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        Build Log PLD-SFUSE1: Servo fuse, filter and signal booster

#1 PLD Nov 03, 2011 10:13 AM

PLD-SFUSE1: Servo fuse, filter and signal booster
The SFUSE sits between the RX and the servo, it's a simple device which can be equipped with 3~4 functions;

* Filter: Capacitor decoupling/filtering, helps abate the glitches when the servos jump around
* Signal booster: Using a small microchip, the signal from the RX can be boosted up to stop it fading along long servo lines
* Fuse: This is a double-option feature, I can either fit a blow-once fuse (field replaceable) or a resetting polyfuse; both have their pros/cons.
* Fuse-blown indicator

The PCBs for the SFUSE have been sent off for prototype production, I'm expecting them back in 2~3 weeks.

The board is moderately small, I deliberately over-sized it a bit compared to what I could have shrunk it to simply to make handling a bit easier and prevent the replaceable fuses being over-exposed to side knocking; about 0.85 x 0.42" in size.

#2 modbuilder Nov 03, 2011 06:58 PM

I am really jazzed about this project! I can't even count the number of pilots I know who are pretty sure they've lost (one or more) aiplanes due to ESC shutdown from an overload caused by a stalled or shorted servo. I lost a super sweet 48" E.F. Edge 540T at the eNall in Sept from a frozen up aileron servo resulting in a total control failure. I might have lost the plane anyway with one aileron servo frozen up, but I really think I could have landed it without damage if I'd still had control of the other servos and throttle. Cannot understand why it's taken this long for someone to offer this kind of product. If you're taking pre-orders -- I'm camping out to be 1st in line.

#3 PLD Nov 03, 2011 07:11 PM


That did remind me, I forgot to add another item to the feature list... the "blown fuse" indicator (LED).

Not currently taking preorders, but I'll be give you "first dibs" on the units that come out. I can't be sure how many boards I'll get back from the PCB factory, but usually for boards this small I will end up with at least 10, sometimes 20. If things "take off" and there's a lot of demand, it only takes another 2 weeks for me to organise a full batch of boards (100~200).

The fuses are planned to be "TR5" types, which look a bit different to the normal glass/ceramic cylinder units, however they more compact and easier to field-replace on small boards like these.


#4 modbuilder Nov 03, 2011 07:52 PM

Just a heads up -- I'm headed over to the crash discussion board and cross-post a link to this thread. Anybody who's suspected a servo as the root cause of losing an airplane, should come follow this project. I also intend to suggest that a couple of i-net hobby stores come check this out. I know you're a direct sales guy, but a little broad exposure couldn't be a bad thing.

My enthusiasm for this product is undoubtedly the death knell for it, but I think it could save a lotta airplanes.

#5 PLD Nov 03, 2011 08:08 PM


All exposure is welcome :)

I do actually service other stores with my other products, though as a "general rule" I tend to try to stick direct sales by default.

I won't worry about your enthusiasm being a death-knell, my own lack of marketing skills tends to be a vastly more powerful force to contend with ;)


#6 PLD Nov 06, 2011 04:41 AM

Just received notice, PCBs have been finished and now shipped.

#7 PLD Nov 18, 2011 03:32 AM

2 Attachment(s)
Here's the first pictures of the SFUSE1 unit.

I've put in the signal buffer unit as well which will pull low signals up to the full rail voltage. The blown-fuse LED comes up bright when the fuse is blown (and the servo is connected).

The one thing that's not quite "correct" in these photos is that the fuse-holder isn't the correct unit (yet). I ordered a batch of proper round fuse holders but they had not arrived as yet, so I used a pair of machined-pin-headers just to test/illustrate.

So, I suppose now we're wondering what the costs are... :)

I think to start with, while I'm dealing with the low quantities, I'll put them up for $13.95

If I can get to the 100+ sort of numbers it'll be able to drop a little more. The biggest costs are from the PCB, since most PCB makers hate boards smaller than 1x1", and this is quite a bit smaller than that - subsequently they penalise me with more fees.

