Archive for BRIT NERF Home of the British NERF Community
 


       BRIT NERF Forum Index -> Modifications
Franksie

The 'Un-named' Rapid Pistol

So being totally shamed into getting my arse in gear by blindgeek on this one I'm now setting off on my own journey to build a rapid pistol.  Firstly, a word of warning - THIS IS GOING TO TAKE SOME TIME. I can be a bit of a perfectionist at times and I'll also be asking loads of questions along the way.

Secondly, a bit about me and my experience so far with Nerf modding...

To date I've completed:
- a stryfe rewire with a jam door operated voltmeter
- an EAT Spring upgrade
- a sidestrike Spring upgrade
- a Rotofury Spring upgrade and stock block install

I'd consider myself still pretty green in the modding department so I thought I'd run this progress log to show everyone what a newbie can accomplish with the help of our great community!

First up, this will be a 180 hellcat build using a mosfet for the Rev trigger circuit. So I knocked up this wiring diagram and fired it off to Boff, who replied within 5 minutes to tell me I'd forgotten the attachment and within 2 hours to confirm the circuit was right and to offer some final pieces of advice.



I then read Toruks post on Rapidstrike wiring and kicked the following diagram around with OldNoob and blindgeek over on Discord:



And then I got my new shiny hacksaw and with a cry of 'no fear' I wrote off any chance of my blaster ever being a Rapidstrike again

Rear first


Then the top rail



Then finally the front



And that's where I left it for my first nights work - next session I'll cut the other side down to size and then file the two sides to shape together
mipevi

I'm totally new at this myself, so grain of salt and all that, but...

I've seen the flyback diode been hooked up to each motor, so they have their own. In the diagram above, one diode needs to handle both Hellcats, right? I guess not a problem if the diode is strong enough. What are you using? I found this video helpful in understanding the whole flyback thing. No diode for the pusher?

I prefer to overengineer, so if you like, use a voltage regulator for your mosfet / the whole control circuit (adding a mosfet for the pusher). I'd expect the mosfet you use can easily take the 3S voltage, but the gate is probably wide open at a much lower voltage already.
Boff

One flyback diode is perfectly fine. We use RGP32 3A diodes which have a decent avalanche current rating. The pusher is in its own braking circuit which is where the back EMF on that circuit goes, no need for a diode. Smile
Franksie

The MOSFET is from the BSUK high powered Stryfe kit, which uses a much more potent IRF3034 MOSFET to give me headroom against a potential Hellcat stall. One wiring loom to rule them all....
mipevi

EDIT: When I say rev trigger, I mean the fire trigger switch... Sorry, was thinking in terms of an isolated pusher system.

Ok, so if I'm reading the diagram correctly (big IF  Laughing ), when you let go of the rev trigger AND the pusher is extended (pressing on the fire control switch), the pusher motor has a closed circuit through which the back EMF will flow, and since that flow is in reverse, it will effectively brake the motor. However, if the pusher is not extended when you let go of the rev trigger, there is nowhere for the back EMF to go, since the previously braking circuit is now open? So the back EMF will try to arc the rev trigger?

Google wasn't able to easily fetch me a datasheet of the RGP32 3A, so I gave up. But Boff's word is obviously good enough for me!

Re: Mosfet, I meant the voltage required to fully open the mosfet gate is less than what 3S delivers. For example at 0 volts it's closed, 1.3 volts partially open, 2.5 volts fully open. But the mosfet gate can take higher voltages than the example 2.5 as well without frying (within limits), the extra voltage just isn't helping any.
Franksie

I will direct you to Toruks page for the answers you seek

http://torukmakto4.blogspot.co.uk...-control-circuits-part-1.html?m=1

Not sure that the Rev circuit can interact with the fire control group as you state though
mipevi

EDIT: When I say rev trigger, I mean the fire trigger switch... Sorry, was thinking in terms of an isolated pusher system.

