Reviving a MOSFET?

[Skladfin]

Hey guys,

if my JG mosfet suddenly just starts letting current through without any warning, does that mean the MOSFET is broken?

basically what happens is if I just connect the deans to the battery, the gun will just start firing automatically and doesn't stop unless i disconnect the battery. This happens a lot of my previous mosfets and im wondering if I can just fix something on the fet itself.

Or is it permanently broken?

[Kos-Mos]

The MOSFET it self (the actual part that does the switching) is toasted. That is typical for N-FETs to burn closed (let the current flow).

You can change it if you know how to solder... most probably surface-mounted though.

But if it burnt, there is a reason. If you replace it with an other one, it will burn again.
A good modification that is seldom done is to install a high power stocky diode across the motor poles. That will soak a lot of the EMF that is harmfull to the MOSFET. Most higher grade FETs have a diode built-in... but when it burns it brings the whole FET with it.

[MadMax]

Originally Posted by Kos-Mos

The MOSFET it self (the actual part that does the switching) is toasted. That is typical for N-FETs to burn closed (let the current flow).

You can change it if you know how to solder... most probably surface-mounted though.

But if it burnt, there is a reason. If you replace it with an other one, it will burn again.
A good modification that is seldom done is to install a high power stocky diode across the motor poles. That will soak a lot of the EMF that is harmfull to the MOSFET. Most higher grade FETs have a diode built-in... but when it burns it brings the whole FET with it.

Kos-Mos is right on the money. Circuit designs without a back EMF diode put a very heavy load on the FET when the motor is depowered. Basically, DC motors have high inductance after they've spun up. They'll output very high voltage spikes when they're depowered because the energy in magnetic field in the armature ends up getting dissapated in the FET in the form of a voltage spike. A reverse biased diode provides an alternate path for this voltage to dissipate.

In simpler terms, the motor tries to maintain a constant current flow after the FET is turned off so a high voltage results when the FET is turned off. The same problem exists with switches which is why TM still uses graphite blocks which are fairly spark tolerant.

[Skladfin]

amazing on the nose explanations from both of you there.

what would typically cause this? With lesser technical terms. This occurred to me right when switched the selector to semi. It was shooting fine before as well.

As well, N-Fet would be the only thing which I would have to replace correct?

[Kos-Mos]

Technically, yes only the FET needs to be replaced.

But if there was some kind of protection on the board and it fried too, that means there is an other problem in your rifle.

You might have a short somewhere, and your selector plate touched the mechbox just enough to spark...

Than and most cheap MOSFET units use a P-Channel fet to avoid changing the stock wire configuration... for any given model of FET, the P-channel version is a lot less powerfull and rated for lower current...

Makes me think that I just got some nice prototype FETs from Vishay Siliconix... got to do some new toys.

[MadMax]

I think it's more likely that you were blasting away on full auto with the motor having reached full RPM. My guess is that an AEG motor would reach a higher RPM while firing a full auto burst than at the end of a semi auto burst. You get a good 20-30 revolutions per shot, but the second shot starts off with the motor already spinning so my guess is that subsequent shots would have the motor spinning faster than the first cycle.

Anyhow, the net effect of this is that the motor is spinning faster just when you turn it off which results in a higher field collapse when it's depowered. Moving the selector switch may be a red herring, unless you're running a MosFet cct which does something different in semi mode. There are microprocessor driven ccts (such as DonP's FET drivers) which offer microprocessor driven functions.

I doubt that static electricity toasted your FET. Most of the connections would be pretty isolated from areas you could touch from the exterior of the gun. Look at your FET for a model number and maker. You can probably find a direct replacement here: digikey.com. I can't recall what their minimum order is, but they are one of the biggest carriers of electronic components that sell in small prototype quantities. You should be able to type the part number (say LM511) right into their search bar. Alternatively, find an electronics component shop in your area. If you were in Toronto, I'd recommend SupremeTronic, but I note you're in Vancouver. Anyhow, I like finding stuff at electronic stores. They tend to be staffed by people who know a bunch about a discipline which is nerdy yet practical. The store keep would be able to help you examine your circuit to see if there is a "reverse biased diode to sink the motor". If not, they'll help you get an appropriate diode and point out where to solder it.

The following is an explanation of what I suspect fried your FET. I've got time to kill because my wife gave me an excellent bottle of bourbon and talking about this sh*t scrapes the rust off of mental machinery I don't exercise often enough. I'll try to be comprehendable, but there be dragons here. The classic "how do magnets work" kind of trouble.

There is a phenomenon with DC brushed motors where they generate a voltage when they're spun. Basically you can chuck a motor shaft in a power drill and spin the motor to generate a voltage. The funny thing is that when you apply a voltage to a motor and spin it up, this generated voltage does not disappear. In fact, it is the reason why a DC motor doesn't spin up endlessly into an infinite RPM and explode when you apply a voltage to it's terminals (with no shaft load). When you apply a voltage to an unloaded motor, it spins up until it starts generating a voltage in opposition to the voltage your applying so it starts to balance against the voltage you're applying. In this condition, the motor exerts very little torque (only overcoming windage and frictional losses) and draws little current.

