1N400x diodes is for different reverse voltage ratings, and has nothing to do with diode load current. ... which is about 1.0 amps. ... don't know where that idea came from.

 

When power is removed, it becomes forward biased, and the voltage across the solenoid is limited to the diode drop,

... as the switch contact is opened, and the DC current is interrupted, the diode is then forward biased, and will generate a standard diode voltage drop. The remaining question is how does the energy stored in the magnetic field of the coil, \( 1/2 LI^2\), dissipate? You are saying that no excess reverse voltage is produced across the diode at any time.

 

Yes. The energy is dissipated by the resistance of the coil, and the diode itself.

Bob

 

Is there a particular reason, reliability maybe, that high voltage arc suppressing capacitors, rather than reverse biased diodes, are typically used on non-electronic, point and breaker automotive ignition systems?

 

This may be informative:
https://www.google.com/url?sa=t&rct...aveforms.pdf&usg=AOvVaw21ZnhrEc7laiQFlsWZEHjh

 

The diode only needs to be a bit higher than the supply voltage and only needs to handle a bit more than the max current the solenoids draw.

Bob

Sorry to disagree with Bob but the kickback voltage in something as inductive as solenoids will be several times the supply voltage due to inductance and the the rate of field collapse.
OP, the current is going to be small by the look of the solenoids and I would suggest using 1N4004 or 1N5404 diodes. These are rated at 400V and there is no cost penalty, these diodes cost pennies whether 50V or 1000V and will largely eliminate your problem. You need to connect them in reverse bias* , parallel with the switch contacts.
*You can read more on bias here: https://www.allaboutcircuits.com/te...chpt-3/introduction-to-diodes-and-rectifiers/

 

Sorry to disagree with Bob but the kickback voltage in something as inductive as solenoids will be several times the supply voltage due to inductance and the the rate of field collapse.

Yes, it will indeed, unless there is a diode to catch it and limit it to under 1V.

 

Is there a particular reason, reliability maybe, that high voltage arc suppressing capacitors, rather than reverse biased diodes, are typically used on non-electronic, point and breaker automotive ignition systems?

Yes:
1) Because "ringing" (ie oscillation) in the tank circuit formed by the coil primary and the condenser helps sustain the spark , which is the ideal .
2) Semiconductor diodes did not really exist in the 1900's when coil ignition became popular in cars and weren't even considered as an option.

 

Yes, it will indeed, unless there is a diode to catch it and limit it to under 1V.

Many datasheets and application notes stress that flywheel diodes should have a factor of three or more voltage rating. Who am I to argue with the manufacturers? This circuit is effectively using the diodes in flywheel mode and I know from long experience working on hydraulic solenoids on agri equipment, that 400V 1N4004 work perfectly to suppress any kickback.

 

This may be informative:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjj7d7h75jgAhUZThUIHXitBnQQFjAAegQIAhAC&url=https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf&usg=AOvVaw21ZnhrEc7laiQFlsWZEHjh

Interesting and thanks for the link. It's from an other era but still very informative, they talk of 7-9KV - today it is more like 30KV. still lots of good information, well explained. Higher compression ratio also demands a higher voltage, that is why an engine may miss at wide open throttle facing a hill.

 

... a reverse biased diode placed across the coil should work, but the 1N400x datasheet shows that the Maximum Recurrent Peak Reverse Voltage of that diode changes with the diode load current designation. You would have to get at least the 4 amp 1N4004 diode to get 400 volts reverse voltage protection, or better the 5 Amp 1N4005 to get 600 reverse volts as specified by the datasheet.

Or a diode across the solenoid - connected so it is reverse biased when the solenoid is energised.

Now i am confused ! Diode across solenoid or contact ?

 

agreed, just diodes across solenoids

Diode across solenoid or contacts ?

 

Diode across solenoid or contacts ?

It won't work across the contacts. It must be across the solenoid.
The voltage across the contact doesn't reverse during the spike, as the voltage across the solenoid does.

 

Sorry to disagree with Bob but the kickback voltage in something as inductive as solenoids will be several times the supply voltage due to inductance and the the rate of field collapse.
OP, the current is going to be small by the look of the solenoids and I would suggest using 1N4004 or 1N5404 diodes. These are rated at 400V and there is no cost penalty, these diodes cost pennies whether 50V or 1000V and will largely eliminate your problem. You need to connect them in reverse bias* , parallel with the switch contacts.
*You can read more on bias here: https://www.allaboutcircuits.com/te...chpt-3/introduction-to-diodes-and-rectifiers/

#30 says across contacts the other across solenoid ?

 

Hello,

There will be no inductive spice accross the contacts.
To protect the contacts a TVS device may be used.
(reducing the maximum voltage accross the contacts to the TVS device voltage).
To protect the driving circuit a diode accross the coil will help.

Bertus

 

Hello,

There will be no inductive spice accross the contacts.
To protect the contacts a TVS device may be used.
(reducing the maximum voltage accross the contacts to the TVS device voltage).
To protect the driving circuit a diode accross the coil will help.

Bertus

Thanks . It makes sense.