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SirBrass

Rapidstrike 180 build debug help needed

So, 3 blade (will be using 3x hellcats and a 3s when I get all my replacement parts together) 180 motors with 16 awg wire, 2s lipo, mosfet on the flywheel circuit. Wired according to the bsuk circuit diagram & using the BSUK kit. Artifact red cage, worker flywheels. Rectifier diode correctly placed.

The mech switch & negative from pusher are on the mosfet source line that runs from battery negative. When I test just flywheels with alkalines & lipo, everything is fine. When I tested trigger & flywheels (pusher won't go without flywheels going) on alkalines, everything is fine.

But, as soon as I test pusher with lipo, that's when the mosfet fails (no outward sign of failure on the mosfet, ie no smoke or device getting hot) & the flywheel motors go nuts.

I think somehow back emf from the pusher is somehow leaking into the mosfet. Looked at my wiring & the circuit diagram and I'm stumped as to how though.

I tested the circuit with c-cell alkalines first & everything was fine, and fine with the lipo too until a few seconds into shooting bursts from a mag of darts. Motors go out of control, but only the flywheel motors. Pusher is fully in control.

Help?
Minky

Whilst not definitely the answer a great place to start would be..
Have a Strategic Cup of Tea. Then come back and consider it afresh. I would particularly look at the wiring of the Mosfet. Have it facing you with the lettering the right way around and the heatsink at the back and wires out the bottom.

Left connection should be your Gate going through your rev switch to your battery positive.
Middle should be your Drain going to your flywheel motors.
Right connection should be your Source and connect back to your battery negative.
Your 10k Resistor should bridge your Gate to your Source.
Your flyback diode should be connecting your Drain negative to the positive into your motors,  with the silver band closest to the positive.

I know it sounds patronising but I've wired 4 and still got 1 wrong and have a 'check card' I keep on my work bench.

EDIT: I don't know whether anyone saw but this is the 2nd time I posted advice on this but first time around I talked myself into thinking I'd got it wrong.. So I deleted it until I could get back to my check card lol
SirBrass

Yes,  I had all of it wired correctly. That was the first thing I checked. And I went through all my transistors trying to debug this. Rectifier diode would have popped if it had been wired with silver band to the negative (I've done it when experimenting with one on my stryfe. Diode went *pop* when wired across the motor lines wrong).

Only thing I didn't do was use a 10k ohm resistor. Instead I spliced in a connection from acceleration switch NC to mosfet source. Does the same thing as the resistor in pulling the gate down to 0 Vref when the flywheel trigger isn't pressed.

As an FYI, my degree is in electrical engineering Wink. That's not a brag, just an FYI. I haven't done circuits since college (over 10 years ago).
Minky

So you went with something like...

...?

Can't handle BSUKs circuit drawings. My failing not their's I'm sure.
SirBrass

I can't really follow that diagram you drew up there. I did this exactly: http://m.imgur.com/NkK2BNT?r

Edit: there was no 10k resistor in there when I was following it. That could be the issue.

Would be nice if old noob or other folks with lots of practical electrical experience chimed in. All my knowledge of circuits was purely theoretical except for circuit theory labs where we used low power currents & non-soldering breadboards. And the linear devices lab had us just measuring gains by using transistors as amplifiers.
SSGT

There shouldn't be any issue with using both contacts of an SPDT switch instead of a resistor other than that the gate pin will be momentarily floating as you switch contacts. That could potentially reduce the life of the FET over time, as the FET may fall into it's semi-on "linear region" causing it to heat up, but the effect with a fast switching switch is likely negligible and certainly shouldn't be causing any issues in normal operation. It's actually perfectly acceptable to use a resistor aswell as both contacts of the switch as that ensures that the FET gate is never floating, and so should (depending on the resistor value and gate capacitance) stay in it's linear region for less time overall, whilst also allowing for a strong (low resistance) pull up/pull down which will do a better job of rejecting noise without the greater current draw you'd get if you used a low value fixed resistor to achieve the same thing.