The fuses can be obtained for under $1 at most places and there's plenty of range from 500mA to 6A and beyond (I recommend not going beyond ~6A, if your servo pulls that much you probably should consider a whole different level of wiring).


#8 PLD Nov 18, 2011 04:21 AM

1 Attachment(s)
Here's what the holder should look like.

#9 modbuilder Nov 18, 2011 07:27 PM

Looking good Paul
Don't know about buying 100 of 'em, but I'm ready to pull the trigger on 10 as soon as you're happy with them. How 'bout a link to a source of those easy to find fuses (preferrably in the states). I'm guessing a 0.5 amp fuse for a factory rated 0.25 amp stall current servo-- whatdayathink?

Does Australia have a celebration akin to our Thanksgiving?

#10 PLD Nov 18, 2011 07:42 PM


No, Australia doesn't have a "Thanksgiving", we've got plenty of other holidays though, but nothing that tears and ties the country together the same way yours does. For us though the Christmas period is our major "Lie around and do nothing" time, very hard to do anything productive.

Getting TR5 fuses shouldn't be too much strife - your domestic "Digikey" suppler has plenty of them on hand - http://search.digikey.com/us/en/cat/...1?k=TR5%20fuse

... not sure though if places like RadioShack or hardware stores would have them, they're not quite that common :p

I'm still trying to decide what the right amperage choice should be relative to the stall current of the servo. Since if in flight, the servo stalls permanently, do you want that to blow or do you want it to continue getting power? It's a tough call. I suspect maybe some experimentation is in order.


#11 modbuilder Nov 18, 2011 09:38 PM

Was just now looking at the "Average Time Current Curves" on the data sheet for a 0.25 amp fast blow fuse at: http://www.littelfuse.com/data/en/Da...e_Fuse_370.pdf
with a thought toward using those operating curves to pick how much overhead I want the servo to have. That .25 amp fast blow fuse will allow 10 seconds of 0.6amps before it blows, but only 0.1 second at 0.8 amp. So just using thumb windage, it seems to me that 10 seconds is about all I'd want to allow a servo to pull twice it's rated stalled current without taking the circuit out. And it doesn't take much more than that to take the circuit out in a hurry. A short circuit would be shut down quickly. A stalling-but-still-fighting servo will be given a little time to work it out before it's taken out. I think you're right on with the trial and error aspect. Sounds like a decent topic for a RCG thread for all those guys who want to start eliminating this cause of crashes :)

#12 modbuilder Nov 18, 2011 09:43 PM

One of the uncertainties is how accurately the manufacturers test/report their stall current. The majors I probably trust to be pretty useful. The cheap knock-offs, not so much.

#13 PLD Nov 18, 2011 10:34 PM

Yes, knowing the real currents is a lot nicer than relying on printed figures with most of the "generic" brands.

I do have a project that I'd love to do, but it's one of those things that's going to cost me a thousand or two to develop. I want to create the ultimate servo tester that'll do complete load/speed/current graphs from no-load to stall-load. The painful bit won't be the logging, rather it'll be trying to invent a suitably generic load/resistance generator for the servo arm. The expense will be in the machining/moulding of those mechanical bits.

#14 PLD Nov 19, 2011 01:44 AM

I've put the available stock for what I can sell now up on the NQRC site - the proper fuse holders and some sample fuses will arrive on Monday all going well, else a couple of days later.


I think it might be useful to others if I carry a small selection of fuse sizes as well.


#15 modbuilder Nov 19, 2011 05:56 PM

Ordered 8 SFUSE's this morning. One for each of four channels on two planes. Would have ordered a couple more, but I think I wiped out your shippable inventory.

Now I'm looking for a SLOGGER to guide decisions on the fuse size, but I'm pretty sure I'm going with 0.25 amp fast blow on the initial fuses until I get data from the SLOGGER. Probably will start up with SFUSE on one channel and get some experience with it, then add a second one, etc. Want to sneak up on this new approach to protecting the plane. Lovin this.

This'll get me through two airplanes 'cause I have CC 10-amp BEC's to go in them, then I'll be ready for a Powerbus - A4 for the next flyer.

You do realize don't you that this is rapidly depleting my Christmas funds :eek:

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