Hermes F3, wrong part...

User avatar
Posts: 1567
Joined: Fri Mar 10, 2017 1:47 pm

Hermes F3, wrong part...

Postby w-u-2-o » Tue Apr 11, 2017 8:59 pm

I received this email from someone who asked me to post it for them, as they are a bit shy about doing it themselves. I don't have an ANAN-10 and therefore can't report on any instability, however his findings are certainly very interesting--73, Scott


Just a heads up. I finally tracked down a hardware instability in my ANAN-10 (TAPR Hermes board/APACHE 10W PA/Case). Symptom: random crash after prolonged period of operation. It seems that the Thermal re-settable "fuse" F3 on the Hermes board is the wrong value. The schematic shows a 2A "fuse". The BOM indicates a part# of MF-MSMF110/16-2CT-ND (indicated as a Bel Fuse part but actually Bourns). The measured current in F3 is ~1.6 amps. The part is spec-ed at Ihold=1.1A and Itrip=2.2A. The part is operating in its transition zone!

They seemed to have chosen it for its trip point and didn't look at its hold rating. My thermal camera shows it running at ~90C! Not good.

I replaced it with a 2A hold part and all is good.

This is just some info that you may want to keep in the back of your mind, if anyone is experiencing random issues with their ANAN-10.
(440.28 KiB) Downloaded 124 times
Sheet5.JPG (371.67 KiB) Viewed 2012 times
User avatar
Posts: 2
Joined: Sun Apr 09, 2017 7:35 pm
Location: Fort Collins, CO

Re: Hermes F3, wrong part...

Postby jsm » Thu Apr 27, 2017 4:31 pm

The problem is that there is no 2A hold part with the same itrip value (probably not possible). The 2A hold part has a 4A trip. So that part may run cooler and work great, but it also may not be providing the needed protection. In the HPSDR email list Phil Harman mentioned that he replaced F3 with a wire jumper. The question is whether the suggested replacement part is actually any better than a piece of wire, i.e. will it trip and protect the circuitry it is supposed to protect, or will the damage be done before it trips, therefore behaving no better than a piece of wire.
Posts: 1
Joined: Fri Apr 28, 2017 2:59 pm

Re: Hermes F3, wrong part...

Postby K3IT » Fri Apr 28, 2017 7:45 pm

I used PTS18126V300 fuse for F3 (3A hold, 5A trip). LT3680 datasheet seems to indicate that it should deliver 3.5A output, good enough to handle PHY 1Gbps 500 ma increase on the 5v rail? May be I'm overlooking something..

Return to “Everything Else: Antennas, Relays, Switches, Power, Grounding, Cooling, etc.”