When you say the mosfet fails/flywheel motors go nuts/out of control does that mean the flywheel motors are slowing down/speeding up/fluctuating in speed when you power the pusher motor and/or does it mean the FET is turning on/turning off when it's not supposed to when you power the pusher motor?

When testing with alkalines was that 4x C-cells (e.g. in an RS tray hooked up to a pack connector)?

What do you mean by the "mech switch & negative from pusher are on the mosfet source line that runs from battery negative"? There shouldn't be anything between the FET source pin and battery -ve other than a length of 16AWG wire and the only thing between the drain pin and battery +ve should be the flywheel motors/diode (given that it seemed to work fine in some circumstances this probably isn't the issue, and it's more likely just a misunderstanding of what you meant on my part, I'm just trying to get a complete picture of what's going on and sanity checking anything that could be wrong).

Would it be possible to measure the voltage at the gate relative to source/battery -ve both with the pusher motor not powered and then again with it powered?

Other than the wiring of the acceleration trigger have you made any other modifications, even minor ones, to the BSUK wiring diagram?
SirBrass

By "going nuts", I mean that the flywheel could no longer be controlled by the acceleration switch. It's like the mosfet gate formed a permanent conductive channel & it won't turn "off". This didn't happen when testing the circuit when testing with the 4 c-cells from the RS battery tray when acceleration switch was engaged and pusher switch engaged. It didn't happen when the lipo was connected and only the acceleration switch engaged. That is, when the acceleration switch was released, the flywheels spun down & when it was engaged, the flywheels sped up. Normal operation. However when the pusher switch on lipo was engaged that's when the mosfet permanently "stuck" to on.

Also, I hear you on not having anything between source & -ve, but look at the bsuk diagram. There's a splice from the pusher moter negative & fire control mech switch normally open that is spliced between mosfet source and battery -ve.

Edit: I made no other modifications to the wiring. Right now I can't measure anything. The motors I used are now wired into my stryfe, and all the mosfets I had are busted (they won't work, period. Permanent channel between source and drain, like a wire, not a semiconductor switch).
blindgeekuk

I'd have made a suggestion that you take pictures of the wiring, and maybe even a video clip of the motors going 'out of control', but it sounds like rather than waiting for assistance, you've just ripped the motors out.

From the sounds of it, something, either the diode, get, or a splice was wired wrong. Easy enough to do. I wired my first motors so one accelerated the dart and the other decelerate it, and I did my first FET back to front.

Never managed to kill a fet though, that's pretty impressive

Edit: the simplest FET diagram to follow (and I should know, I'm blind and I followed it) is Franksies, as per the unnamed rapidpistol thread, http://m.imgur.com/lqzDS2n?r
Franksie

I'm not sure why you are putting the pusher circuit through the FET.  A better option would be to take a splice off the main positive and route it directly to the pusher trigger.  Back EMF shouldn't have entered the FET if you have correctly placed the diode
Minky

The splices on and off the negative describe the half bridge circuit via the mech and trigger switches.  Looking at it I guess the motors could be introducing back emf positive to the gate due to your splice from rev NC.. But I don't know enough (period Smile ) (...and this shouldn't happen if the motor braking circuit is sound).

It's going to be hard to fix a problem without having the original circuit available to look at/test nor a Mosfet to install.. If the change from the BSUK circuit isn't at fault and the circuit diagram is sound then what logically follows is that it was mis wired/soldered.

Edit: Franksie it's not through the Fet it's a separate feed from the rev trgger. Acts as an electronic lock to avoid pusher activation without flywheel activation, I think. My circuit drawer lumped them together as it could be a splice if you really wanted it to be.
Edit2: Cleared up my description to franksie.
Edit3: Further cleared up description and added last sentence to first paragraph and wrote myself a note to stop posting from my phone whilst walking. Apologies for the poor posts Mods/Brass/Everyone.
SirBrass

Franksie wrote:
I'm not sure why you are putting the pusher circuit through the FET.  A better option would be to take a splice off the main positive and route it directly to the pusher trigger.  Back EMF shouldn't have entered the FET if you have correctly placed the diode


It isn't being put through the FET.

