HOWTO Wiki Fuse Mod
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Introduction
When the blade (or any brushed heli) crashes and stalls the motor, it creates a stall current. What this does is send too much current through the ESC, and can end up blowing it out, then you are stuck either replacing the mosfet that drives the ESC (which requites very good soldering skills and the right iron), or buying a new 3 or 4 in 1. If you just add a fuse to the line that is below the max sustained current rating of the mosfet, then the fuse will fail first saving your ESC. You do need to make sure that you properly select the fuses so that it blows before the ESC does, and also so that it does not blow when you do not want it to, such as regular flight. You also want to make sure to use the spade type automotive fuse because it is designed for vibration, where as a glass fuse might trigger on vibration alone. You can also use other fuses that are not sensitive to vibration, but I chose the mini spade connectors, like many others, because of their weight, size, availability, and low cost.
If you decide to go with an alternate tail, such as DD, the dual tail, the ducted system, ect then you will really want to make sure they are fused because they draw extra current. The one exception to this might be brushless. If you go brushless, you can fuse it, but usually a good ESC will have features such as current limiting, so you don't need a fuse. If you do a fuse though on a brushed system, make sure to put it on the power leads to the ESC and not on the leads from the ESC to the motor.
If you only want to do the fuse mod on the tail, you can have pretty good protection on the main motor by just installing the Tips Wiki Paper Clip Sheer Pin Mod because it will sheer when you get in a crash, and then the motor wont be bound against anything because the main shaft will just spin inside the head and it wont be able to draw a whole bunch of current.
I measured how much current the different motors take with my ICE charger on the motor break-in function, but it only goes up to 8v which is quite below the 11.1v nominal of the 3s lipo. It should give you a good ball park though of what the different loads on the motors require. The stock motor with an 8T pinion flat blades and full collective draws 3.67A at 8v and 1.71A at 0 pitch. A higher T pinion, and a higher voltage should increase the amp draw motor, and symmetrical blades should decrease them. The stock tail motor draws 0.91A, and the dual tail draws 1.26A. The resistance of the stock main motor is 0.6 ohms, the single tail is 1.7 ohms and the dual tail is 1.0 ohms as I measured it. The max current draws of each system are 20A, 7A, and 12A @ 12v, anyone of which could melt the ESC that powers them in a stall condition.
also add gws dd current draws - resistance is 1.2 ohms
There is also a section at the end dedicated to alternate versions of the fuse mod, such as adding resistors, diodes or capacitors.
This mod with all the heat shrink solder and fuses ended up only adding 2g to the total weight of my heli.
This is the IRF7451 Datasheet, which is probably the mosfet used in the ESC on the 3 or 4 in 1. It doesn't really make a lot of sense to me because it says the max continuous current is only 3.6A. The tail has one of these mosfets for 3.6A max current draw, and the main has 2 of the stacked for 7.2A max current draw. That doesn't sound quite right, so if anyone else knows how to read that sheet, please contact me.
Parts List
or
- You can just reuse the wires to both motors that you already have on your heli
- 7.5A AST mini spade automotive fuse for the main motor available here
- 2A or 3A AST mini spade automotive fuse for the tail motor(s) (depending on what tail setup you will be using) available here
- Heat shrink tubing and something to shrink it with (I used 5/32" heat shrink, and a butane flame stick)
- 4 small spade connectors to match the fuses like this, or a pre-made 90 with an insulation boot
- 1 or more zip ties to hold the wires back and keep them from getting caught in something, also to hold up the main fuse above the main motor (see final pic)
If you don't want to use the connectors, you can also check out an alternate fuse mod with the fuses soldered right in line here, and is done by SSG Scott of RCgroups. The main reasoning for that is so that the connector doesn't oxidize or something, and create a bad connection or so the fuse doesn't come out somehow. The one downside to this mod is if you are out flying and don't have a mobile way to solder, then you'll be packing up and heading home.
Here are the two wires that you get in the motor wire set, the mini spade connectors with crimp on ends, and the fuses. The reason there are a few fuses is to hold them up so you can see the sizes, and also for spares if the fuses pop.
Some more supplies and tools: heat shrink, zip ties, razor blade, scissors, tape measure, and a clamp.
Instructions
This picture has a lot of information all crammed into one pic. You're going to need to cut off the insulation on all of the spade connectors. The way I did this was to hold the metal end with the pliers being careful not to smash it, and using the razor at the top to cut through because there is a split in the metal there and the razor can go all the way through the plastic.
