Hi,
I have some radio gear that needs 13.8 VDC running off two linear PSUs. I understand if the voltage regulation fails then a higher voltage can appear at the outputs and burn up whatever is attached to the PSU.
I haven't been able to find a clear presentation of a crowbar circuit that shows how to calculate the component values for practical applications.
What are my options for building a black box than can go between the PSU and each radio?
If my question is too open-ended it is because I have been away from electronics for a long time and am getting back into it recently.
Thanks!
I have some radio gear that needs 13.8 VDC running off two linear PSUs. I understand if the voltage regulation fails then a higher voltage can appear at the outputs and burn up whatever is attached to the PSU.
I haven't been able to find a clear presentation of a crowbar circuit that shows how to calculate the component values for practical applications.
What are my options for building a black box than can go between the PSU and each radio?
If my question is too open-ended it is because I have been away from electronics for a long time and am getting back into it recently.
Thanks!
Have a read of -
A Complete 13.8 Volt, 20 Amp PSU – Warrington Radio Club
It includes an --MC3423 IC
Or a simple circuit of a crowbar -
Practical Electronics/Crowbar circuit - Wikibooks, open books for an open world
A Complete 13.8 Volt, 20 Amp PSU – Warrington Radio Club
It includes an --MC3423 IC
Or a simple circuit of a crowbar -
Practical Electronics/Crowbar circuit - Wikibooks, open books for an open world
A basic crowbar consists of a thyristor across the supply rails. Select a high current type.
A Zener diode of appropriate voltage sets the trip value, so a 15V Zener would be a good starting point. A small cap (say 0.1uF) prevents spurious triggering. The resistor would be around 470 to 1000 ohms.
The supply MUST be fused before the thyristor.
A Zener diode of appropriate voltage sets the trip value, so a 15V Zener would be a good starting point. A small cap (say 0.1uF) prevents spurious triggering. The resistor would be around 470 to 1000 ohms.
The supply MUST be fused before the thyristor.
Attachments
Thank you guys. I'll go over the article again several times but I think I'm still missing info on how to spec the individual component values.
Are these something there are calculations for or are people figuring it out by trial and error?
Are these something there are calculations for or are people figuring it out by trial and error?
For the diagram I posted there isn't all that much to calculate tbh but lets see where we get...
Zener diodes begin to conduct when their breakdown voltage is reached... great in theory but in practice they leak a bit before that point. That's no good in itself because the thyristor would then fire before the marked Zener voltage is reached.
So we add the resistor. Now enough current has to flow to generate enough voltage across the resistor to exceed the thyristors turn on voltage of about 600mv and then to increase enough (the current) to reach the thyristors gate trigger current.
You could calculate all this if you know the Zener diode characteristics and what the current flow is as it approaches its marked voltage... and how that varies with temperature... and tolerances...
So we just use a one size fits all that covers all bases and in practice anything from a 100 ohm to probably a couple of kilo ohm would be fine.
The cap is just to stop random unwanted transient firing. Make it to big and you add a time constant that delays firing when needed, to small and it is ineffective. So something like a 0.1uF would be a suitable value.
Zener diodes begin to conduct when their breakdown voltage is reached... great in theory but in practice they leak a bit before that point. That's no good in itself because the thyristor would then fire before the marked Zener voltage is reached.
So we add the resistor. Now enough current has to flow to generate enough voltage across the resistor to exceed the thyristors turn on voltage of about 600mv and then to increase enough (the current) to reach the thyristors gate trigger current.
You could calculate all this if you know the Zener diode characteristics and what the current flow is as it approaches its marked voltage... and how that varies with temperature... and tolerances...
So we just use a one size fits all that covers all bases and in practice anything from a 100 ohm to probably a couple of kilo ohm would be fine.
The cap is just to stop random unwanted transient firing. Make it to big and you add a time constant that delays firing when needed, to small and it is ineffective. So something like a 0.1uF would be a suitable value.
This is a simulation of the crowbar. The resistor shows the current in the triac (or thyristor) and you can see that a suitable fuse in the supply would blow.
The voltage here goes from 13.6 to 18v in 20 milliseconds (left hand scale) and the current is on the right hand scale.
The voltage here goes from 13.6 to 18v in 20 milliseconds (left hand scale) and the current is on the right hand scale.
