Hello,
I started designing a power protection circuit and it became a side note of another thread, but i think that it is time to give it it's own subject and thread and this will maybe help some other people as I trial and error my way through this. Anyway, I am trying to protect the input to a smps (24V) from any voltage over 26ish volts up to and including 120VAC. The first circuit I tried is below (powerprotection.jpg). This did not work for 120V and I believe it to be that the TVS diode's clamping voltage was less than 120V. The problem is that I could not find a TVS diode with a breakdown voltage of around 28V with a clamping voltage of over 120V. Scrapped that design.
After some research, i came to the standard "crowbar" circuit (powerprotection2.jpg) And I think that this could work. Here is what i have so far, but am a little unclear about how to choose some of the components:
B1 - Rectifier in case someone puts AC voltage on the input
Type - Schottky, Schottky Silicon Carbide, Avalanche or Standard
?? (Doesn't Matter??)
Current Rating - Greater than the fuse Rating (1A)
Voltage Rating - At lease the maximum voltage to protect against (120V)
Speed - ?? (Doesn't Matter??)
Reverse Recovery Time - ?? (Doesn't Matter??)
F2 - Blows to protect Circuit
Type - Fast
Current Rating - Slightly Greater than Current draw of System Being protected (1A)
Voltage - minimum 2x maximum protected voltage (120V)
I2t less than I2t of SCR
C10 - Reducing Input noise
Capacitance - 100uF used in past seemed to work
Don't know if this will work in this case or not. Can try different types
Voltage - Greater than max protected voltage (120V)
Type - Electrolytic
D4
Breakdown Voltage - Slightly Greater than the normal working voltage (26, 27,28V)
Reverse Current - Greater than the draw from SCR and R12 (in parallel at breakdown) (???)
Power - Reverse Current x Max Voltage (???)
Max Voltage - Greater than the max protected voltage (120V)
R12 - provides current to the gate of the scr when D4 breaks down, current flows from + through D4 to SCR and through resistor to ground
Resistance - (???)
Power - (??)
C11 - Prevents thyristor from being triggered at power on. (called snubber)
Capacitance - High enough to snubb D4 (), Low enough not to create high state at the gate at power on. (??)
Voltage - Greater than max Voltage at SCR Gate which will be max protected voltage ( > 120V)
Type - (???)
D6 - SCR
Voltage Rating - Higher than the max protected voltage (120V in this case)
Current Rating - Higher than the Fuse rating (1A)
To Do:
Select SCR, then R12, then D4, then we can just try C10, C11.
SCR: MCR12DSMT4G (ON Semi)
Let me know if I am way off track here and if you have any suggestions, let me know. I will update this as I go for anyone who has the same problem in the future.
I started designing a power protection circuit and it became a side note of another thread, but i think that it is time to give it it's own subject and thread and this will maybe help some other people as I trial and error my way through this. Anyway, I am trying to protect the input to a smps (24V) from any voltage over 26ish volts up to and including 120VAC. The first circuit I tried is below (powerprotection.jpg). This did not work for 120V and I believe it to be that the TVS diode's clamping voltage was less than 120V. The problem is that I could not find a TVS diode with a breakdown voltage of around 28V with a clamping voltage of over 120V. Scrapped that design.
After some research, i came to the standard "crowbar" circuit (powerprotection2.jpg) And I think that this could work. Here is what i have so far, but am a little unclear about how to choose some of the components:
B1 - Rectifier in case someone puts AC voltage on the input
Type - Schottky, Schottky Silicon Carbide, Avalanche or Standard
?? (Doesn't Matter??)
Current Rating - Greater than the fuse Rating (1A)
Voltage Rating - At lease the maximum voltage to protect against (120V)
Speed - ?? (Doesn't Matter??)
Reverse Recovery Time - ?? (Doesn't Matter??)
F2 - Blows to protect Circuit
Type - Fast
Current Rating - Slightly Greater than Current draw of System Being protected (1A)
Voltage - minimum 2x maximum protected voltage (120V)
I2t less than I2t of SCR
C10 - Reducing Input noise
Capacitance - 100uF used in past seemed to work
Don't know if this will work in this case or not. Can try different types
Voltage - Greater than max protected voltage (120V)
Type - Electrolytic
D4
Breakdown Voltage - Slightly Greater than the normal working voltage (26, 27,28V)
Reverse Current - Greater than the draw from SCR and R12 (in parallel at breakdown) (???)
Power - Reverse Current x Max Voltage (???)
Max Voltage - Greater than the max protected voltage (120V)
R12 - provides current to the gate of the scr when D4 breaks down, current flows from + through D4 to SCR and through resistor to ground
Resistance - (???)
Power - (??)
C11 - Prevents thyristor from being triggered at power on. (called snubber)
Capacitance - High enough to snubb D4 (), Low enough not to create high state at the gate at power on. (??)
Voltage - Greater than max Voltage at SCR Gate which will be max protected voltage ( > 120V)
Type - (???)
D6 - SCR
Voltage Rating - Higher than the max protected voltage (120V in this case)
Current Rating - Higher than the Fuse rating (1A)
To Do:
Select SCR, then R12, then D4, then we can just try C10, C11.
SCR: MCR12DSMT4G (ON Semi)
Let me know if I am way off track here and if you have any suggestions, let me know. I will update this as I go for anyone who has the same problem in the future.