26 replies to this topic

[oddron]

I wanted to modify my Stryfe, but I didn't want to wait to order a FWC, so I decided to make my own. I tested this in the superstock rounds at APOC.



In testing, I noticed that my darts have a tendency to curve or even whirlybird. I also lost one dart when the head was ripped off, leaving the remainder to jam the blaster. I am going to add a guide to help center the darts as they enter the FWC.

As for the rest of the blaster, I used a relay to run the motors, and I used copious amounts of duct tape to hold the wiring. The relay coil is controlled by the stock trigger switch.



I was advised to replace that relay with a transistor and internalize my wiring. The puzzle is finding a transistor that won't require a heat sink, and I think I have a solution.  After I test my blaster with a transistor, I'll post an update.

Spoiler

I was advised to use an N-channel MOSFET. To find out how hot it will get, I need three pieces of information: the internal on-resistance, the thermal resistance, and the load current.

I am currently looking at the AUIRFB8409 in a TO-220 package, which has an internal on-resistance of just 1.3 mΩ.

http://www.mouser.co...g4noEfhEQ3IYw==

The TO-220 package, with no heat sink, has a thermal resistance of 62.5°C/W.

To be paranoid, I'll calculate the temperature at 48 A continuous. My stryfe has 2 hellcat motors, and their stall current is 24 A each. At 48 A, this MOSFET will generate 3 W of heat.

With no heat sink, 3 W will make the transistor 187.5°C warmer than the surrounding air. If the surrounding air (inside the blaster) is 30°C, then the transistor will heat up to 217.5°C. Not surprisingly, it will burn out at this temperture. In other words, if the flywheels are completely jammed, and I hold the trigger anyway, then the transistor will burn out.

Realistically, I'm only drawing 48 A momentarily when I start the motors, so the transistor will not get as hot as my calculation would suggest. To calculate how hot, I would need to know how long is "momentarily" and I would need the thermal mass of the transistor.  Alternatively, I could just test it and find out.

Finally, MOSFETs can be safely operated in parallel, and parallel configurations are regularly used in power supplies to produce higher output current. Sticking with my paranoid scenario of 48 A, I could use two MOSFETs in parallel. If they're perfectly balanced, each one would generate 0.75 W of heat and become 47°C warmer than the surrounding air. The on-resistance of a MOSFET increases as it warm up. After accounting for this fact, the transistors will reach 91°C each, which is well below their burn-out temperature of 175°C. Even if the motors burn out, as long as they don't short out, a parallel pair of transistors would survive.

 

[dudifer]

so, is it a canted FWC? that helps with accuracy. (with elite darts you need all the help you can get.)

if so, can i have one?

[CCBall]

Was it canted? If it was, try a brass dart guide.

[DjOnslaught]

My big question is why did you need the relay? Was it because of the motor being a size above 130 (appear to be 180s maybe) or is it cause of your battery?

[oddron]

so, is it a canted FWC?

Was it canted?

Sadly, no, it wasn't canted. As I continue to develop the design, I plan to experiment with canting angles.
 

My big question is why did you need the relay?

I found a relay before I found a high-current switch. The motors (not pictured) are Hellcats. I am going to replace the relay with a transistor circuit, and then I'll get to test how hot the transistor gets.
 

can i have one?

It's still under development. Just answer these three simple questions, and I might send you a prototype. 

1. Are you or have you ever been a member of the Communist Party?  :kappa:
2. Are you willing to help me test it?
3. Do you happen to live anywhere near Philadelphia, PA?

[Draconis]

Sadly, no, it wasn't canted. As I continue to develop the design, I plan to experiment with canting angles.
 

I found a relay before I found a high-current switch. The motors (not pictured) are Hellcats. I am going to replace the relay with a transistor circuit, and then I'll get to test how hot the transistor gets.

 

Using a MOSFET or relay instead of a a high-current switch is both costly (by comparison) and over-complicated.  Really, the circuit just needs to be battery, motors, and switch.

 

I guess I don't understand why you made a new cage if you didn't change the design.  Was the original damaged?

[CaptainSlug]

Draconis is right. If you get the right kind of switch nothing else is needed. Except for maybe a fuse for safety.

 

Snap-acting switches only cost between $7 and $18 and are typically rated for 15 amps to 25 amps. See McMaster-Carr page 932. Most flywheel motors stall current at only 8 amps maximum. The ceiling seems to be Hellcats which are 24 amps max.