Basically I spliced both mech switch NO & pusher motor negative onto the wire going to battery -ve instead of splicing pusher mech NO onto a splice running from the battery -ve wire (which connects to FET source aa in the diagram) to motor negative. I did so because I was running out of space, basically.
Franksie

Can you post an internals shot?
SSGT

SirBrass wrote:
Also, I hear you on not having anything between source & -ve, but look at the bsuk diagram. There's a splice from the pusher moter negative & fire control mech switch normally open that is spliced between mosfet source and battery -ve.


Gotcha. The way I read it it sounded as though there might have been components in series with the source-battery -ve line (which didn't make sense from what you were saying). Electrically it shouldn't matter where they are connected just they are all effectively connected to the same rail.

So all the FET(s) (all killed in this RS?) have failed short circuit? FET failures can be quite tricky to diagnose but some of the more common failures include over/under-voltage at the gate, overheating/excessive power dissipation or avalanche failure of the body diode across drain-source. Avalanche failure isn't that likely given that the flywheel motors have a flyback diode across them so I'd guess either overheating or gate over/under-voltage. Back EMF in of itself shouldn't be an issue as it should never exceed the voltage originally applied to the motor by the battery - it could hold the gate voltage high when you stop sending power to the pusher but you'd need a direct connection from motor +ve to the gate whilst the motor was spinning down to do that (which shouldn't be possible as releasing the trigger switch disconnects the motor from the supply that's feeding the gate aswell).

It's unlikely that the FET overheated through exceeding IDS(max) but if you ran a 2S system with three diodes as per the BSUK circuit then that could potentially cause the FET to overheat when the pusher motor is switched on. With zero current flowing through the diode the forward voltage drop will also effectively be 0V meaning the gate will receive full pack voltage of 7.4V and turn on fully (current will flow as it "charges" the gate but it'll be for a such a small amount of time that it likely won't affect the rise time of the FET significantly). As soon as you turn on the pusher motor you're drawing current through the diodes and now the voltage at the gate will drop down at least to 5.3V. That should be plenty to switch on the FET fully although it may drop lower than this as the current draw from the motors at startup will be close to stall which will cause the pack voltage to sag slightly aswell as cause the voltage drop across the diodes to increase slightly. If it drops much below 4V this will be a problem as if the gate voltage drops below the threshold voltage the FET will move into it's linear region where it may end up dissipating quite a lot of heat. This may not be your issue since you mentioned that it didn't feel as though the FET had heated up significantly (although it's possible it killed it so soon after you powered on the pusher motor that it had cooled down by the time you touched it) and since I would expect it to act similarly with alkaline cells owing to the fact the nominal voltage of 4 C cells is only 6V and that would sag much further than a 2S LiPo would under load (although that would also mean the power dissipated through the FET would also be lower as the current draw would be reduced so that may have been it's saving grace on the alkalines).

Gate over/under-voltage is a real possibility but I can't quite see how it would have happened. The lack of a resistor across gate and drain would cause the gate to be floating as the contacts in the acceleration switch physically switch over but that shouldn't have caused a failure when the trigger switch was depressed (as previously stated I wouldn't expect it to be an issue in normal operation only that it may reduce the FET life in the long run). It's possible a voltage spike from the pusher motor raised the voltage at the gate or the source causing the FET to exceed it's VGS(max) (±20V for an IRF3034) but that is only likely to happen when the trigger is released (although excessive bounce in the switch contacts could cause a fairly large spike on startup). Depending on wire-routing it's also possible that EM interference from the motor induced a high voltage at one of the FET pins.

In a future build I'd suggest trying one or more of:

  • Using a resistor across gate and source (either instead of or in addition to using both switch contacts - if space is an issue you can wire it around the FET like this) - this will ensure the gate is never floating even when the switch is between contacts. A resistor in series with the gate, whilst often not strictly necessary and unlikely to be the cause of this failure, would also help kerb the inrush current to the FET's gate.

  • Placing a diode across the pusher motor to kerb any voltage spikes that may occur as current to the pusher is interrupted.

