Fuse change

[theroc]

Please let me know if there is any problem with this analysis, thanks.

Current draw from the battery (with AC power failed) is less that 1/10 Amp.
Inrush current from battery is approximately 2/10 Amp. (measured after 12 Volt buss has fully discharged)
The Alarm strobe lamp draws 2/10 Amp when active.
Horn alarm current is NOT drawn through this fuse.
Total current is 1/2 Amp with strobe lamp operating.
The specification parts list specifies a Picofuse, Fast Acting, 1A 125V.
This is adequate protection with enough overhead to allow for normal inrush surges.
Use of a 5 Amp fuse (as shown in the original OEM drawings on CM server) would risk damage to circuit board traces under a fault condition.

 

[VEBill]

Perhaps the circuit (including the circuit board traces) was designed for 5A capability in case the horn ended up being wired through the fused circuit in the final production units. It is the sort of flexibility that a good designer would include.

Or (other way around), perhaps the selected (or possible) horn ended up drawing more than the 5A design capability and that is why the horn ended being wired to bypass the fuse. This is the sort of mix-up that is all too common.

It is likely that the 5A fuse is the correct value and matches the circuit's capability (an unlikely mistake). You would have to examine the whole circuit including Cu thickness. 5A isn't THAT much current. And it probably has a 3 or 5 minute time-out.

 

[automatic2]

A smoothing capacitor will give you substantially more than the 2/10 inrush if not resisted.

 

[lcsjk]

No. 1. Your analysis is probably correct.
No. 2. You have a pico fuse, soldered in. It is not easy to replace so it needs to be heavily over-rated. Nuisance failures here translate to lost customers. Over rating is used to protect by taking the circuit off-line, and is not intended to protect the circuit board or parts from damage.

5 amps may be overkill, but probably doesn't hurt.