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Flywheels out of control

 
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Flywheels out of control
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BerlinNerfer
New Member


Joined: 05 Apr 2017
Posts: 7
Location: Berlin



Post Flywheels out of control  Reply with quote
Hi guys,

this is Chris. I am a fairly new member.

I have run into to me rather strange scenario where my flywheels start running as soon as any source of power is connected. But I am getting ahead of myself. I recently bought a used RapidStrike with the intention of modding it and using it in our office Nerf battles.

I got my setup from BSUK consisting of two hellcat motors for the flywheels and one honey badger for the pusher. Also I got Mike's "High Power Rapidstrike Wiring & Switch Upgrade Kit".

I went over the circuit over and over again and I can't figure out where I messed up. What is also weird is that when I first soldered it all together it worked fine. I was able to shoot some 30 darts and then I hit a runaway pusher. The cause for that was the mechanical switch underneath the pusher arm. I adjusted it and that problem was solved. Almost immediately the flywheels started running and wouldn't stop.

So to summarize, i connect power and flywheels start to run instantly. The mechanical switch is taped shut as to be open and not let any current through. I then push the trigger and nothing happens. I press and hold down on the flywheel switch (which has no effect on the flywheels whats so ever, they just keep on spinning) and then press down on the trigger switch, and the pusher arm starts moving as it's supposed to.

So I am a bit at a loss here. I think, the Mosfet could be fried. Meaning it is constantly closed and the connection between drain and source is permanent regardless whether the gate gets activated or not. If that is the case, I ask myself, what did I do wrong to create this situation. I use a 3s 2200mAh Grafine Lipo with 65c / 130c. could that be too much for the mosfet in any dimension?

I attached some pictures of the built. I appreciate your feedback.

Thank you
Chris

Edit: I am using this Lipo:  Turnigy Graphene 2200mAh 3S 65C LiPo-Pack w / XT60. I don't know the proper word for it, but when the hellcats are spinning, i can see a little lightening inside the motors. Is that normal?

wiring diagram i used:














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Thu Apr 06, 2017 8:19 pm View user's profile Send private message
SSGT
Nerf Addict


Joined: 12 Jan 2013
Posts: 1265
Fav. Blaster: 'Rave-N' CS-18, Elite Alpha Trooper (EAT) CS-12
Location: Durham



Post Reply with quote
EDIT

Images resized to 640x480 and/or thumbnails. Click to see full-size images. Please try to keep individual images to within 640x640 pixels, especially if there's a lot of them. For guidance on posting images, see the "How to add images to forum posts" thread in Q&A and New members. -SSGT



I'd agree and say that the FET has probably failed short. A similar thing has happened at least once before and it seems to happen only during/after cycling the pusher. The LiPo itself shouldn't be the cause as the IRLB3034 is rated to 40V across drain and source and 20V across gate and source whereas the LiPo only outputs 11.1V - the discharge rating won't matter as the pack won't force it's max burst current (supposedly 286A) through the circuit unless you short the motors.

This post from that previous thread outlines what I thought could have been potential issues on that occasion and how to rectify them in future builds. Since you didn't wire the switch the same way as SirBrass (it looks like you used a pull-down resistor across gate and source as per the diagram) the first bullet point doesn't apply (other than potentially adding a gate resistor to kerb inrush current) and I don't know whether or not you used diodes to reduce the voltage to the pusher circuit (from the photos it looks like you haven't) so the last bullet point may not apply either. Since it seems to happen during/after cycling the pusher, and unless both yourself and SirBrass zapped your FETs with ESD whilst handling them (or baked and damaged them whilst soldering), it suggests that it may be caused by a voltage spike across the pusher motor when you interrupt it by opening the circuit via either the trigger and/or cycle control switch. If that is the case then an additional diode across the pusher motor (as per the diode across your flywheel motors) should prevent future failures.

Alternatively ditch the FET and run mechanical switches only. If you do that I'd personally recommend swapping in a full size 21A switch for the flywheels although, that said, the old Cherry DC2 has been shown to take a fair bit of punishment (they've been used to switch 180 motor builds almost as long as the RS has been available).

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Thu Apr 06, 2017 9:13 pm View user's profile Send private message
BerlinNerfer
New Member


Joined: 05 Apr 2017
Posts: 7
Location: Berlin



Post Reply with quote
Hi SSGT,

thank you for the link to SirBrass' Thread and your feedback.

The symptoms are indeed very similar.

While I posted in the forum, I was also exchanging thoughts with Mike (BSUK). He's read your post and basically comes to the same conclusion.

My go to solution now is to replace the mosfet.

I am also considering adding a flyback diode onto the pusher motor as you suggested. Would I use one that is equal to the one I already soldered onto the flywheel motors?

