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Topic: Mosfet question (Read 980 times) previous topic - next topic

moncureww

Jul 01, 2012, 07:42 am Last Edit: Jul 01, 2012, 07:45 am by moncureww Reason: 1
I am looking to drive a number of 1157 automotive bulbs utilizing a 12v DC MOSFET configuration paired / breadboarded to my Uno.

1157 bulbs are typically used in automotive applications - you know, night time lights when you're driving and then they brighten and become brake lights.

In my application I need to creatre a functional circuit controlled by a switch that mirrors this wattage brightening effect - I am assuming that MOSFET controlled would work best here.

Like in automotive night time mode, I need the bulbs to run at the standard luminescence that is effected at 10W, the brightened mode at 27w, and I need to turn them off completely. I'd like to code this (seems it would be fairly easy) but the difficulty is determining the hardware to effectively run this circuit (or one of the circuits) from my Uno.

Essentially, what I'm looking around for is a circuit setup that will allow me to control my 12v light/lights (in tandem, two) much like the effect you would get of stomping on your brakes at night by utilizing a momentary normally off switch with instructions coded through arduino. Any advice or tips? I looked around a great deal and couldn't find anything much on this specific topic.

Thanks for your help!

CrossRoads

Those bulbs do two brightnesses by using two filaments.
Are you planning to control the two elements?
Controlling 2 Transistors to do would seem pretty straightforward.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

moncureww

Why specifically a transistor? And which sort of transistors would be best for this application? Again, this is a project I've taken on as an educational endeavor - but if a MOSFET configuration would be possible I'd like to proceed in that direction. I have just grown comfortable with pre-coding most of my project using MOSFET's so far - and I just haven't jumped into utilizing transistors for this application.

CrossRoads

#3
Jul 01, 2012, 09:12 am Last Edit: Jul 01, 2012, 09:24 am by CrossRoads Reason: 1
MOSFET is a transistor. Good for controlling higher currents while keeping cool due to very low internal on-resistance.
N-channel would be good, can switch 12V on & off using 5v control levels from the Arduino.
Search at digikey.com:
N-channel mosfet, fets-single, in-stock, logic-level, thru hole, will come up with ~12 pages of choices. Sort by price, then scroll down to find lowest Rds part you can (down in the 50 mOhm (milli0ohm) range) in the package you like and the cost you can accept.

That method yields this one as a good example I would think.
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

CrossRoads

Wire 12V to one side of  the light, other side to MOSFET, other side of MOSFET to GND.
Called Low Side switching. Lots of examples in the forum and playground.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

MarkT

In the UK these bulbs typically have 5W and 21W filaments, taking 0.4A and 1.75A respectively when on.  However bulb filaments take many times more current at startup from cold (when the resistance is much less).  The switching device must be able to carry these higher start-up current pulses safely - assume about 8 times the hot current, so lets assume about 15A peak pulses.  The FETs need to be rated at least this for current if the power supply is capable of supplying it (and the power supply probably needs to be able to give at least 3 or 4A to guarantee bring the filament up to temperature BTW).

In order to run cool they should also not dissipate lots of heat.  Power dissipation in a FET that's on is I-squared-R for R=on resistance.  An on-resistance of 0.05 ohm would mean about 150mW dissipation for 1.75A, which is reasonable.

So, we're looking for two _logic level_ n-channel MOSFETs rated at 15A or more, Rds(on) of <= 0.05 ohm for each bulb.  Voltage rating of >= twice the supply suggests 30V or more for the voltage rating.

Wiring is Arduino-pin to 150 ohm resistor, other end of resistor to the MOSFET gate.  MOSFET source to 0V (ground).  MOSFET drain to bulb filament, other side of filament to +12V supply.  Adding a 10k resistor between gate and source will hold it switched off while the Arduino resets.

And don't forget it needs to be a logic-level MOSFET.
[ I won't respond to messages, use the forum please ]

CrossRoads

So you're in agreement this part is ok?

FET Type MOSFET N-Channel, Metal Oxide

FET Feature Logic Level Gate

Drain to Source Voltage (Vdss) 60V

Current - Continuous Drain (Id) @ 25° C 20A

Rds On (Max) @ Id, Vgs 39 mOhm @ 10A, 10V

Vgs(th) (Max) @ Id 2.5V @ 250µA

Gate Charge (Qg) @ Vgs 15nC @ 10V

Input Capacitance (Ciss) @ Vds 675pF @ 25V

Power - Max 36W
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

dc42


So you're in agreement this part is ok?

FET Type MOSFET N-Channel, Metal Oxide

FET Feature Logic Level Gate

Drain to Source Voltage (Vdss) 60V

Current - Continuous Drain (Id) @ 25° C 20A

Rds On (Max) @ Id, Vgs 39 mOhm @ 10A, 10V

Vgs(th) (Max) @ Id 2.5V @ 250µA

Gate Charge (Qg) @ Vgs 15nC @ 10V

Input Capacitance (Ciss) @ Vds 675pF @ 25V

Power - Max 36W



Since you will be using logic level gate drive, the Rds(on) that matters is the one quoted at the Vgs you will actually be using, not the one quoted at 10V. If it really is a logic level mosfet, then it will have a specified Rds(on) for Vgs = 4.5V or thereabouts as well as the one for 10V.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

CrossRoads

Datasheet says 50mOhm at 4.5V.
So between 50mOhm and 39mOhm at 5V.
Fig 3 & 4 show even lower at low currents.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

dc42

50mOhm is the value you should use then, at drain currents up to the one that value is quoted at. Ignore the graphs unless you are feeling lucky, they show typical values, not guaranteed values.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

MarkT

So yes, that ought to be fine.  That's the NTD5867NL mentioned already by the look of it.  There will be many other possibilities but few will be much cheaper.
[ I won't respond to messages, use the forum please ]

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