help with current/voltage limiter

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Izy

Member
Hello,
I hope I came to the right place
I'm a hardware engineer in a company that manufactures outdoor radio equipment.
We have now demand to pass certification tests for hazardous locations (class I div 2).
The equipment is powered by 48V DC, rated 5A.
We need to design som sort of circuit that can limit the current/voltage in case of short-circuit.
We were told by the testing laboratory that we can't use any ICs, and the protection must be done by discrete components only (resistors, diodes, etc.). We were told that we couldn't use circuit breakers and fuses - because their operation is not "instant". They suggested us to use series resistor, but it appears to be huge in size (rated 140W), so it's impactical.
Is there anyone faced this kind of issue?
 

Izy

Member
The problem is not designing such a circuit, don't embarass me, I'm an engineer :)
The problem is that the laboratory won't accept using fuses or off-the-shelf circuit breakers, althouth they are certified for class I div 2. They don't provide me any explanations, they just say that "they are not instant acting". So what is the "instant acting" according to the regulation? Why circuit breaker or fuse is not "instant"?
 

Besoeker

Senior Member
Location
UK
Hello,
I hope I came to the right place
I'm a hardware engineer in a company that manufactures outdoor radio equipment.
We have now demand to pass certification tests for hazardous locations (class I div 2).
The equipment is powered by 48V DC, rated 5A.
We need to design som sort of circuit that can limit the current/voltage in case of short-circuit.
We were told by the testing laboratory that we can't use any ICs, and the protection must be done by discrete components only (resistors, diodes, etc.). We were told that we couldn't use circuit breakers and fuses - because their operation is not "instant". They suggested us to use series resistor, but it appears to be huge in size (rated 140W), so it's impactical.
Is there anyone faced this kind of issue?
In the case of a short circuit I wouldn't have expected the need to limit voltage..short = no voltage.
Current limit can be implemented using discrete components. If you are permitted to use discrete transistors you can pretty much implement many IC applications. I'm old enough to have been there and done that.
But why would anyone want to do that when modern technology can make circuit design so much simpler and more reliable?
 
The problem is not designing such a circuit, don't embarass me, I'm an engineer
Since you are new here I will try to be nice.

:)

If your power supply was a transformer that maxed out at the desired amount of current, you would have the current limitation required using nothing but a discreet component.

Imagine a 300 mW transciever powered by a 600 mW power supply. That approximates a 50 percent transmitter efficiency. No matter what happens inside the radio, no more than 600 mW will flow.
 

Izy

Member
Since you are new here I will try to be nice.

:)

If your power supply was a transformer that maxed out at the desired amount of current, you would have the current limitation required using nothing but a discreet component.

Imagine a 300 mW transciever powered by a 600 mW power supply. That approximates a 50 percent transmitter efficiency. No matter what happens inside the radio, no more than 600 mW will flow.
I didn't understand what is has to do with the power supply? My power supply is external, and it can be a battery, DC/DC or AC/DC. I don't have any control on the power supply, so I have to implement the limiter circuit inside my product.

Still, if you understand the standard, please explain me why using fuses or circuit breakers is forbidden and why I must use only discrete components.
 
I didn't understand what is has to do with the power supply? My power supply is external, and it can be a battery, DC/DC or AC/DC. I don't have any control on the power supply, so I have to implement the limiter circuit inside my product.

Still, if you understand the standard, please explain me why using fuses or circuit breakers is forbidden and why I must use only discrete components.
Without being able to get input from the one requesting the design specifics, I can tell you that circuit breakers and fuses will allow more than their rating through for a very short time without opening. Perhaps that is what the issue is.

If so, what I have heard effective is the use of a crowbar circuit. You still need some sort of over current protection. The crowbar circuit will limit flow to the radio by shorting out the power supply, instead of relying on the OCPDs to open.

The crowbar can be internal to the radio and the OCPD can be external.

Does that sound like what the requirement is intending?
 
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