This is the phenomenon is called back EMF and unfortunately it is not the thing that I suspect is killing your FET. Back EMF does not reach a very high voltage. At best, it can only match the voltage of your battery pack, albeit in a reverse polarity, but most power FETs should be fine with this. It's been a longass time since I wrote this exam...

There is another phenomenon which I suspect is the culprit. Kos-Mos is aiming in the right direction with diode protection. We just use the wrong word to describe the voltage spikes that could be toasting your FET.

DC motors have coils which establish a magnetic field when you pass a DC current through them. The ugly thing about coils wrapped around iron armature plates is that they have a lot of inductance. That is: they can store a significant amount of energy in the magnetic field they establish when current is passed through. This energy has to be dissipated somewhere when the field collapses (motor is disconnected) and it MUST be dissipated in order to obey the law of conservation of energy. Hellooo FET!

The thing about inductors is that they do not allow sudden changes of current flow through them. In mathematical terms, the voltage across an inductor terminals is represented by: V = L (inductance in Henries) * di/dT (rate of change of current). Soooo when you open circuit and disconnect the motor from voltage, your rate of change of current goes infinitely negative which results in very high voltage. Since infinites don't actually occur outside of CERNs Large Hardon Collider, voltage rises until it can find a current path which unfortunately is through your FET so it's basically welds the junction closed in a quantum sense.

Putting a diode across the motor terminals, such that it is reverse biased in regards to the usual polarity applied to the motor, will draw no current when the motor is powered (wrong polarity so the diode does nothing). However, when you depower the motor, the diode provides an alternate path for current to flow. The high inductive spike that would have occurred now has an easy path to discharge though.

I do suspect that it would be helpful to put a low resistor in series with the diode though. While inductive discharge can reach very high voltages (olde cars used inductors to generate spark gaps!), back EMF comes back into play here (I though MadMax said he was wrong!). Shorting a motor that is already turning brakes it rather suddenly. That can be a bit hard on a motor as it causes brush arcing. A small resistance will throttle the stop slightly and reduce brush wear.

I hope this helps somehow.

[coach]

Originally Posted by MadMax

If you were in Toronto, I'd recommend SupremeTronic,

Okay good. So I'm not the only one that goes there. Though I miss it when it was on Queen and when they moved to the south side of College. Sharing space with Home hardware is kind of weird and way too compact. (harder to find stuff)

I had some interesting conversations with the old Chinese guy with glasses over the years. Much better service and quality over Active Surplus.

[Kos-Mos]

For replacement parts, especially MOSFETs, the best place to go is the manufacturer's website.

Some of them will have a in-house shop that cost a lot less than any other place.

Thing with digikey is they charge 20$ flat shipping, regardless of what you order...

Once you know what FET you need, you can probably find an alternative from a company that GIVE samples (Vishay Siliconix are awesome).

The FETs I just receved are for an update on a trigger control unit...
http://www.vishay.com/docs/90377/90377.pdf

As you can see, this one runs 60V@50A as long as the cooling is sufficient to cover the heat generated. It also supports burst (30% PWM) of 290A.
And the internal diode can sink 4.5V/ns. That is a decent rating for protection, but I would still add an extra diode across the poles.

Check the board if there is a resistor as MadMax said. It should be included in the design to de-gate the FET when the trigger signal is removed. If not, the FET will enter triode for a small moment at every turn-off and that is bad for heat. *edit* sorry, it should be across G/S or G/D depending on the FET used.

[MadMax]

Originally Posted by coachster

Okay good. So I'm not the only one that goes there. Though I miss it when it was on Queen and when they moved to the south side of College. Sharing space with Home hardware is kind of weird and way too compact. (harder to find stuff)

I had some interesting conversations with the old Chinese guy with glasses over the years. Much better service and quality over Active Surplus.

Yeah, I loved it when SupremeTronic was on Queen. I used to lump my trips to Active and Supreme together. The guys at Active aren't electronics wizards. The guys at Supreme have all the equivalence lookup tables and they actually know about electronics design.

As to Kos-mos's comment, I forgot about sinking the FET's signal input. Truth is I've never seen a circuit which didn't do that. Disconnecting a FET gating input and letting it float to zero would work, but it'd be kind of goofy. I really meant to say that a reverse biased diode should be in series with a low value resistor so you didn't short the motor when voltage was removed. Braking a DC motor is common in cordless drills, but if you watch them suddenly stop, you see quite a lot of arcing which would eat up brush and commutator life. It doesn't matter as much in cordless drills because the on-off cycles aren't quite as high as in our AEGs. I put out probably 300 semi shots or short bursts in a game day. That's a lot of on-off cycles on a cordless drill.