Yeah, looks like I mixed up when the fire control switch button is being pressed. It happens when the pusher is fully retracted (as opposed to being extended), then braking the motor using its back EMF. Still, if the pusher is extended when you let go of the rev trigger, there's nowhere for the back EMF to flow (until the pusher is fully retracted, provided the motor slowing down is enough to achieve that).

EDIT: When the pusher is not fully retracted, the back EMF would seem to flow through the (released) rev switch, into the fire control switch, where it will try to arc to complete the braking circuit. If the fire control switch is robust, this might not be a big problem. Of course the rev switch will also need to handle the back EMF voltage/current.

When I get to my Rapidstrikes, I'll probably be running mosfets and diodes for flywheel and pusher motors, with a 3-burst Arduino system. Smile
SSGT

Just to clear up a few things (it looks like you've worked a couple of things out while I was writing this but I'll leave them in for posterity), the pusher depresses the cycle control switch when it is fully retracted as opposed to extended, also the rev switch in that circuit is completely independent of the pusher motor (although you might have meant fire trigger/main trigger) - if you release the flywheel rev trigger the pusher circuit will continue doing whatever it is currently doing. If you release the main "fire" trigger when the pusher is extended, the pusher motor will continue to spin (and be powered) until the pusher is fully retracted and depresses the cycle control switch. When both the main trigger is released and the cycle control switch is depressed, there is a closed circuit shorting the terminals of the pusher motor which should give the current that was flowing through the motor before power was removed a path to flow through and decay, before providing a path for the current generated by the motor as it spins down to brake the motor. This current flows in the opposite direction through the motor as the polarity of the generated EMF, and induced current through the motor, is the opposite as per Lenz's law (diagram below showing current flow through a motor, and motor speed, after power is released and whichever relevant "braking" circuit is applied - note that a flyback diode cannot provide dynamic motor braking). This is the "back-EMF". Back-EMF is generated by the motor whenever it spins as DC motors also act as generators. It is the back-EMF that is responsible for the reduced current draw at no-load (it effectively opposes the supply voltage thereby reducing the current demand even though resistance largely stays the same). Back-EMF is just the EMF (voltage) generated by the motor - the back-EMF itself doesn't flow but it can generate a current which will.


Click for full size.

I said previously that the shorting circuit should provide a decay path for the current but, in reality, because the switch between contacts isn't instantaneous, there will still be arcing in whichever out of the trigger and cycle control switches switches last. That said, unless the pusher motor has undergone stall, the current being drawn should be pretty low and so the arcing should be fairly minimal and, since the pusher controls are mechanical switches as opposed to FETs, likely not as damaging either. If you want peace of mind though then yes, the pusher motor could do with a diode across it. If you used a pair of FETs in a half-bridge to control the pusher motors then you probably wouldn't need a flyback diode as the FET across the motor would perform that duty for you (they effectively have a built in reverse-biased diode). The issue is that it's actually quite tricky to put FETs in a RS pusher circuit. To make the half-bridge you need an N-channel FET in series with the motor to turn it on and off and a P-channel FET in parallel with the motor to actively brake it. It's doable (even with a three switch setup and no electronics) but if you get it wrong you can have both FETs conducting at once resulting in a short across the battery. To be honest, if you run a decent 10A microswitch with a diode across the pusher motor, you'll probably be fine switching pretty much any pusher motor (most switches are de-rated for motor loads due to the inductance causing arcing issues - if you reduce the arcing with a diode you'd likely be able to re-rate your switches back close to resistve load ratings).

