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Minky

Variable Resistors.. Possible?

Soo..  I want to make the bestest build ever. I have a plan to build upon build eking ever last bit of performance out of my blaster until I'm prepared for the Independence Day aliens..  But what's the point of a toy gun you can't play with?  Does anyone know if it's possible or a good idea to fit a variable resistor in order to be able to de tune your blaster should you break the 130fps that's (sensibly) set on the games? .. And even if you can, will the war organisers allow a blaster that could be instantly tuned back up to dangerous levels?

Please bear in mind that my knowledge of electronic engineering extends as far as what my dad taught me and what I learnt at school,  I had to check whether a variable resistor was an actual thing..  So forgive my blatant ignorance.
Dazzle740

basically what your describing is a potentiometer which will allow to turn down or up the voltage that reaches your motors, so yes it is doable, whether you could get you to fit nicely - unsure.
And again would event organizers allow such a mod would i guess on the organizer themselves.
FDT OldNoob

You need a Pulse Width Modulation controller.
Resistors in a circuit with high current loads is a very bad idea, as they are non linear, when they get more current through them they shed it as heat, increasing their resistive qualities further.
TLDR they throttle the motors on the wrong way to be useful, get hot and are generally woeful.
SSGT can explain the why better than me.
130fps is set only on GC (eyepro & character reasons) and at indoor events or public parks. Any closed field event is usually 170fps. You can play much higher, the 12ftlb air rifle limit equates to over 600fps with a Nerf type dart.
We usually make a plug in motor cage, that way you just swap out your uber wheels and motors for something tamer. If you are making springers or air blasters then lower springs and lower air pressure has the same effect.
PWM controllers have been made for/by several people on this forum but don't pass all event rules because they can be adjusted. They also take up valuable battery space for no real gain vs swapping motors.

Arduinos can't directly handle Nerf currents usually. PWM controllers can be made or bought but given your knowledge state I would stick to a deans connector on your motor wires. I do.
Minky

Like with with Arduino PWM outputs?  (Yes.. I have been reading lots and lots of mod guides..  Some of it may have rubbed off)
SSGT

Variable resistors (more commonly potentiometers used as variable resistors) are bad for two reasons. Firstly they are a form of resistor - a device that experiences a voltage drop across itself which is directly proportional to the current flowing through it (Ohm's Law - V=I*R). Put simply, even if the resistance stays the same, a higher current draw will result in a greater voltage drop across the resistor. This is an issue since motors will draw anything between tens of milliamps (no-load/full speed) and tens of Amps (start-up/stall) - that's two orders of magnitude difference. To reduce the velocity you need to bring the motor speed below "critical speed" for that setup (around 25,000RPM for Nerf flywheels and darts - you normally choose a motor with a no-load speed greater than this to account for speed drop under load of firing). This is relatively simple since motor speed at a given load is proportional to voltage - to halve the speed you halve the voltage. Given you want a balls-to-the-wall build you'll probably either be using Hellcats or XP180s - since XP180s are expensive, harder to get hold of (long shaft variety is currently a BlasterTECH exclusive) and, more importantly, there's very little technical data for them I'll use MTB Hellcats in the example. A pair of MTB Hellcats will draw 0.92A at no-load on 3S (11.1V nominal) and yield a speed of around 33600RPM. You can bring that down to 22400RPM by dropping down to 2S (7.4V nominal) at which point a pair will draw 0.62A at no-load. So, to calculate the resistor to give you that drop you need to rearrange Ohm's Law to R=V/I and plug in the numbers giving R=3.7V/0.62A so R=5.97Ω or approximately 6Ω (in reality you can't easily get a 6Ω resistor off-the-shelf but since you'd be using a potentiometer we'll go with it). At no-load that's nice and simple - 6Ω resistance gives you a nice simple 3.7V drop reducing the speed to 22400RPM - however it effectively limits the maximum current the motors can draw to 1.8A (coil resistance of 2 Hellcats in parallel is 0.25Ω, add that to the resistor to get 6.25Ω and then back to Ohm's Law with the voltage across both - I=V/R -> I=11.1V/6.25Ω -> I=1.8A). You physically couldn't exceed that without increasing the pack voltage or reducing the resistance - you'd also be limiting the max torque of each motor to only 2.7mN.m and the effective voltage across the motors would be only 0.44V! You'd basically nuke the response time from a cold start as you have to wait for the motors to start turning and start to speed up for the current demand, and therefore voltage drop across the resistor, to reduce. You then realise that all this power is going somewhere - it's all being converted to heat.

High power diodes would be better since a diode isn't a linear resistive device and doesn't follow Ohm's Law - for our purposes we can assume the voltage drop per diode stays within 0.7-1V regardless of current (i.e. four diodes in series equals around a 3-4V drop). Diodes are still relatively inefficient though (they're still throwing that energy away to heat), and appropriately rated ones can be expensive, so a switch mode regulator/PWM supply would be better still (for a step-down converter, a converter that drops the voltage, the current coming out is actually greater than the current going in - this doesn't defy physics since the power (P=I*V) in/out is the same - you still lose a little power to heat but almost none compared to the alternatives).