Also I want to integrate some voltage drop diodes for the pusher motor. The observed ROF doesn't seem beneficial for my usecase (which is mainly indoors / office space). As not to run into any issues when the voltage of the Lipo itself drops the best place to put them in I concluded from SirBrass' thread is after the trigger switch.

maybe reduntant now, but for reference. This is a video of the setup in operation I made for mike: video

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Fri Apr 07, 2017 11:40 am View user's profile Send private message
SSGT
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Joined: 12 Jan 2013
Posts: 1265
Fav. Blaster: 'Rave-N' CS-18, Elite Alpha Trooper (EAT) CS-12
Location: Durham



Post Reply with quote
That diode should be fine. The spec of the flyback diode isn't critical as long as it can handle the peak current and it's reverse voltage rating is greater than that of the pack voltage. If the same diode can handle a pair of Hellcats it'll easily handle a single Honey Badger. If you wanted to add additional protection to the FET you could also (or instead of the flyback diode) add one or more zener diode(s) between drain and source and/or gate and source and/or drain and gate to prevent a high voltage building up across the FET pins (5KP18A zener diodes are commonly used across drain-source with IRLB3034 FETs in airsoft AEG circuits although, if you want to add one or two across gate-source to protect the gate from voltage spikes, you'd probably want something like a 5KP12A zener diode). A flyback diode across the motor should be enough to prevent the motor from generating any voltage spikes but the zener diode(s) would offer additional protection from other sources (such as ESD)/if the flyback diode somehow failed. Probably not necessary but definitely another option. You can go nuts and end up with something like this (from the equivalent circuit for a FET with integrated protection) but that really is overkill for our use.

Voltage drop diode placement shouldn't be as much of an issue for you as it could have been for SirBrass since you're using a 3S pack to begin with - if you use three diodes to bring that voltage down it'll still be around 9V (possibly as low as 6V with a very flat pack and a very high current draw through the diodes) and certainly never below the gate threshold voltage for a logic level "IRL" FET. If you think you might run a 2S pack for lower velocity games then it could potentially become an issue but you'd still need to have a pretty flat pack and be drawing a fairly high current through the diodes for the FET gate voltage to drop far enough to start causing a problem. If you did want to change that you'd effectively need to wire it up as per the BSUK diagram but move the acceleration switch before the diodes and effectively parallel to the pusher circuit (i.e. you could run the pusher without pressing the acceleration switch). Basically make this circuit with diodes replacing the red connection.

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Fri Apr 07, 2017 5:10 pm View user's profile Send private message
OldNoob
Foam Data Collector


Joined: 07 May 2013
Posts: 4823
Fav. Blaster: Xtreme 180 Rapidstrike
Location: In the Boonies



Post Reply with quote
Ditch diodes on the pusher. If you want slower ROF use a slower motor or develop trigger discipline.

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Big_Poppa_Nerf wrote:

Boff whats the damage? I have spent over 3 times my Nerf budget this month already. Part of me is trying to be a responsible parent/husband/house owner. The other half is just says 'Ahhhh, Screw it!'.
Fri Apr 07, 2017 5:30 pm View user's profile Send private message Send e-mail
BerlinNerfer
New Member


Joined: 05 Apr 2017
Posts: 7
Location: Berlin



Post Reply with quote
@SSGT Ok thank you.

If as you say, that when using a 3s pack the voltage should pretty much always be high enough for the fet, i'll put the diodes as per BSUK wiring after the fork to the flywheels and before  the fork to the mechanical switch.  
I found those diodes recommended in a different thread: BY500-800, Datasheet.
Do I read it correctly, the have 1.3v forward voltage drop? I would then put in two of those. Voltage then is still higher then the recommended 6v for the Honey Badger but still high enough for the mosfet to properly function.


@oldnoob

yes you are right. And i definitely will have to develop a good trigger finger. edit: A couple of reasons, why I am gonna stay with the Honey Badger. First off, I already have it, no need to buy another motor. Second, it's designed for 6v. With the current setting I run it at double the recommended voltage. Lowering the voltage will hopefully result in a longer lifespan. Another reason I already mentioned, I don't need that ROF. Last, and maybe most important in terms of functionality, I want to realize cycle control with life center. At the speed the motor is currently running it is really hard to accomplish.

Does this reasoning make sense to you? I am open to your feedback.

I have since read, that it is generally hard to realize cycle control with a 130 motor and easier with a 180 because their torque brakes the motor rather effectively. I didn't have that information when I ordered the setup i currently have.

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Sat Apr 08, 2017 12:08 pm View user's profile Send private message
SSGT
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Joined: 12 Jan 2013
Posts: 1265
Fav. Blaster: 'Rave-N' CS-18, Elite Alpha Trooper (EAT) CS-12
Location: Durham



Post Reply with quote
I honestly wouldn't read too much into the 6V "recommended voltage" for the Honey Badger. Ryan from MTB always intended for it to be used primarily as a very high ROF 3S pusher motor and to replace the "Ranson" motor (specced at 68000RPM @ 12V) which was a popular 3S pusher in Oz but was becoming harder to find. The fact that it could be used as a 2S flywheel motor was always a secondary concern.