[oddron]

See McMaster-Carr page 932.

Thank you. I never though to check McMaster-Carr for switches. Listed on page 932 is part number 7510T12, which is rated for 25 A and costs $5.69 each.
 

Using a MOSFET or relay instead of a a high-current switch is both costly (by comparison) ...

Now that CaptainSlug has pointed me to these switches, you are absolutely correct that a switch is cheaper.
 

... and over-complicated.

I used a relay because it seemed like the "next best" solution when I couldn't find an appropriate switch.

I am considering a transistor because of a post on Reddit, and also because on Saturday (at APOC) someone specifically advised me that I should use a MOSFET because high current microwitches supposedly don't exist. (I don't remember who it was.)

In this Reddit article, one user discussed switch ratings and the idea that the rating for a motor load is significantly lower that for a resistive load. In particular, they discussed an Omron 21 A switch that can handle an inrush current of 42 A maximum. This would not be enough for a pair of Hellcats, which would have an initial inrush current of 48 A (2 x 24 A). McMaster-Carr doesn't specify if their 25 A rating is for a resistive load or for a motor load. Another user pointed out that if I use a MOSFET then I can use any switch I want. Through my own calculations, I have determined that if I use two AUIRFB8409 MOSFETs (either in parallel or one per motor), then I don't need heat sinks, even if the motors are both stalled.

The same post also mentions extra complexity, with at least two users suggesting that a switch would be better. No one mentioned flyback diodes; this extra complexity is essential to avoid damaging the transistor when turning off the motors.

If I assume that the 25 A switch from McMaster-Carr will handle a 48 A inrush current, then it is clearly the simplest option.
 

I guess I don't understand why you made a new cage if you didn't change the design.

I am making a new cage because I intend to experiment with the design. For example, I can change it to angle the motors or adjust the distance between them.

[Draconis]

You need to understand how motors work.  The stall current is an instantaneous maximum measurement, based entirely on the resistance of the wire that the coil is made from. The current drops significantly as the motor begins to move, because the permanent magnetic field begins to induce counter-EMF, in this relationship:

 

(current) I = [V_applied - V_counterEMF] / [R_wire]

 

Just like a mod to a sprung plunger blaster, you are going to have breakage and wear, which will require periodic maintenance.  You might end up having to replace your microswitch down the road, but it isn't really that big of a deal.

 

I am making a new cage because I intend to experiment with the design. For example, I can change it to angle the motors or adjust the distance between them.

 

Certainly a good enough reason.

Edited by Draconis, 10 August 2016 - 01:23 AM.

[dudifer]

1. Are you or have you ever been a member of the Communist Party?  :kappa:
2. Are you willing to help me test it?
3. Do you happen to live anywhere near Philadelphia, PA?

 
1. umm, I don't think so....
2. sure, I have a stryfe that is willing and running stock motors, if you need that tested.
3. if you consider upper NC close, then yes, I do.

 

(Could you send me the .STL file? I could print it myself and save you on shipping.)

Edited by dudifer, 11 August 2016 - 09:08 PM.

[oddron]

So I worked some more on my Stryfe this weekend.

First, I can take advice when I feel like it. I removed the relay and replaced it with a switch.



Second, I modified my FWC to angle the flywheels. One flywheel is angled 5° forward, and the other is angled 5° back.



Before adding the brass barrel, I tested the blaster, and I found that every dart came out whirly-birding. This means the canted flywheels are doing their job. After adding the barrel, and based on indoor testing, the darts seem to come out fairly straight. On the other hand, I had a few misfires and some chewed up darts, especially if I try to shoot fast.



I strongly suspect my barrel is part of the problem. Too bad I already glued it in place.



Since I have never cut brass before, it took some experimentation to find a way to cut out the holes for the flywheels. Using a dremel, here is what eventually worked for me. First I used the black disk attachment to cut a slot in the brass, and then I used a 1/8" drill bit to cut sideways. Too bad I didn't know it would leave rough edges inside. What would be a better way to cut holes in brass?

Regarding the design, I think there are still a couple of issues to work out. In particular, when I make space for the flywheels, it seems to cut slightly into the walls. I would be interested in sharing my design files, which I created with OpenSCAD. What would be the easiest way to do that?

 stryfe_fwc_5deg_version_0.stl   4.17MB   225 downloads

 stryfe_fwc_5deg_barrel_template_version_0.stl   1.2MB   181 downloads

[DjOnslaught]

If I'm not mistaken you should have only used the brass at the ejection site of the dart out of the flywheels, not threw the entire length of the cage.