You're also going to want to fold over the connectors as shown after you get the insulation off. It's really hard to describe how to do it, but really easy to do. I'll try my best to explain it, but you'll probably need the explanation and the picture. One of the connectors just needs the crimp on connector cut off, one of them needs bent 90 degrees along the wider side (the one on the 7.5A brown fuse), and 2 of them need to be bent to 90 degrees along the shorter side for an inline fuse for the tail motor (the ones on the purple 3A fuse). If the connectors crack while you are bending them, just reinforce them with solder being careful not to let the solder run up into the connector where the fuse is going to go. Also widen the connectors if you need to so that the fuses will slide in fairly easily, but will still stick firmly in place.
This picture also shows that you need to cut one of the wires (can be either the red positive wire, or the black negative wire. The reason that works is because unlike a car or something similar, it does not have an electrical connection to the frame, both of the electrical connections are isolated, so if either one breaks then no current flows) at 3" in length including the connector. This will put the tail fuse right at the base of the main shaft. Also strip the ends about 1/8" and tin the edges (that means just get the solder to bond to the copper wire before you try to bond the wire to something else). The spade connectors also need to be tinned at the crimp on connector, basically just fill the hole with solder making sure not to let solder flow into the connector where the fuse will go.
This picture shows the spade connector that was attached to the 7.5A fuse, and how it attaches to the main motor. To do this you need to take off the two capacitor ends that link to the negative pole and then solder on the connector, and then solder the capacitors back to the spade holder.
The easiest way I found to solder this is once the two ends of the caps that were attached to the negative pole are moved out of the way, touch the iron to the negative pole just enough so that it flows (making sure to watch the temp so you don't melt anything inside the motor or the plastic that holds the pole in place), then quickly take the pliers and the spade connector bent at the 90 along the wide side. Put it on top of the pole, and touch the iron to the side of the crimp on part of the spade connector until the solder flows and it slides over the pole. Pay real close attention to the heat to not over heat the motor or the plastic, and also to the angle that the connector needs to be so that the canopy can still be put on, and the fuse doesn't get in the way of the servo.
This picture shows the capacitors soldered back to the pole on the motor. It's also another view of the angle the connector needs to be at in order to hold the fuse, keep the servo from binding, and to be able to put the canopy back on.
In this mod I also included the mod to prevent a main motor wire short just because it was easy to do while I had it apart and also it's a good mod to prevent damage to the 3 or 4 in 1 from shorts that wouldn't be protected by the fuse. That is available here HOWTO Wiki Prevent Main Motor Wire Short
This picture also shows the spade connector (the one with the crimp end that has been cut off) soldered to the negative wire that goes to the main motor. You'll have to watch the orientation of the end of the connector where the fuse goes in order to make it go the right direction (the connector end needs to be on top and the open end towards the wire, which is opposite of how they are normally connected with the connector facing away from the wire). Just solder the lead to the back, and be real careful not to let the solder flow in to the other side of the connector where the fuse will go.
This picture shows the main fuse attached after I have added heat shrink. It's best to let the heat shrink go all the way over the end of the connector, and then cut the heat shrink down about as close as you can get with a pair of scissors. When you heat the shrink, then it will cover up the end of the metal, but still leave enough room for the fuse to go in. Also when I was heating the shrink that goes on the wire (vs the one on the pole) I left enough room for it to hang over the end and then I just heated it up and then pinched the end shut with my fingers or the pair of pliers, whichever is easier which makes it so the shrink bonds together at the end and you don't get something poking through the back side. Also you may have to put a small slit into the heat shrink that goes over the 90 in the spade connector to get it to go over all the way. Just make sure that it is flat when you heat it up, and of course not to get too much heat on the motor.
This is the final picture of both fuses. Make sure on the main fuse that you position the zip tie underneath the fuse to hold it up and keep the stress off that solder, pole joint. If it gets too much torque to it, it could move the brush on the inside of the motor.
Also on the tail fuse I divided the heat shrink in to 3 different lengths to get it around the 90. What I did was cut a short length and heated it right on the joint, and then I got the other two lengths and slipped them over the top and heated them. I had to cut a few small slits in the first piece of shrink to get it sitting on the 90.
Make sure to put an extra twist in the wire that goes between the 3 or 4 in 1 to the first part of the fuse so that it twists the fuse back into the frame. With this setup I haven't had a problem with it shifting in flight, although if you do extreme 3d you might want to think about securing it better. Also the stock canopy fits on this mod very well =) and now you can change out your fuses at the field without a soldering iron.
This pic shows the fuse mod with the stock canopy and configuration.
