[Shop-talk] Fuses

[JohnT Blair]

 On 01/12/2024 6:21 AM EST Moose wrote:
 
>This is brilliant (figuratively AND literally) 8>)   But is this safe?  Couldn't it melt wires if there
>is a short?
 
 
Moose,
 
Yes it's safe if you do it correctly.  You want to use a good socket, if it's not plastic wrap it with electrical tape.  We use this for finding a short in a piece of electronic equipment.  Put a 60W light bulb in a box in series with the outlet that you plug the equipment into.  This will limit the current to 1/2 amp. 
 
I also suggest (someone else mentioned this) that you make some sort of adapter to plug into the fuse socket.  For the old
glass cylindrical fuses, solder a piece of insulated wire on each end so you can connect clip leads to the wire to connect to the light bulb or to the circuit breaker.  For the spade type, take a couple of spades and solder insulated wire to the spades. 
 
The secret is to use something to use limit the current to less that the original fuse, ie. if the circuit that has the problem has a 10A fuse, limit the current to 5A.  How?  Use ohms and Watts law,
 
Ohms law:  E = I * R     E - Voltage,  I - Current,  R - Resistance.
 
Watts law:  P = I * E      P - power (watts)
 
So for a 10 A circuit in a 12 V system, R = E / I, you'll need
R = 12V / 10A = about 1 ohm of resistance.
 
You also need to account for the power:  P = I * E or
P = 10A * 12V or 120 W. 
 
This is why you use light bulbs.  But they should be closely matched to the power used by the circuit being worked on.
 
If you go too low of wattage, the light bulb will burn out,  If you go to high, then the light bulb won't limit the current enough to protect the circuit.
 
>This is a really great idea I never hear of, so thanks for sharing!
 

Yes, I use it for electronics.  But you need incandescent bulbs.
 
https://www.instructables.com/Light-Bulb-Current-Limiter/ 
 
For automotive I prefer to have different size circuit breakers, 1, 5, 10, 20, 30A breakers and clip leads.
 

JohnT,

John T. Blair  WA4OHZ     email:  jblair1948 at cox.net

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