[CaptainSlug]

You just need to use a rat-tail or round needle file to clean the cut edges of the brass before assembly.

[MAV13]

Any way I could get a look at the project files for the cage? I'd love to move the screw ports around and adjust it to fit in my Desolator.

[oddron]

Any way I could get a look at the project files for the cage? I'd love to move the screw ports around and adjust it to fit in my Desolator.

 
I guess the easiest way would be to put the OpenSCAD design files on GitHub.  Keep in mind, this is very much a work in progress.  Good luck!
 
https://github.com/o...n/flywheel-cage

[MAV13]

Wow, as someone with a computer science background, OpenSCAD is a revelation. I've been able to make any adjustment I can think of in a matter of seconds having never used it before. Thanks a ton!

[oddron]

Wow, as someone with a computer science background, OpenSCAD is a revelation. I've been able to make any adjustment I can think of in a matter of seconds having never used it before. Thanks a ton!

You're very welcome!  I remember about a year or two ago I looked at a long list of CAD software and tried to pick out the "best" one.  I have chosen wisely.

 

It looks like I'll only be able to work on the design on the weekends, so I will be curious to find out how other people improve it.
 
MAV and DjOnslaught, I wonder when the next war in Maryland will be. Baltimore, NYC, the NJ shore, and York, PA are all within a 2 hour drive from Trevose, PA.

[Montymarks]

Springers? Anyone? Firestrikes and Crossbows? Anyone?

But seriously, that is awsome. I would have no clue where to start with making one of them for myself.

-Montymarks

[DjOnslaught]

You're very welcome!  I remember about a year or two ago I looked at a long list of CAD software and tried to pick out the "best" one.  I have chosen wisely.
 
It looks like I'll only be able to work on the design on the weekends, so I will be curious to find out how other people improve it.
 
MAV and DjOnslaught, I wonder when the next war in Maryland will be. Baltimore, NYC, the NJ shore, and York, PA are all within a 2 hour drive from Trevose, PA.

Look up southern md house of nerf on facebook they have a war this saturday

[MAV13]

You're very welcome!  I remember about a year or two ago I looked at a long list of CAD software and tried to pick out the "best" one.  I have chosen wisely.

 

It looks like I'll only be able to work on the design on the weekends, so I will be curious to find out how other people improve it.
 
MAV and DjOnslaught, I wonder when the next war in Maryland will be. Baltimore, NYC, the NJ shore, and York, PA are all within a 2 hour drive from Trevose, PA.

 

I do most of my nerfing during HvZ at my college. It's at UMBC on Mondays and Thursdays at 7 if you ever want to come by. We have a larger invitational event in the spring as well.

 

Would certainly like to go to Apoc one day though.

Edited by MAV13, 19 August 2016 - 10:29 PM.

[oddron]

Look up southern md house of nerf on facebook they have a war this saturday

 
I went to this war (in Waldorf, MD) and had a blast, but not with my Stryfe.  During round 1, I learned the hard way that I shouldn't use hot glue to secure a microswitch.  Fortunately, I had a secondary.  I fought the entire war with my Strongarm and two pockets full of rolled up socks.

[DjOnslaught]

 
I went to this war (in Waldorf, MD) and had a blast, but not with my Stryfe.  During round 1, I learned the hard way that I shouldn't use hot glue to secure a microswitch.  Fortunately, I had a secondary.  I fought the entire war with my Strongarm and two pockets full of rolled up socks.

I've had zero issues with hot glue holding micro switches. What happened to yours? Daniel (the head of the group, might have been able to help also)

[shandsgator8]

I haven't had a problem using hot glue to hold in a microswitch either, as long as the I modified the shells so that they were both "sandwiching" the microswitch with the hot glue merely acting like something to hold it in place for the shells to do their work.

[DjOnslaught]

I haven't had a problem using hot glue to hold in a microswitch either, as long as the I modified the shells so that they were both "sandwiching" the microswitch with the hot glue merely acting like something to hold it in place for the shells to do their work.

That's how I did it too in my rayven

[DjOnslaught]

 
I do most of my nerfing during HvZ at my college. It's at UMBC on Mondays and Thursdays at 7 if you ever want to come by. We have a larger invitational event in the spring as well.
 
Would certainly like to go to Apoc one day though.

The southern md house of nerf does monthly battles anyone in the area is welcome.