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Help with mosfet circuit needed

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chownsy

New Member
Dear all,

Im brand new here so be kind, I have a couple of questions and some help needed with a mosfet circuit im building for my airsoft rifles.

basically the system is an Active braking circuit.

I have been supplied some schematics from a well known supplier of these products but it seems wrong (works when constructed but im not sure if the braking circuit is working correctly).

Also i have a few questions about the Fet's im choosing to use.


So here is the details.


https://extreme-fire.com/SupportDocuments/AEG-FET-Switch-Schematics.jpeg

the circuit im using is the top SW-AB system.

Ignore the mosfet part numbers on the diagram as im using these in replacement:

P channel FET - IRF5210S replaced with SPP80P06P
N channel FET - IRL1404ZS replaced with STP75NF75

the capacitor part number is 222206036109

the resistors are carbon film

100R = CFR16J100R (0.25W)
220R = CFR100J220R (1W)
4.7K = CFR100J4K7 (1W)

the TVS diode is shown as a bi-directional device but this is an DC circuit so that is another part that is confusing myself.


Would someone be kind enough to check the circuit and tell me if it would activly brake the motor. Basically in airsoft rifles single shot tends to overrun on higher voltages or miss trigger pulls, this system completes the cycle and leaves the spring uncocked because it does not overspin.

any more information needed ask and i will try to supply as much as i can.

Regards

Richard

EDIT: spelling and link
 
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MikeMl

Well-Known Member
Most Helpful Member
No comment on the Fet replacements; just print out both data sheets and compare specs.

It takes one FET wired to the power source to start/run the motor; it takes a second FET and a resistor wired across the motor to accomplish dynamic braking. The two are independent; but the external switching can combine them so that the motor is always braked whenever it is not driven.

The TVS is used for transient suppression. Inductive ringing can cause either polarity to appear across it.
 
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chownsy

New Member
that sort of does not answer the question of does the supplied circuit diagram work. Or does it need modifying?

Thanks you for the reply so far.
 

MikeMl

Well-Known Member
Most Helpful Member
I know nothing about Airsoft.

What is the trigger signal? Hi Voltage? Lo Voltage? Duration? How often?

Battery Voltage? Motor voltage? Current?
 
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chownsy

New Member
I know nothing about Airsoft.

What is the trigger signal? Hi Voltage? Lo Voltage? Duration? How often?

Battery Voltage? Motor voltage? Current?

Battery voltage ranges from 7.2v to 15v.
Current draw usually upto 50amps.
On normal system with out mosfet the trigger is in series and takes full load. With ab mosfet it runs very low voltage due.
The circuit diagram on the link shows the circuit.

The mosfet system does 2 things.
1 stops high voltage on the trigger contacts and sends all the power to the motor instead.
2 the active braking stops the gearbox over spinning so on single shot its ready to fire again. And on full auto fire it stops the motor over running after a long burst.

Trigger can be pulled loads in rapid succession or held for long 5sec to 20 sec or more durations.


Hope this is a bit more helpful.
 

MikeMl

Well-Known Member
Most Helpful Member
So one end of the trigger (switch) is connected to BAT+, while the other end is connected to the TRIGGER input shown on the schematic?

The motor starting current transient might be 50A, but it is doubtful that the running current is that high. What is the battery? Type? Ah capacity?
 

chownsy

New Member
So one end of the trigger (switch) is connected to BAT+, while the other end is connected to the TRIGGER input shown on the schematic?

The motor starting current transient might be 50A, but it is doubtful that the running current is that high. What is the battery? Type? Ah capacity?
I dont know the running ampage of the motor.
The batterys can be nicd nimh and lipo. All mah from 600 to like 4600mah.
If you go to www.extreme-fire.com that is where i got the information from.
Regards
Richard
 

MikeMl

Well-Known Member
Most Helpful Member
I dont see any obvious improvements.
 

marcbarker

New Member
Check out the Gfs [parameter in datasheet] . It's a bit low on the ST MOSFETS chosen.

Really, to diagnose this problem, and improve on their design you need to set up a storage scope triggered to record the voltages and current pulse. To see what's actually really going on.
 
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chownsy

New Member
Check out the Gfs. It's a bit low on the ST MOSFETS chosen.

