EXP Bus - voltage drop across polyfuses

[Neal@F14]

The RV-6 I recently bought is built with the EXP Bus II system (which I'm really not too thrilled with), and after installing my Garmin 196 GPS into the panel with a new Airgizmos dock to replace an old Bendix monochrome SkyForce unit, I noticed that the avionics bus connection I used to power the GPS with, seems to be showing almost 1 full volt of drop between the battery and the "AB2" terminal on the EXP Bus board that I am using to power the GPS. The built-in voltage measurement you can put on the screen of the Garmin 196 has generally been dead-nuts accurate, I've checked what it displays against my Fluke DMM in the past and they both matched to the tenth of a volt.

Has anybody else out there who runs the EXP Bus also noticed a significant voltage drop across the polyfuses? I haven't yet tried measuring across all the polyfuses on the EXP Bus circuit board, since I just noticed the low voltage on the GPS display first, and I had already reassembled the EXP Bus into the panel. I'll do more testing this weekend, but it sure seems to me that a 0.8-1.0 volt drop in normal operation for a GPS that draws less than 1 amp, and is connected to a supposedly 3 amp polyfuse is too excessive to be good.

[Dean Pichon]

I have an EXP II bus in my -4 and have had no problems with it. The voltage drop across the fuse should be almost zero. I checked several of mine when I installed my Pmags and found no significant drop, though I do not recall a specific value. Several times over the 10+ years I have been using the EXP II bus, I blamed it for nuisance problems, but in the final analysis, fault lay elsewhere. Along the way, I received excellent technical help from Tim Fontenot at Control Vision. I suggest you talk with him.

To help de-bug, you may consider moving the GPS load to a different polyfuse by just rearranging the Fast-on tabs in the EXP bus. I used the AUX switch/fuse several times for de-bugging.

Good luck,

[Neal@F14]

There's still one more unused 3 amp polyfuse circuit (AB3) on the board's avionics bus. I was going to try that approach next time I go out to the hangar. Maybe it's just got a bad polyfuse, or it's a symptom of the lower-amperage polyfuses. I've been googling for "polyfuse voltage drop" today and have found a lot of references on the web about low amperage polyfuses in computer USB power circuits that are dropping the 5v down to 4v and certain USB devices not running right at the reduced voltage. If it proves to be the case that the low amperage polyfuses just have a naturally high volt drop, then I'll resort to putting an inline normal fuse in the GPS power line and connect it to one of the unused higher-amperage AB terminals on the EXP Bus circuit board. I'd really like to be able to use the voltage display on the GPS to monitor the health of the ship's charging system, like I did when I had the 196 in my old Cherokee. Just plugged into the cigarette lighter outlet, I could easily see a nice, happy 13.8 volts displayed on the GPS screen at normal cruise RPMs to let me know my alternator was doing a good job. I do have the EXP Bus display panel in the RV-6, but the LEDs on it that show a pseudo-ammeter are pretty much worthless and are so dim, you can only see them in the dark.

[AZtailwind]

Originally Posted by Neal@F14

Just plugged into the cigarette lighter outlet, I could easily see a nice, happy 13.8 volts displayed on the GPS screen at normal cruise RPMs to let me know my alternator was doing a good job. I do have the EXP Bus display panel in the RV-6, but the LEDs on it that show a pseudo-ammeter are pretty much worthless and are so dim, you can only see them in the dark.

I have had problems with the Master relay supplied on the board until I soldered the jumper: removed it out of the system and allowed a real solenoid do the work. We also replaced Q1 IC on another board that failed. We have 500 hours on this system and I like it if nothing else it has the over-voltage crowbar circuit built in for spikes. I haven't seen a full 1 volt drop across the poly's yet. Wouldn't doubt it could happen though and easy to test the other outputs to see if it's only one output trace or a full board/gremlin problem... FWIW