V1 Rapid Decay - Project-Stinger/Main GitHub Wiki
What is rapid decay?
The solenoid current defines with how much force the solenoid is extended. More current = more force. After extending, you therefore want the current to drop to zero as fast as possible. On Stinger V1, the current decays slowly, because the only thing acting against the circulating current is the resistance of the solenoid coil together with the flyback diode.
The theory
In phase 1 (extend), the MOSFET is turned on, and the current rises. It peaks at about 5A, and since the diode is in reverse direction, we can ignore it here. In phase 2 (decay), the MOSFET is turned off, and the current circulates through the diode, back into the coil and so on and so forth. The current decays slowly, because the reverse voltage is pretty low. The retract time is about 40ms.
Now, we introduce another diode, placed inverted compared to the original diode. In my example, it is a Schottky diode (see below for why I chose it). An eager eye will notice that in theory, the current cannot recirculate like this and might say that this will destroy the components. However, Schottky diodes are known to have a low reverse breakdown voltage. That means, we use the diode in reverse by design, to get a higher reverse voltage. This way, the current decays much faster, and the retract time is under 15ms, similar to Stinger V2.
Why and why not?
- Pro: Faster retract time -> faster rate of fire (2x as fast)
- Con: High transient voltage at the MOSFET's drain -> choose a diode with a low reverse breakdown voltage, so that VBat + VDiode < 55V (the breakdown voltage of the MOSFET). Proceed at your own risk. In my view, the only thing that can break is the MOSFET, and that is cheap to replace (IRLZ44N recommended (yes, correct, irLz, not irFz), less than 1$).
How do I get it?
Go to a site like AliExpress or LCSC and buy a TVS diode with a reverse voltage of roughly 20V. The diode should be rated for a peak reverse current of at least 5A. Choose a form factor that you feel comfortable with. At that point, I would also get a spare MOSFET if something goes wrong. I had a Schottky with 40V breakdown voltage at home. Theoretically, that is too high, but thankfully I did not think it through and went ahead with it. It works perfectly fine. I would recommend a 20V diode, though.
Also get some 50V 100nF ceramic capacitors with solder leads. They can be used to suppress the transient voltage at the MOSFET's drain. Maybe this is the reason why my MOSFET did not break. :man_shrugging:
[!WARNING] Proceed at your own risk. I am not responsible for any damage to your blaster. The upgrade is not necessary, and the blaster works perfectly fine without it. Nevertheless, I would rate the upgrade as low risk. Please read all the instructions first, before starting the upgrade. If questions arise, ping me (Bastian) in the Discord server.
Step 1: Open the blaster
Remove the battery, if you did not already do this. Remove the screws of the top left alu bar using a 2mm hex driver. Locate the MOSFET-Diode-Resistor-package and undo the electrical tape. It should look something like this:
Step 2: Solder up the new diode
Separate the old diode from the MOSFET, but leave the other side untouched. Solder the new diode in series to the old one, but with reverse polarity. See the next picture on how it is to be done. The schematics can be found above (second picture from the top). During the whole upgrade, only the middle MOSFET pin needs to be touched. The other MOSFET pins are to be left untouched.
Step 3: Solder up the capacitor
Solder the capacitor across the two diodes (i.e. from the middle MOSFET pin to the thick red wire with VBat, see the second picture from the top). The polarity does not matter. This is how my installation looked. Arguably, not the best soldering job, but it works.
Step 4: Wrap it up
Make sure all open contacts are individually covered with electrical tape. Then, wrap the whole package in electrical tape. Now is also a good time to test if nothing explodes.
Reassemble the blaster and test if everything still works. If it does, you are done. If not, look for any shorts or bad solder joints. If you cannot find anything, ask for help in the Discord server.
Step 5: Enjoy
While you have now enabled Stinger V1 to have rapid decay, auto timing is still not available on Stinger V1. That means, you need to manually decrease the retract time in the settings (Firing -> Expert -> Min. retract ms). A retract time of 20ms or less should be possible. Then, increase the Darts/s in the Firing Menu. On a full charge, I was able to get 40dps. You will probably not get these very high numbers because I used a rather high breakdown voltage diode. But you should be able to get 30dps or something similar.