Really, to diagnose this problem, and improve on their design you need to set up a storage scope triggered to record the voltages and current pulse. To see what's actually really going on.
How do i do this and how much will it cost.
I really want to try and make the best product i can
Regards
Richard
 

marcbarker

New Member
Well traditionally, if you are lucky enough to have access to lots of test equipment, you start off with digital storage scope with 4 channels, add a clip-on current probe to it, and some means of recording the data recorded on the scope. Often the scope has a built in FDD to save images on, or nowadays it's a USB memory stick.

Then you connect up all the probes so that you can record the events all going on at the same time, things like motor current, motor voltage, gate voltage, drain voltage.

Set the DSO to capture SingleShot only. Trigger the capture off one of the channels, usually the gate voltage.

From studying the voltages and that you can suss out what's going on, whether the FET is wimping out, or whether the gate isn't being driven hard enough and all that stuff.
 

chownsy

New Member
sounds complicated. I will ask my dad if he has access to that stuff. If not then i will just have to carry on using the current (no pun intended) setup.
So just to clarify the circuit will brake the motor when the trigger switch is released.

Thank you all for the help so far :)

Regards
Richard
 

marcbarker

New Member
yes the circuit looks about right. It's a nice 'minimalist' style of design. the P-fet allegedy clamps the current, according to the way it's drawn. Whether it does in real life I'm not sure until scope traces are seen.

The Gfs parameter on the 'alternative' fets looks a bit low compared to the original, maybe it's critical. If you suspect the p-fet isn't clamping that well, you can add another p-fet in parallel to boost the effect. That's ever so easy to try out.
 

chownsy

New Member
sorry to sound dumb, but what is GF's value?

also i went for the newer part numbers because they had higher voltage, ampage and wattage max value (so i thought it could handle higher ratings)

My supplier is Welcome to rswww.com
 

chownsy

New Member
I have all the parts i need for this except the 2 mosfet units,

can some one advise me on one to use.

the origional part numbers are

N channel - IRL1404ZS
P channel - IRF5210S

for my 1st venture into this i used:

N channel - IRFZ48NPBF
P channel - IRF4905PBF

but i have now found these:

N channel - STP75NF75
P channel - SPP80P06P

will they work better than the origionals.

you mentioned the GFs of them, does the time (S) have to be as low as possible for better use?
 

marcbarker

New Member
I've taken off the attachment because something had gone wrong somewhere I'd shown the wrong MOSFET curve for the first Pfet.

I'll make another posting...
 
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marcbarker

New Member
The parameters on the data sheet that are important for you right now are getting a low Rds(on) and a reasonably high gfS. Check there's enough Id, there should be enough, because a low Rds(on) is usually hand-in-hand with high Id. Luckily you don't have many volts to worry about. Nor is any dynamic parameters much concern to you.

Automotive 'fifth generation' FETS seem to gravitate in this direction, also TrenchFet might be the holy grail of RollsRoyce of fets for you. The curves on a datasheet are really useful, hardened engineers study them closely, and in their head these curves 'come alive' to them and they can watch them animated.

Bear in mind that paying more for a lower Rds(on) is diminishing returns, because there is still the resistance of the connecting wires, solder joints, and of course the electric motor winding resistance. 20 mΩ is sufficient, but you could get a better feelgood choosing an 8 mΩ one if it's the same price.

Rds(on) is the lowest possible resistance possible when the FET is under ideal conditions. gfS is forward transconductance, it's a fancy way of saying "how many Amps for each Volt". The two together vary the shape of the curves.

Once you've built it with FETs you believe in, and the circuit still doesn't work after that, it's still going to be difficult to tell what's really going on without recorded scope readings of the voltages and currents in the actual circuit. You'd need to set up a triggered scope and capture waveforms for studying.

PS. in a test setup, you are allowed to try things like connecting Fets in parallel to boost power.
 
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chownsy

New Member
in my 1st test setup that works but the braking P-fet circuit dont work as great as ive seen them.

I will carry on looking at the Fets that are in the range that im looking and see if i can see one thats low, if i think its low and then post on here and get you to double check, then im going to go ahead and test that one works.

Regards

Richard


EDIT: i have found this P-fet with what i beleive a low RDS(on) - STP80PF55 and this N-fet - STP80NF03L
 
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marcbarker

New Member
Sometimes in product development, it's beneficial to put your brain on a shelf, and stop thinking too hard. You can get a collection of trial parts, and then go through them all trying each one in the circuit and seeing what happens. Thomas Edison used to work this way. You often find this for a critical opamp circuit, so you have an IC socket and a tray of different opamps to try while having a coffee. It might be that you find something that's awful, but you might find something that works really well. Later on you can study the data and see why.
 
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