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Topic: MOSFET Recommendation (Read 692 times) previous topic - next topic

tack

Looking for someone with a bit more experience to recommend me a logic level, SMD (could be through hole if required) MOSFET, for the following application:-

Source voltage would be 7.2-12V, more than likely form AA's, but could be from a 12V SLA or a LiON.

I need to switch that directly across a thermo igniter, essentially an electric match. These igniters are then used to fire pblicly available pyrotechnics, such as a smoke grenade or flashbang.

Now, with AA's in series, maximum current should remain the same so they should not be able to deliver more than a couple of Amps, and it will be for fraction of a second, maybe 200-500ms transient. The igniters are essentially self destroying and go open circuit anyway, but I'll still pulse the output.

I could insert a resistor into the circuit, but I'd need one that could cope with the current, voltage and transient time and continue to operate for 1000's of operations.

My initial thoughts are that a well rated MOSFET could just switch these from AA's, but the complication comes if a battery is being used which can source a higher maximum current.

Measured resistance of e-match is 1.5 Ohms, so a 12v source would give a maximum transient current of 8A, then dependent on the batteries ability to actually deliver that.

It would more typically be 9v (via 6 x AA in series), so theoretical maximum 6A.

The datasheet on Duracell Procell AA's doesn't state maximum discharge, but just says that it could deliver 1A with a discharge curve indicating less than 1 hour. (Datasheet link attached).

Does anyone know typical short circuit currents for different battery types and can you advise on a suitable MOSFET or supplementary circuit to switch this type of transient load?

The same power source will provide 5v and 3.3v to AVR and other items, via linear regulators, so my other concern is voltage dip affecting BOD. Could this be mitigated a little with some large reservoir caps on the input to the regs?

Thanks in advance for any help.

MarkT

Do you need to switch high-side or can you get away with low-side switching?

If low-side OK then a logic-level MOSFET with Rds(on) of 0.05 ohms or less will be fine,
just a 150 ohm resistor from Arduino pin to gate, source to GND, drain to
"match", +ve supply to other end of "match".

High-side switching needs a small NPN transistor to level shift and a non-logic-level
p-channel MOSFET.  Search here for examples.
[ I won't respond to messages, use the forum please ]

tack

The voltage reaching the 'match' is irrelevant, so I can't see any issue with low side switching.

CrossRoads

http://www.digikey.com/product-detail/en/AOI516/785-1568-5-ND/3152480

low-side switch.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

tack

Thanks CrossRoads

I've not used any FETs before now.

CrossRoads

Watch out ESD wise, discharge yourself by touching something metal before touching any pins.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

MarkT


Watch out ESD wise, discharge yourself by touching something metal before touching any pins.


Helps not to have nylon carpet or nylon clothing (and live in a damp country like the UK!)
[ I won't respond to messages, use the forum please ]

JoeN


http://www.digikey.com/product-detail/en/AOI516/785-1568-5-ND/3152480

low-side switch.


I've never heard of this company before and they appear to be relatively new.  How did you hear about them and this component in particular?
I have only come here seeking knowledge. Things they would not teach me of in college.

CrossRoads

Digikey? They are one of the bigger parts distributors out there.

"Since its founding in 1972, Digi-Key has been committed to offering the broadest selection of in-stock electronic components, ..."

"Digi-Key is an authorized distributor of electronic components for more than 650 industry-leading suppliers. This means Digi-Key customers can rest assured that the product they order is authentic and comes to Digi-Key directly from the manufacturer. "

As an electrical engineer I know of lots of prime distributors, a few I use regularly including Digikey are listed below.  As an electrical engineer, I also am concerned about counterfeit & fake electronics. So you won't see me buying questionable parts from e-bay; a good part of that is also repeat purchasability.  My active electronics all come from Digikey & Mouser, and MAX7219's from taydaelectronics as they seem to have working parts (so far), altho I have e-mail from Maxim saying tayda is not an authorized reseller.

Digikey
Mouser
Avnet
Newark/Element 14
Arrow
Farnell

Then there's the secondary market that has much smaller selection:
DipMicro
TaydaElectronics
Jameco
AllElectronics
PartsExpress
MPJA

I use the filtering capabilities of Digikey all the time to find a part.
In this case:
N-channel mosfet, in stock, logic level gate, through hole.
Sort the list by price, look for a part with Low Rds, Low Gate capacitance.  That's one of the first parts with Low Rds that appears with low price.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

rmetzner49

The decision whether to use high-side or low-side switching is generally one of convenience and safety.  If you switch low-side then one side of all your "matches" need to be tied to the battery+ and you energize them by connecting them to "-" via the FET.  In a machine tool or a car, that's a concern because if your switched wire happens to fall of and hit "ground", it will cause the actuator (or "match") to fire.  Probably undesirable to have your pyrotechnics go off unexpectedly.  There are some really neat "high side drivers" available for this reason.

JoeN


Digikey?


No.  Sorry, I know all about them.  I mean Alpha & Omega Semiconductor Inc and their part AOI516.  What have you used this part for in the past and how did you come across it?  I have never heard of AOS.


I use the filtering capabilities of Digikey all the time to find a part.
In this case:
N-channel mosfet, in stock, logic level gate, through hole.
Sort the list by price, look for a part with Low Rds, Low Gate capacitance.  That's one of the first parts with Low Rds that appears with low price.


OK, I think I get it now.  It just looked good and you gave it a shot.  Nothing wrong with that for a small buy.
I have only come here seeking knowledge. Things they would not teach me of in college.

CrossRoads

I have not used the AOS part yet.
I have used N-channel MOSFETs with similar characterstics - those just happen to  be the best price   right now.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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