As for FET ratings, the spec you're looking for that determines whether or not you need a regulated voltage control on the gate is the gate-source voltage (VGS) which is the max potential difference between the gate and source pins (source for an N-FET is usually ground so VGS is effectively your max pack voltage) which is usually around 20V for non logic-level FETs and 16V for logic-level FETs (a lower threshold voltage, Vth or VGS(th), FET designed to be switched directly by a logic level/μCU signal and usually identified by "IRL" as opposed to "IRF"). Note also that practically all TO220 FETs have a package limitation (usually listed below the main table) of 75A continuous - the max continuous rating, if higher than that, assumes an infinite heatsink.
Franksie

In summary, does that mean I'm ok with what I've got? Laughing
mipevi

Wow, a very good and well thought-out post by SSGT, thank you! Even using proper nomenclature as not to confuse anyone (sorry again Laughing ). However, I think that while

SSGT wrote:
If you release the main "fire" trigger when the pusher is extended, the pusher motor will continue to spin (and be powered) until the pusher is fully retracted and depresses the cycle control switch.


is true in the original RS system (what Toruk calls a live center circuit), in the OP diagram there is no power for the pusher motor aftere the main "fire" trigger is released (Toruk's dead center circuit).

Also, re: mosfet, yes gate-to-source voltage is how you determine if you need a regulator as to not fry your fet, but gate threshold (max) voltage is how much voltage is necessary to fully open the gate. I'm just thinking that using higher voltage than needed could unduly strain the fet gate, even if it is within what the fet is capable of taking. But I suppose in reality much more strain comes from the current actually flowing through the fet... So prolly just me talking out of my arse because, you know, overengineering.
Franksie

Just to be clear, I don't believe I'm trailblazing any new ground here - is there any chance a mod can split this thread please?
mipevi

Yeah, I'd say you're good to go, Franksie. Personally I'd work a flyback diode into the pusher motor, but as SSGT stated, not really necessary. Smile
SSGT

mipevi wrote:
However, I think that while [quoted text] is true in the original RS system (what Toruk calls a live center circuit), in the OP diagram there is no power for the pusher motor aftere the main "fire" trigger is released (Toruk's dead center circuit).


Thanks for pointing that out, I missed that. I saw mention of following toruk's post on RS wiring (which would have been live-centre) and didn't bother checking the diagram closely enough. So yeah for dead-centre, if you release the main trigger while the pusher is extended, the motor will just coast until it the pusher is fully retracted or coasts to a stop, whichever comes first. In that case even with a switch that could switch contacts instantaneously there would still be arcing. Again, a diode there probably isn't necessary but the switch would still likely benefit from it.

As far as FET gates go, I take max ratings as read. If it says it can handle 20V between it and source it will be fine, especially with only 7.4-11.1V. A voltage regulator, if not required, is additional complexity, an additional source of inefficiency and another point of failure. It's the inrush current to the gate that more commonly causes a failure - without a series resistor you can be sending one hell of a lot of current into the gate over that short period of time it takes to charge (the gate is effectively a capacitor). Regardless, in most cases I'd personally prefer a full-size 21A microswitch as the rev switch - a decent one is more than capable of handling 42+A inrush current at 7-12V.
Franksie

So I may have trimmed a bit too close....



Tonight I'll be seeing if this is salvageable, I'm hoping that with a small shim I won't have to start over but if not Davera is coming to the rescue with a new Rapidstrike shell.

Looks quite good still....




OldNoob

Please use 600x400 max size photos!
Although the pics need to be smaller, your fly motor holes need to be bigger.
Franksie

Sorry about the pics - now resized.  Ignore the sharpie lines - they are toss
Franksie

Things have moved on a little.  The flat top has now been completed and everything has been sanded level.  I've also done the cut outs for the 180 motors









Next, I wet sanded all of the 3D printed parts with 240 grit so they are nice and smooth



Things are started to look pretty good.  The next challenge is to shim the front block to perfect the barrel alignment and then level out the fore grip
The Dark Kitten

Was good to see it in person at ZHoM
I could see it was clearly very well made!
I can see this really becoming a nasty sidarm and really ripping through kids and zambies.
Its nice to see that Coop and Bobos design is really being sorted out British style

       BRIT NERF Forum Index -> Modifications
Page 1 of 1
Create your own free forum | Buy a domain to use with your forum