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batusai2425 · 12th March 2008, 02:10 AM

Mosfet????? An airsoft gunsmithing told me to put a "mosfet" to my AEG to lessen the arcing on the trigger switch.The AEG uses 8.4v - 12V Depending on Motor voltage capacity.IS IT TRUE.
IF NOT HOW???

PLS. HELP

dknguyen · 12th March 2008, 05:09 AM

What's an AEG? From the very vague information you have given I would guess that you would want a fast diode instead of a MOSFET (but a MOSFET has a very slow, large, parasitic diode in it which may be what the guy is talking about).

Oznog · 12th March 2008, 05:52 AM

Airsoft Electric Gun. Fires those plastic pellets.
Some people amp up their guns, sometimes use more voltage than the mfg intended, and it can cause probs with not only the trigger but just about everything else in there. People upgrade the sector gear and all and yeah it shoots real fast.

dknguyen · 12th March 2008, 05:55 AM

A capacitor in parallel with the switch might help with the arcing. THe diode thing I was talking about is onyl for flyback spikes caused by disconnecting current to inductive loads which doesn't sound like it applies in your case.

I still don't know what the MOSFET thing is unless he was talking about replacing your a mechanical electrical switch with a MOSFET (which won't arc since it's a solid state switch).

Oznog · 12th March 2008, 06:12 AM

It's inductive, it's motor spikes. A diode may help.
He's probably talking about just that, using the switch to pull up a MOSFET gate to switch current rather than running the current through the switch. Unless you have a problem with the switch breaking on this model, I see no reason to bother doing this.

batusai2425 · 14th March 2008, 02:53 AM

hi thank's for the reply.I thought that you're familiar with the AEG.So i sent you another mail.Hope i'm not irritating.
the AIRSOFT ELECTRIC GUN is powered by a LI-POLYMER or NI-CAd. battery.
Here is the stock connection, from the positive terminal of the battery its is directly connected to the mechanical switch(+postive terminal) is then directly in series of the dc motor or dynamo (8.4v-12v).the other negative terminal is in series with a 15 amp. fuse and is then connected directly to the other terminal of the dynamo.

As per your info. it is possible to place a mosfet to the ckt. using it as a solid state switch to terminate the motor arcing..if so given the info can you please help me design a suitable ckt. and components for my AEG.

p.s. i hope the info i gave hepls us

Thank you for your time,
sai

dknguyen · 14th March 2008, 04:36 AM

Forget about the MOSFET. Just place a diode in ANTIPARALLEL with the motor (so there is no short circuit between the battery. the diode should be placed with the anode to the negative terminal of the motor and the cathode to the positive terminal of the motor).

Just like this (the resistance and inductor represent the inductance of the motor coil and the resistance of the coil's wire):
http://hades.mech.northwestern.edu/w..._schematic.gif

The arc is produced by suddenly disconnecting current through an inductive load (the motor). THe inductance tries to keep the current flowing and only lets it change gradually. To keep it flowing, a voltage spike is produced and this voltage spike will rise until a spark is formed somewhere to allow current to continue to flow (and collapse gradually). This spark is your switch arcing. It would happen even if you used a MOSFET instead of a mechanical switch (like a relay or anything else). THis is called the motor's flyback. THe diode gives this flyback current a path other than sparking across the switch when it opens.

EVen if you had a MOSFET, you would still need a diode in parallel with the motor to protect the MOSFET from the motor's flyback voltage spike. Unlike the switch which arcs and keeps on working, a MOSFET will BURN AND DIE when the spike happens if you don't have the diode. So leave the switch alone and add a diode.

PS. It's best to make it a low-forward voltage, fast recovery diode like a schottky diode. Make sure it can handle the peak pulse current of the motor and of course it has to be able to withstand the voltage of the battery in the reverse direction. You probably relax the low-forward voltage, fast recovery diode part since you are using a switch which has been working just fine with the arcing. If it was MOSFET on the other hand, it'd be more important since the MOSFET dies to overvoltage (which causes the arcing) more easily.

batusai2425 · 14th March 2008, 06:59 AM

thanks for the info the small feature of the diode will not compromise the secluded area inside the AEG.And it will remove the arcing on the switch hopefully.I'll look for it in google.
thanks a lot,
sai

batusai2425 · 14th March 2008, 07:04 AM

Hi does a schottky diode acts as a zener barrier which allows specific voltage to pass through????

sai

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12th March 2008, 02:10 AM	(permalink)
batusai2425	Mosfet Mosfet????? An airsoft gunsmithing told me to put a "mosfet" to my AEG to lessen the arcing on the trigger switch.The AEG uses 8.4v - 12V Depending on Motor voltage capacity.IS IT TRUE. IF NOT HOW??? PLS. HELP

12th March 2008, 05:09 AM	(permalink)
dknguyen	What's an AEG? From the very vague information you have given I would guess that you would want a fast diode instead of a MOSFET (but a MOSFET has a very slow, large, parasitic diode in it which may be what the guy is talking about).

