Low Current Overcurrent Protection Methods

Thread Starter

RAMBO999

Joined Feb 26, 2018
216
For example, how do you protect a TQ2-5V relay from current overload? The nominal current is small at about 28.1 mA. If it's a 5V relay. I can limit the Vcc to 5 V that's no problem. But if I wanted to put a fuse in the circuit to limit the current through the coil how could I do that. Are there any fuses that can work at that level?
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
216
You don't need to protect the coil, just the load through the contacts..yes you can get fuses rated at 10mA upwards..
Thanks for the response. Could you post a link to an example of these fuses? eBay or Amazon would good. See what's available.
 

Hymie

Joined Mar 30, 2018
743
I’ve never seen a circuit where a low power dc relay coil is provided with dedicated overcurrent protection.

The relay coil will be a fixed resistance and if you limit Vcc to 5V, the only way an excessive current will flow through the coil is if there is a partial (or complete) short within the coil – which is very unlikely.

It is more likely that any snubber reverse diode across the coil will fail – but unless there is some critical reason that the 5V rail must not be dragged down by a fault in the relay, I would venture that dedicated fuse protection of the relay will not add to the reliability/safety of the circuit.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
216
I’ve never seen a circuit where a low power dc relay coil is provided with dedicated overcurrent protection.

The relay coil will be a fixed resistance and if you limit Vcc to 5V, the only way an excessive current will flow through the coil is if there is a partial (or complete) short within the coil – which is very unlikely.

It is more likely that any snubber reverse diode across the coil will fail – but unless there is some critical reason that the 5V rail must not be dragged down by a fault in the relay, I would venture that dedicated fuse protection of the relay will not add to the reliability/safety of the circuit.
The coil itself is polarised on the TQ2-5V and I am filtering the supply through a cap so there is a diode in the circuit to prevent any reverse polarity doing any damage. But I was curious as to whether OC protection is necessary and how you would cater for it physically. It's relatively easy to control the voltage. But I tend to think more about the power limitations of the components. In the case of this relay it's 140mW = 28.1 mA at 5V. And some of the resistors ar only 125mW. So some kind of OC protection might be worth considering. Most probably not routine I agree **. Just my thoughts on the subject.

** Although having looked at the availability of micro fuses it suggests to me that OC protection might not be such an unusual idea.
 
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