  • Moving the acceleration switch, and therefore the connection to the FET's gate, before the diodes (or just remove the diodes entirely - they aren't necessary with 180 pushers as they're more than capable of stopping themselves quickly enough at that sort of RoF) to ensure that the gate always receives full pack voltage. Alternatively you could use a logic-level FET (VGS(th)(max) for a logic-level FET is a max of 2V) - their IDS(max) will tend be a little lower but you can still find logic-level FETs that are more than capable of switching a pair of 180s. If you are moving to Hellcats in the future this should no longer be an issue as the voltage drop across the diodes shouldn't bring the gate voltage anywhere near as low as threshold voltage.
SirBrass

Franksie, Not really but when I get home from work I'll do a "back of the napkin" sketch of what joins where made where, take a picture & upload it. Electrically it looks like the bsuk circuit diagram, but how things are actually wired look different in real life often enough. That's the tricky thing with electricity.

When I get a new kit (I'll need one for reasons other than this... One of which being an awkward CA glue job on the mech switch to the loom), I'll put the FET in the front of the blaster below the flywheel cage. That'll let me run fewer 16awg wires all the way back to the trigger group as the wires for the acceleration trigger will be a simple 20 or 22awg signal wire that'll run from acceleration trigger common to FET gate. FET will definitely have a 10k resistor this time.

Another possibility here is that I was sent bad FETs. I sourced the wire, diode (from RadioShack), & FET independently & bsuk only supplied the switches & loom. I could only find the FET needed on ebay. Could be the seller sent me a bad batch?
OldNoob

If you used IRFZ44N which is the common Britnerf fet and an IN5400 series flyback diode then I have never had a bad fet, only done a bad job of installing it! There's a reason we standardise components and wiring looms, it's so this stuff is easier. Reputable component sellers are also a good idea.
Franksie

The reason I asked about internals is because I wondered if I could help you with 'packaging'.  You said you were running out of space....
SirBrass

I used a 3034 fet. And while I didn't do a great soldering job, the joins were flat (no gaps between joined surfaces) & there was no spillage between pins. Soldering the FET was a pain in the starts-with-"a"-ends-in-"se".

Would like to know if there's a good supplier here in the US that doesn't do "bulk" orders for businesses & the like. Had to be bad FETS. Unless my iron heated up the FET too much, but that's hard to avoid when you've got to melt all that solder in a 16awg wire onto a thin metal strip. Wiring the 20awg signal wire was quick, easy and probably my only professional looking join.
OldNoob

We get our parts from RS Components or Farnell and I use Bright Components on ebay for smaller quantities. You didn't get a bad fet. Move beyond that premise.
We have proof a registered blind person can solder them. People have given you several solutions to try, I would start with those. There will also be some new mosfet based stuff to look at in the next few weeks if you keep an eye on here that may make the job simpler.
You can spread the pins a little further apart to solder them, tin the components and wires first so you don't have to sit the iron on as long. I would ditch a mosfet on the pusher unless you are running a microcontroller, it's totally unnecessary and adds nothing to the build on its own.
SirBrass

Thanks. I'll go with SSGT's suggestions, but I do wonder how I had to have screwed up badly enough to cause the FET to fail each time.
OldNoob

You have wired it wrong and it cooked, we have all done it. I popped one last month because my flyback diode wasn't properly soldered to the motor wiring, it happens.
SirBrass

That's what I thought the first time. So I double checked all my connections (including the diode), and wired in a new FET. Same thing. Rinse, repeat. I may be a n00b at soldering but I checked and triple checked that I had the right wires going to the right pins. Tested with alkalines first each time too. Gate fused open every time I pulled & released the trigger. I had no voltage drop diodes as the pusher is the same motor as the flywheels, only the flyback on the flywheel motors. It hadn't gone pop either (I checked).

So if I had the circuit right (I did. Absolutely 100% certain of this), and I didn't have a bad FET, then it has to be that I cooked the FET with the iron each time (distinct possibility). That sucker got burning hot to the touch several times trying to get a good join with 16awg wire.
OldNoob

You have got the circuit wrong, that I guarantee.
Boff

Pfft... If you've cooked the FET using your iron I'll eat a hat. I just baked several FETs at 275C for 5 minutes in an oven for reflow soldering and they're functioning fine.