Other things to consider
Some other things that people have done in combination with the fuse mod are putting an inline resistor, diode, electrolytic capacitor. These are all to reduce the stress on the motor somehow in order for it to last longer. Here I will try cover most of the pro's and con's and the info you will need to know before you get to installing them or trying them out.
Another option to reduce tail motor failure is to look at replacement tail options
HOWTO Wiki Choosing a Tail Drive Setup
Background
The ESC's on the blade are just your regular so8 n-channel mosfets. The way they work is, for example, if your heli's tail was at half throttle, it would make sense that the motor was running 50% throttle 100% of the time, but this logical way just isn't the way these mosfets work. They actually provide 100% of the power 50% of the time, so you end up with voltage spikes, and those spikes end up being hard on a motor because it revs then coasts and repeats every couple hundred microseconds. The idea is if you get the pulse fast enough and short enough, then you won't even be able to tell the difference, kind of like still images on a TV coming so fast that you cannot even tell they are not actually moving. With all of these mods it is a good idea to install them on the motor side of the fuse, otherwise you could end up in an over current situation. For example with the resistor that is 2 ohms and between the ESC and the fuse on the 3s setup at full right rudder could by itself draw about 5.5A, and if you add the 3A it takes to make the fuse fail, you could be looking at 8.5A before the fuse would fail, but likely the mosfet would fail before the fuse would.
Electrolytic Capacitor
The idea of the capacitor is to smooth out these pulses by storing and releasing the electricity as needed by the motor in order to make it last longer. This idea is very good in theory, and can work in practice too. The only trouble with this one is that it can cause excessive power draw. SSG Scott over on RCgroups has done some tests and finds that on the stock single motor tail that normally draws the 0.91A or less that with a 220uF to a 470uF electrolytic capacitor would pop a 5A fuse in flight. The caps can also get very hot and vent or explode, and could result in an open circuit failure which would leave you piro'ing back to earth. The main draw back though is the excessive current draw on the already taxed ESC.
Various Resistors
The other thing is that the stock n30 motors are only rated for 6 or 7 volts, and with a 3s lipo, on the high end, there can be 11v or more going through the motor. Based on that you can see why that could make it fail. Some people have come up with a mod involving a 2 watt, 2 ohm carbon film, or sandbar resistor. These actually do work quite well in taking down the voltage, and can get a drop of around 2v or so. The thing to be concerned about is they make the duration longer in the PWM as seen in this O-scope. If you look at the second one and compare it to the first, you will notice that the spikes have larger plateaus. What this means is to keep the tail from drifting and oriented at the lower voltage, you will have to apply a greater duty cycle by turning up your pot, trim, or gyro. So instead of running at 100% power 50% of the time, you would be running, for example, 100% power 60 or 65% of the time. The extra duty cycle means more current, and since it isn't spinning the blade more or faster, it's just going to heat, both in the ESC and the resistor, which can be quite significant. This would reduce flight time, but more importantly resistors fail in an open circuit situation which would leave your heli spiraling out of control. Resistors wouldn't necessarily fail, especially the more rugged sand bar resistors (which would add more weight than the carbon film). Basically what happens with this mod is you save the tail motor from burning up, but instead get extra heat from a resistor, and higher current draws to the ESC, which would make the 3 or 4 in 1 more likely to fail. If the heli is properly fused, that would protect the 3 or 4 in 1 from going out, but because of the higher current draws, it could be feasible to pop a fuse in regular flight.
The how to for the fuse mod with resistor how to is available here
This concept with a carbon film resistor and a zener diode is commercially available here
If you had a 2 ohm resistor in series with the tail motor and said that 0 was your min volts and 11 was your max volts, and 5.5 was your average (over a period of time, because it's actually going to be small blips of full current full voltages), you would end up with 0 watts, 32.7 watts, and 8.2 watts TDP of the resistor with the resistance of the tail motor added in (P = V^2/R, if I figured it right).
Diode
A diode might be a good option, because they usually provide about a 0.7v drop, which will make it easier on your tail motor, but not too hard on the mosfet. The drawbacks would include the extra heat, an extra load on the ESC, and the diode can fail in either an open circuit or a short and if it fails in the short it will have the same drawbacks as the resistor. Overall this is could be a good fix to lower the voltages to the tail and keep them within acceptable limits for the ESC. Just make sure to get a diode rated higher than your fuse. So for example with a 3A fuse, you could go with a 4A diode.
mrasmm
PS feel free to PM me either on RCU or RCgroups if you have any questions or comments
