Mosfet switch for small flashlight

Discussion in 'The Projects Forum' started by steveomiami, Apr 20, 2013.

  1. steveomiami

    Thread Starter New Member

    Apr 20, 2013
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    I want to build a mosfet switch into the tailcap of a small regulated flashlight. I am good at soldering, I have all the tools and all of the solder and irons. However, I am not good with the technical side of electronic circuits. The flashlight is small, takes 1 3.7 V battery. The + of the battery touches a spring in the head. The - touches the spring on the on/off switch. In the head of the flashlight there is a circuit regulated that will draw 10 amps from the battery. The on/off switch connects the spring to the body of the flashlight. The problem is this much current will destroy pretty much any small flashlight switch.

    I want to build a Mosfet switch in the tailcap that works the same as the switch thats there now, pretty much it will open and close the circuit between the flashight body and the spring that goes to the main battery -. Theres no room to wire anything to the + on the main battery. I want to include a flat lithium battery in the tailcap to provide the power to the mosfet to open and close the gate. The inside of the tailcap is about 18-20 mm. I'm doing my best to explain this please let me know if you need some clarification.
     
  2. KMoffett

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  3. shortbus

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    This makes no sense. Where are you finding a "small" battery that is capable of 10A at 3.7V? What type of "small" bulb needs 10A?

    The headlight bulb of a car only uses around 4.6A. You may have your values wrong here.
     
  4. wayneh

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    Sep 9, 2010
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    Something is not right.
     
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  5. absf

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    Dec 29, 2010
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    A 1W power LED is only 330mA at 3.3V. I used 5 of these to make 5W energy saving lamps for home use.

    Allen
     
  6. steveomiami

    Thread Starter New Member

    Apr 20, 2013
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    The values are correct but they dont really matter for the question I am asking. I think my first post must be a little confusing. The battery that is driving the flashlight will not be involved in the circuit I am asking for. The battery driving the light is an AW IMR 18650 3.7V lithium Ion battery that does supply a steady 10 amp supply. There are others that can do up to 20 A. As far as the LEDS go there are some new leds such as the Cree MTG2 which can handle 10 amp for short periods of time if properly heatsinked. The flashight has modes so low is only like 150ma and the High will actually be 9A or 6.6A depending on which driver I end up using.

    That part is already figured out. What I'm needing help with is a mosfet switch in the tailcap. There will be a permanant small flat lithium ion battery installed in the tailcap, a round flat buttontop battery. This battery needs to open and close the gate thus connecting/disconnecting ground to the battery when a small switch on the end of the flashlight is switched off and on. I know this can be done and works because theres a guy on another forum who use to make and sell these switches but I'm not able to get my hands on one since he left the forum.
     
  7. steveomiami

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    Apr 20, 2013
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  8. wayneh

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    Sep 9, 2010
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    That seems like a fine approach. What are you asking about?

    You won't need the resistor divider on the gate, since your voltages should all be well below the max voltage allowed there (15V for many MOSFETs). In fact you'll probably need a logic-level MOSFET that is FULLY turned on by the gate voltage you want to use.

    This is key - at the current you have, you MUST drive the gate fully on to reduce Rds(on) to its minimum. Otherwise the MOSFET will drop too much voltage and dissipate too much heat.
     
  9. steveomiami

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    Apr 20, 2013
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    Im asking what Fet should I use and how should everything be connected? The diagram is a little complicated for me. I guess im asking for a english version of the diagram.
     
  10. wayneh

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    Sep 9, 2010
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    The MOSFET has 3 pins, Gate, Drain and Source or G,D & S. For this application, Source will be connected directly to battery ground. Drain connects to the low voltage side of the load. The Gate is like a light switch controlling current flow from Drain to Source. Putting the right voltage (with almost no current) onto the Gate (relative to the Source pin = ground) provides a very low resistance path to ground, from Drain to Source. An inadequate voltage on the gate will fail to fully reduce the resistance and heat will be generated.

    You have many choices for MOSFETs. You'll want to find an N-channel MOSFET rated to maybe 40A or more, to provide safe margin against overheating. A "standard" MOSFET like an IRF540N requires at least 10V on the gate, so that won't work and you need a "logic level" MOSFET that will be full on at 3V or so. Just go to Mouser or another supply house and search around. If you don't want surface mount, be sure to look for a package type you can use, like TO-220 for instance.
     
  11. steveomiami

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    Apr 20, 2013
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  12. wayneh

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    Those look reasonable, as long as you're OK with SMD devices. Otherwise add through-hole to your search criteria.

    You may also want to look at even lower Vgs voltages. I'm not sure what your battery voltage might look like as it ages. You wouldn't want to burn up your MOSFET because of low battery.

    Be sure to also do a real power calculation and compare it to the dissipation max for your package: I^2*R where your I is 10A I think and R =Rds on of the MOSFET you choose. The max current specs tend to be overly optimistic, but if you design for the power rating, and allow yourself maybe 2X for a safety factor, you'll be fine.
     
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