These solutions don't, however, fix the second problem with a potentiometer since, if you can easily control them, they fall foul of the same issue. If you're employing a knob or switch to switch between power levels for safety, or to meet a game's velocity limits, there's almost nothing stopping you from accidentally (or "accidentally") ramping up the power when no one is looking. IMO the best option would be to use a second, lower voltage, pack - this also happens to be the most efficient and reliable way to drop the voltage as there are no additional components to fail/convert useful energy to heat. At the very least, make the method of varying the voltage something that is inaccessible during normal game use - preferably requiring a tool to access/vary.

Whatever way you reduce the voltage though you will, unfortunately, have to live with the fact that running motors below their critical speed for that flywheel/cage/dart will likely mean less consistent performance since motor speed variation between shots will have a more noticeable affect on velocity. The "ideal" would be to have a separate blaster (or flywheel cage for that blaster) for separate velocity limits although if you don't go to a wide range of games, with a wide range of velocity limits, I agree that probably isn't practical.

tl;dr:

Using resistors to control the max speed of a motor will throttle performance under load and increase spin-up/response times.

Using anything with an easily accessible knob/switch on it for the purpose of reducing performance to meet safety rules is bad and potentially unsafe - a second (or even third) lower-voltage pack(s) for lower-velocity games would probably be the easiest option (ideally with a second/third blaster or flywheel cage tuned to provide consistent lower performance although this isn't essential) although if you can't get to the performance bracket you want that way at least make any velocity capping system impossible to switch on-the-fly.
Minky

That is an absolutely brilliant reply. Explains exactly why they aren't mounted on the side of all Nerf guns. Thank you so much for the time that must have taken to write. I had thought about dropping down the battery power but had convinced myself that these packs really,  really didn't like a hungry draw and that's why you had to monitor their charge so closely.  Clearly have so much to learn..  Which is great!

I have just thought of another potential solution.. Mount the appropriate amount of barrel extensions to bring down the FPs  Very Happy  easily monitored..  If you're not walking around with 5ft of gun you're booted!

Y'know..  You really should consider being an engineer. I'd trust you.

Thanks again for everyone's input. I will learn.  Promise. [/u]
FDT OldNoob

Never be afraid to ask questions, it's a vital part of learning for all of us and detailed replies like SSGT's are useful for everyone on the forum or future searchers.
Correctly sized and specified Lipo will take far more current draw than most set ups will require, especially hybrids like Lihv or Graphene lipo. The reason you need to check the charge is not because they go flat. You can put a charged pack at the start of a game, play all day and discover it's lost about 0.3v total over 100's and 100's of shots. You can charge a larger pack and it would do a whole years games on one charge.
What ISN'T a good idea about that is leaving lipo fully charged in storage or at too low cell voltage, that's what monitoring is for. Stick above 3.6v per cell for storage and never above 3.8v. In use anything between 4.2v per cell and 3.5v per cell is totally safe.
I have never had a pack drop below 3.6v in use, despite using some of the most current hungry motors like XP180's.
Broz

That's very useful to know. I brought a couple of volt meters but had no idea what numbers I needed to worry about, just rather assumed that when the alarm went off I would have a few seconds to throw the thing into the nearest body of water before the battery suffered the equivalent of a warp core breach...
Boff

Broz wrote:
suffered the equivalent of a warp core breach...


I need to use this in the safety briefing at GC. Very Happy No, a battery alarm going off usually means you've got plenty of head room to salvage it.

I might do a PWM tutorial at some stage, I think it'd be an interesting and valuable addition to the community. It's not optimal, as SSGT says, to have motors spinning at less than full duty cycle but it is one option for Nerfing your Nerf.
Northwind

If you were a cyclist, you'd see that the correct answer here isn't to have one blaster and adapt it for different jobs, it's to have more blasters. N+1, where N is the number of blasters you already have.
BobCatMax

that's a formula very close to my heart.

I fear that it no longer applies though and s-1 is more appropriate.
tbr

Northwind wrote:
the correct answer here isn't to have one blaster and adapt it for different jobs, it's to have more blasters.


That's similar to what I was going to say. Currently my main blasters are a Stryfe with Hellcats, a Stryfe with Rhino's and a couple of stock motor Stryfe's with IMR's. I use the blaster that best fits the situation I'm in. (plus I enjoy making blasters)
Minky

Northwind wrote:
If you were a cyclist, you'd see that the correct answer here isn't to have one blaster and adapt it for different jobs, it's to have more blasters. N+1, where N is the number of blasters you already have.


Yeah,  I feel you.  However the likelihood is that I won't be able to get to an event until after Xmas (I work for a brewery so this time of year it's crazy)  so I didn't want to rock up in spring with my new bested blaster and find that I had no hope of using it.  Though in honesty,  with the kit that I need to buy,  the amount I can budget for it,  the time I'm likely to have,  and the blaster I'm using this isn't likely,  but it would still be a kick in the balls if it were the case.

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