It would really, really help if we could get some photos and possibly a circuit diagram of what you're trying to achieve. It's next to impossible to assist with just text descriptions.
SirBrass

OldNoob wrote:
You have got the circuit wrong, that I guarantee.


Only if I'd somehow messed up the switch wiring.

Boff, I'd do that if I hadn't scrapped most of the wiring connections afterward. Really regretting doing that. I'll work up a circuit diagram of what connections I made where according to what I know I did (took me several afternoons so I remember all of that work).
SSGT

SirBrass wrote:
Gate fused open every time I pulled & released the trigger. I had no voltage drop diodes as the pusher is the same motor as the flywheels, only the flyback on the flywheel motors. It hadn't gone pop either (I checked).


If you didn't use a train of voltage drop diodes that pretty much rules out excessive power dissipation from having the FET not being switched on fully (other than possibly whilst the gate was floating but that seems unlikely). If everything else in the circuit was fine a gate over/under-voltage seems the most likely either through EMI or a voltage spike from interrupting current flow to the motor (it doesn't necessarily need to have been at the gate either - you can exceed VGS(max) with a voltage spike at the source pin). If it failed on main trigger release then it could very well have been the latter.

A diode across the pusher motor (to kerb voltage spikes - motor and FET do share a common ground rail) and a resistor across gate-source (if the gate isn't floating EMI is less likely to charge the pin) would definitely be the first things I'd try in the next build. Nowadays DOA FETs are pretty rare - one bad FET is a possibility but two or more in a row is very unlikely. It's possible a whole batch was faulty but that should have been picked up by QC if they do their job properly and test a random sample from each batch. It's also possible they were handled carelessly (not necessarily by yourself, it could easily have been the reseller) and damaged through static discharge.
SirBrass

How much accidental static discharge would it take to damage a FET?
SSGT

Not a lot (not necessarily enough to feel the discharge) although you have to be working in an environment where you're able to build up a static charge in the first place. A lot of people have handled FETs without any special precautions and had no issues whereas I also know others (including toruk) that have had at least one FET blow mysteriously (IIRC also a 3034) via what they figured must have been ESD (if you replace a FET and suddenly everything is fine then failure due to ESD is probably the culprit). Then again Alice-Coatduck has a video where she's working with FETs which quite happily turn on/off just by touching them (implying she is transferring charge to the gate) and seemingly without suffering any damage at all so YMMV.

If you replace a FET and it fails again due to ESD either you're very unlucky or you/the supplier are doing something very wrong when handling them. You can add one or two appropriately rated zener diode(s) across the gate and source pins to limit gate-source voltage to a pre-set threshold but if you're installing the FET somewhere it'll never be touched it's usually unnecessary (although I guess you could solder them across before installing to help protect against ESD during assembly - some FETs come with this "ESD protection" on the package).
OldNoob

I have never taken any measures to protect from static, the only thing that's killed my fets is mistakes in wiring or EMI from motors.
SirBrass

EMI was my first & primary suspect.

Tonight I'm going to take my stryfe cage (should fit the RS just fine, right?), stryfe wheels, and wire up the RS with my hellcats just using the 10A switches that came with the basic "switch and loom only" kit. Since I saw SSGT's spreadsheet (thanks to your 2017 motors YT video, OldNoob), I'm more sanguine about letting the switch handle just the flywheel motors' current. I'll just keep the pusher activation off the acceleration switch to keep the current through the acceleration switch  down during freewheeling, since when  freewheeling, 3 hellcats will exceed 10A continuous.

