Aimtrak solenoid clicks soon as power is applied

[meltman]

Hey everyone - I'm struggling with a bizarre issue. As soon as 36v is applied to the gun the solenoid clicks closed and will try to stay there.

At first I thought the Mosfet was to blame - it seemed to measure bad. I replaced it but the symptoms persist. I plug it in (power only, not usb) and instantly the solenoid closes on.

Any ideas? This circuit is not especially complicated. I dont understand what is activating the fet. Does the USB board need to be attached? Maybe I need a pulldown or something? I thought that is what the resistor on that board was doing.

[Titchgamer]

I had this problem twice, Originally thought the polarity was wrong but after confirming that was ok it left me and Andy stumped.

Afraid I never fixed it and now just play without the recoil but if you ever find a solution please let me know!!

[meltman]

I have two guns now that both suddenly exhibit the behavior. I’m playing without recoil now as well.

[Mike A]

Return them.

[meltman]

Mike, you are a troll sometimes.

[Mike A]

Are you outside the return window?
I was under the impression that these guns were new. If not then call Ultimarc and demand that they make things right. You might not get what you want, but it's worth a shot.

[meltman]

So some observations as I troubleshoot...

First, the mosfet chosen for the guns, the FQP33N10, has a Rds(on)Vgs of 10v which makes it a poor choice for this application.

The drive circuit is a microcontroller that can only supply 5v to turn on the fet, so the fet is never "fully ON". In fact it takes 10v to turn on the FET fully. This makes it generate way more heat via internal resistance every recoil fire.
This, coupled with no heatsink on the fet leads to premature failure of the part - least that's my thought.

Ok with that out of the way, there is also no pulldown resistor on the gate. This kind of leaves the gate "floating" - and since it's floating the device seems to be fairly unstable. I added a 1k resistor between gate and ground and the device would stay off until triggered.
https://electronics.stackexchange.com/questions/45494/what-happens-when-a-mosfets-gate-is-left-open


I'm going to dig into and test further. It may be as simple as adding a pulldown resistor to keep the fet from coming on, though this does not solve the issue of only partially turning on the FET with a Vgs of 5v.

So for me, I'm going to replace the FET with one that is meant to be triggered by logic levels. I've chosen the IRLZ44N which has a Rds(on) Vgs of 5v. The IRLZ44N has a lower max voltage of 55v but that's well over the 36v of the recoil, plus it has extremely low internal resistance when fully on (cooler running). Another suitable part would be the IRL540N which is more similar to the FQP33N10, but with an appropriate Rds(on) voltage of 5v.

I'll let you all know how I do.

[Titchgamer]

Interesting!
I look forward to seeing how you get on!

[meltman]

This is the circuit implemented in the Aimtrak recoil guns (replace the lamp with the solenoid, add the flywheel diode).


The only thing they didn't implement is the Rgs pulldown resistor.

Source https://www.electronics-tutorials.ws/transistor/tran_7.html

I should have some replacement FETs on Friday so this weekend I'll likely continue to experiment. I'll post back with pictures and success/failure.

[meltman]

Replace your MOSFET with IRLZ44N.
Bridge the outer legs with a 10k resistor.
Pray your solenoid isn't fried.

Of my two guns, one has a bad solenoid (measures in the kiloohm range). GAH. Anyone know where to source a new one? I'll email Andy as well.

[AndyWarne]

The issue is not the MOSFET specification, the 33N10 does not overheat and does turn fully off/on.

The pull-down resistor is not needed as the MCU drives both high and low.

The issue is most likely a rare failure of the output of the MCU rather than MOSFET or other issue. Although the cause might not be in this case, I have come across damage happening when the recoil power was connected with reversed polarity.

[Titchgamer]

The issue is not the MOSFET specification, the 33N10 does not overheat and does turn fully off/on.

The pull-down resistor is not needed as the MCU drives both high and low.

The issue is most likely a rare failure of the output of the MCU rather than MOSFET or other issue. Although the cause might not be in this case, I have come across damage happening when the recoil power was connected with reversed polarity.

Regarding PSU’s Andy,
Can I make a suggestion to you to ship the guns with a PSU’s in future as I always thought it was a little odd not getting one with the kit.

[meltman]

I disagree Andy, the MCU is putting out pulses every time I hit "test". I verified by putting an LED on it.

How is it possible to drive a FET to full open when it wants a 10v gate and it's only getting ~5v? The datasheet of that FET shows that it is not fully "ON" in that configuration.

There is a thermal fuse inside the solenoids that blows - rendering the solenoids useless unless you remove it - if you've experienced the "stuck on" scenario your solenoids probably blew this thermal fuse.

[meltman]

Youtube of BEFORE swapping the FET, verifying MCU pulse for recoil with LED. Yes the 36v PSU was connected for this test.


Youtube of AFTER swapping the FET and adding 10k pulldown.

[AndyWarne]

If there were a design issue we would have a huge number of failures but we dont. I have never come across a failed MOSFET but it does appear this is the case in the video. The threshold voltage is less than 5V and the on difference between 5V and a higher voltage on the gate is very small.

There is not a thermal fuse, its a trip which resets, or should do. Its possible it might not be able to withstand a sudden over-temp caused by a solenoid stuck on permanently though.

The reason for not supplying PSUs is product liability especially in the USA, and also complxity of supplying the correct power cord etc. Also high shipping costs which the China suppliers are able to bypass with their subsidized postage.

We have only experienced two issues with recoil on a very small number of guns which are:

Breaks in the 2 power wires in the USB cable, where it exits the gun: Cured with a cable re-design (thicker outer sheath) and larger packing box such that the cable does not have to be tightly folded in packing.

Machining marks on a conical section of the recoil plunger and the internal part of the solenoid which sometimes caused "hanging-up" of the plunger such that is does not spring back. Cured by correction to machining process.