BY500-800 diodes should be fine for a single Honey Badger. In fact any of the BY500 series should work, even a BY500-50, since you're not going to be applying more than 50V peak reverse voltage and especially if you'll be adding a diode across the pusher motor to kerb any voltage spikes. The forward voltage drop is rated as being less than 1.3V, in reality it'll probably be around 1-1.1V in use (it'll only be as high as 1.3V at a current draw of around 20A). If you did still use two that's still a 2-2.2V drop which would still bring you down to a nominal 8.7-8.9V. Even at stall current, with a practically flat pack, that shouldn't drop below 6.4V so the FET's gate voltage should always stay healthily above the max threshold voltage in use. That said, whilst the FET would be fine if you did stall the pusher, if it draws stall current for more than a second or two you'd likely find that one or more of the voltage drop diodes would fail. You could protect them by adding a PTC (or physical e.g. blade) fuse somewhere between them and the motor however, if you did use a PTC fuse, it'd be a good idea to keep it after the acceleration switch otherwise you end up with a potential divider which would also act to reduce the voltage at the FET gate (a PTC fuse is effectively a temperature controlled variable resistor) - this might not be too much of an issue, as the PTC fuse should trip fairly quickly and remain relatively low resistance the rest of the time, but it's something you might need to think about and will affect the suitable range of pull-down resistor values (you'd probably still be OK with a 10kΩ pull-down resistor).

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Last edited by SSGT on Sat Apr 22, 2017 3:56 pm; edited 1 time in total
Mon Apr 10, 2017 5:37 pm View user's profile Send private message
BerlinNerfer
New Member


Joined: 05 Apr 2017
Posts: 7
Location: Berlin



Post Reply with quote
Hey SSGT,

happy easter and thank you for your support so far.

After what was posted last I've done some more reading around here and i have a better understanding on 130 motors, torque, braking, pusher overrunning and the sense or nonsense of lowering the voltage. At least I think I do Shocked .

From what I could gather, the Honey Badger, by having very little torque, will most likely overrun because when no longer supplied with current there is too little torque to effectively brake it down in time before the switch is released again. This can be counteracted by moving the mechanical kill switch a litte to the front, so it engages sooner and disengages later. That is if I keep a this cycle control circuit alive.

Otherwise it would probably make sense to get a motor with higher torque and lower rpm.

In terms of flyback diode, could I use a BY500-100 as a flyback diode, or would that be to little to compensate for voltage spikes? The two hellcats have a BY500-800 as flyback diode soldered on i think.

The new mosfet should have arrived by the time i get home tonight. can't wait to get the RS running Smile

Cheers
Chris

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Mon Apr 17, 2017 1:54 pm View user's profile Send private message
OldNoob
Foam Data Collector


Joined: 07 May 2013
Posts: 4823
Fav. Blaster: Xtreme 180 Rapidstrike
Location: In the Boonies



Post Reply with quote
Honeybadger is trash. You won't find a single decent player in the UK using one.

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Big_Poppa_Nerf wrote:

Boff whats the damage? I have spent over 3 times my Nerf budget this month already. Part of me is trying to be a responsible parent/husband/house owner. The other half is just says 'Ahhhh, Screw it!'.
Mon Apr 17, 2017 2:52 pm View user's profile Send private message Send e-mail
SirBrass
Junior Member


Joined: 06 Mar 2017
Posts: 55
Fav. Blaster: 180 Stryfe & 180 Rapidstrike
Location: California, USA



Post Reply with quote
While it sucks to see happen, I'm glad I'm not the only one with the issue.

Perhaps flyback diode across the pusher motor should be part of the standard wiring diagram.

And ON is right on the hb. You could rectify the situation by going with a rhino as the pusher until you're ready to out in a 180 in that spot. It won't give you the insane rof of an hb, but trust me, at Nerf ranges, the rof a rhino provides will allow you both sufficient control and rof to throw a small dart spread that will usually bracket and tag an opponent within your darts' effective range.
Mon Apr 17, 2017 10:56 pm View user's profile Send private message
BerlinNerfer
New Member


Joined: 05 Apr 2017
Posts: 7
Location: Berlin



Post Reply with quote
Hi there,

So I think I got the situation under control. After frying two more mosfets, I added a flyback diode (BY500-800) direktly onto the pusher motor. Since I have emptied a couple of magazines and it keeps running strong. So far I am going to assume, that this is the solution, though time will tell I guess.

Edit: Also I wanna add, I had no runaway pusher anymore. I was able to permanently position the "life center switch" forward enough so it fulfils its designed function. I had to make room for the metal lever of the switch to move freely up and down by removing some material inside of the RS shell just underneath the opening for the pusher itself. So the opening through which the pusher pushes into the chamber has gotten a litte bigger. If anybody is interested I am happy to post some pictures.

I run into a different issue with the pusher box, but that has nothing to do with the circuit and hence I don't wanna discuss it here.

I wanted to thank you guys for very productive feedback and also for some, that send me off following a few different ways of thought. And also Mike from BSUK who showed more patience with me than I deserved.
So thanks Smile

On a side note, the gun probably is not gonna make it into battle at the office. After a demonstration yesterday at the office it was rated as too high powered for office wars. In fact all battery powered Nerf guns will be removed. Also our Ops got a little nervous with the fire hazard of the lipo and asked me to keep the RS at home.
I did earn the respect of our tekkies though Smile


Cheers
Chris



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Sat Apr 22, 2017 3:15 pm View user's profile Send private message
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