That should do till I get a high power kit and matched workers (if there's still any available) from bsuk & Tacticoolfoam gets more red cages in stock.
OldNoob

RS cage is the same fit. Be aware the ribs on the inside of the RS cage need shaving down to fit mod motors unlike the Stryfe one.
We just make plug in motor blocks, then we can switch cages between looms whenever.
You are misunderstanding the circuit if you have the pusher anywhere near the rev switch. Also the 10A Cherry DC2 is easily able to cope with nearly 60A burst. I have had one on a test build where kids repeatedly tried to rev a double Xtreme 180 flywheel set up when it had a dart stuck, it fused the 60A fet, switch was fine. I have NEVER had a single DC2 failure in 3 years and over 50 top end 180 builds.
Most of my first few tri blades used a 5A switch since that was in the very early kits, all are still working, including BSUK RS number 001 which has the prototype kit in, running very basic switches of nowhere near today's standards.
You can simply follow my RS wiring video, add a plug in motor block in the front, then you can put a FET in later. It will take inrush current of 2 Hellcats easily. Use proper 3 switch wiring and the cycle control for a perfectly useable, reliable build.
SSGT

Note that a "10A" microswitch is rated to 10A continuous for a resistive load at 240VAC whereas we use them for motor loads at 8-12VDC. Thankfully, for low DC voltages, the resistive loads don't tend to be derated by much (if at all) and the motor loads may even be slightly uprated. A full size 10A Omron microswitch (datasheet - their submini datasheets don't have as much in the way of ratings) has a motor load rating of 3A @ 8VDC and 30VDC. Inrush rating is listed as 6x the continuous rating which results in a rating of 18A burst. For a single 130 or 180 motor (or even a pair of 130s in the case of Rhinos) that's probably sufficient but for a pair of 180s with a combined inrush current of over 40A I'd go for a full-size 21A switch instead if at all possible (a full size 21A Omron microswitch has a motor load inrush rating of around 40A at 11.1VDC if you interpolate between 8VDC and 30VDC values).

That said, and as Oldnoob has mentioned, many have used the humble DC2 to switch pairs of high current motors such as FK180SH-3240s/XP180 (and in the case of the BSUK wiring scheme sometimes a third for the pusher, although not necessarily all starting up at the same time) since the RS first came out with apparently little to no issues. They seem to be more than capable of switching that load they'll just likely have a significantly shorter lifespan.
OldNoob

I would say several years is a pretty significant duty cycle. If they were failing I would have had a ton of angry customers beating a path to my door.
SirBrass

OldNoob wrote:

You can simply follow my RS wiring video, add a plug in motor block in the front, then you can put a FET in later. It will take inrush current of 2 Hellcats easily. Use proper 3 switch wiring and the cycle control for a perfectly useable, reliable build.


Where would I stick the internal xt60 (only plugs I have & the hobby store closes same time I get off work)?

And good to know about those DC2s.

Edit: Upon reflection, the answer is obvious: battery tray. I got the 3s 65C 1AH Graphene lipo, which is much more compact than the 2.2AH 60C 2s lipo I'd originally intended to use when I wanted to triblade.
OldNoob

I would use deans. See my Rapidpistol internals video. If you must use XT60 the only place that will fit is in the mag well in the wiring cover with a big section cut out the top between the screw posts.
SirBrass

OldNoob wrote:
I would use deans. See my Rapidpistol internals video. If you must use XT60 the only place that will fit is in the mag well in the wiring cover with a big section cut out the top between the screw posts.


If I had deans on hand, I'd use them.

One additional item. There's a Singaporean modder by the YT name of Tungsten EXE. He's made a nice project RS that's arduino controlled but the real interesting thing is that he relocated the cycle control switch to be mounted on top of the pusher box. He attached some plastic to the top of pusher arm that sticks out of the box and that trips the dc2 lever upon pusher return. Much better idea seems to me than trying to have the cycle control switch itself be physically detached from the pusher box itself. And it makes use of all that space above the pusher box.
OldNoob

The space above the pusher box is for a fire control module. Then the battery box can fit a proper battery. You can put it on the back of the pusher box too. The issue with those locations is you add more wire and it makes plug in pusher boxes harder.
Fun fact- the location of the cycle control on the BSUK switch kit WORKS REALLY WELL, with thousands of units sold worldwide its had more test hours than almost any other mod.

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