12th March 2008, 05:52 AM	(permalink)
Oznog	Airsoft Electric Gun. Fires those plastic pellets. Some people amp up their guns, sometimes use more voltage than the mfg intended, and it can cause probs with not only the trigger but just about everything else in there. People upgrade the sector gear and all and yeah it shoots real fast. __________________ I thought what I'd do was I'd pretend I was one of those deaf-mutes.

12th March 2008, 05:55 AM	(permalink)
dknguyen	A capacitor in parallel with the switch might help with the arcing. THe diode thing I was talking about is onyl for flyback spikes caused by disconnecting current to inductive loads which doesn't sound like it applies in your case. I still don't know what the MOSFET thing is unless he was talking about replacing your a mechanical electrical switch with a MOSFET (which won't arc since it's a solid state switch).

12th March 2008, 06:12 AM	(permalink)
Oznog	It's inductive, it's motor spikes. A diode may help. He's probably talking about just that, using the switch to pull up a MOSFET gate to switch current rather than running the current through the switch. Unless you have a problem with the switch breaking on this model, I see no reason to bother doing this. __________________ I thought what I'd do was I'd pretend I was one of those deaf-mutes.

14th March 2008, 02:53 AM	(permalink)
batusai2425	hi thank's for the reply.I thought that you're familiar with the AEG.So i sent you another mail.Hope i'm not irritating. the AIRSOFT ELECTRIC GUN is powered by a LI-POLYMER or NI-CAd. battery. Here is the stock connection, from the positive terminal of the battery its is directly connected to the mechanical switch(+postive terminal) is then directly in series of the dc motor or dynamo (8.4v-12v).the other negative terminal is in series with a 15 amp. fuse and is then connected directly to the other terminal of the dynamo. As per your info. it is possible to place a mosfet to the ckt. using it as a solid state switch to terminate the motor arcing..if so given the info can you please help me design a suitable ckt. and components for my AEG. p.s. i hope the info i gave hepls us Thank you for your time, sai

14th March 2008, 04:36 AM	(permalink)
dknguyen	Forget about the MOSFET. Just place a diode in ANTIPARALLEL with the motor (so there is no short circuit between the battery. the diode should be placed with the anode to the negative terminal of the motor and the cathode to the positive terminal of the motor). Just like this (the resistance and inductor represent the inductance of the motor coil and the resistance of the coil's wire): http://hades.mech.northwestern.edu/w..._schematic.gif The arc is produced by suddenly disconnecting current through an inductive load (the motor). THe inductance tries to keep the current flowing and only lets it change gradually. To keep it flowing, a voltage spike is produced and this voltage spike will rise until a spark is formed somewhere to allow current to continue to flow (and collapse gradually). This spark is your switch arcing. It would happen even if you used a MOSFET instead of a mechanical switch (like a relay or anything else). THis is called the motor's flyback. THe diode gives this flyback current a path other than sparking across the switch when it opens. EVen if you had a MOSFET, you would still need a diode in parallel with the motor to protect the MOSFET from the motor's flyback voltage spike. Unlike the switch which arcs and keeps on working, a MOSFET will BURN AND DIE when the spike happens if you don't have the diode. So leave the switch alone and add a diode. PS. It's best to make it a low-forward voltage, fast recovery diode like a schottky diode. Make sure it can handle the peak pulse current of the motor and of course it has to be able to withstand the voltage of the battery in the reverse direction. You probably relax the low-forward voltage, fast recovery diode part since you are using a switch which has been working just fine with the arcing. If it was MOSFET on the other hand, it'd be more important since the MOSFET dies to overvoltage (which causes the arcing) more easily. Last edited by dknguyen; 14th March 2008 at 04:48 AM.

14th March 2008, 06:59 AM	(permalink)
batusai2425	thanks for the info the small feature of the diode will not compromise the secluded area inside the AEG.And it will remove the arcing on the switch hopefully.I'll look for it in google. thanks a lot, sai __________________ It is never humiliating to ask questions.......

14th March 2008, 07:04 AM	(permalink)
batusai2425	Hi does a schottky diode acts as a zener barrier which allows specific voltage to pass through???? sai __________________ It is never humiliating to ask questions.......