Best way to switch a small (~3W) load

Hi guys,

i have some experience with electronics, but i'm a little bit lost right now. This is the first time i want to turn on a load i can not run directly from the digital output.

I have a battery pack with a charging curcuit which can provide up to around 4-5 Watts. I want to run a LED strip with a maximum draw of about 3 Watts. All the MOSFETs i found are overkill in regards of voltage and/or current (i think they won't even work) but i'm not sure which a transistor can handle the load.

I hope you can either point me to a fitting tutorial or just give me a part number of a component i can use with a supply voltage of 3.7V and max. 1 Ampere current.

Thanks!

All the MOSFETs i found are overkill in regards of voltage and/or current (i think they won’t even work) but i’m not sure which a transistor can handle the load.

I’m not sure why you’re concerned about “overkill”. 1A not insignificant, and a “power MOSFET” is appropriate. The MOSFET will “work” if you get a “logic-level” MOSFET That can be turned-in by 5V, and if you can switch the ground-side. [Here is a simple MOSFET driver circuit]](Gammon Forum : Electronics : Microprocessors : Driving motors, lights, etc. from an Arduino output pin). You can leave-out the diode since LEDs are not inductive.

Are you sure your LED strip runs off 3.7V? Do you have a link to the specs? There are all kinds of LED strips, including some that have built-in drivers. Most of them run off 12V or 5V.

A similar transistor driver circuit would also work, but most regular (BJT) transistors will get hotter and with 1A you’d probably need a heatsink. (A MOSFET might require a heatsink too.)

Alternatively you could use a relay. You can get a solid-state relay that runs directly off the Arduino’s 5V/low-current output, but make sure you get a DC solid state relay that’s rated for low-enough voltage. (AC solid state relays don’t work with DC, and power-line-voltage solid state relays usually won’t switch low voltage.)

A regular electro-mechanical relay would also work, but you generally need a driver circuit for the coil, or you can get a relay board with the driver and relay built-in.

Regular electro-mechanical

Hi,

most of the MOSFETs i found require 12V or up, i wasn't sure if that was the general direction when using them. What kind of MOSFET would be appropiate in my case? The heatsink won't be a problem, i have the perfect case for that :). It's the casing of a dead IP65 LED flood light from China.

From what you wrote transistors are not suited in this case, but a relay also sounds fine since i think i already have one somewhere. Would that be more power efficient? I guess that i won't run into any overheating issues when it can be triggered by digital pin of the Arduino.

There are enough cheap LED lights that run with 3 AA batteries (so they will be able to handle the maximum voltage of 4.2V), i'll just clip of the battery case. Or i'm going to run two or three of them parallel, not sure yet.

I'm going to make my own over the top solar powered party lights since everytime i buy one it dies after a couple of month. At least i can turn them on with my smartphone that way and can replace any defective components :slight_smile:

Also thanks for the reply, i really appreciate it.


I also found this solid state thingy: http://www.ebay.de/itm/G3MB-202P-5v-1-Kanal-OMRON-SSR-Solid-State-Relay-Modul-fur-Arduino-/172257446878?hash=item281b580bde:g:X7QAAOSwwo1Xcl2e
That sounds perfect for my purpose since i have more power than i need during the day. Electronics are complicated :wink:

FYI

Outputs.jpg

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larryd:
FYI

Outputs.jpg

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Well, that’s unfortunatly above my pay grade :/. I have a very specific problem and that does not help me at all, sorry.

Don't look at the whole image at once.

Just pick one output pin and follow the path to the load being controlled.

For example, D7 drives Q3 which then energizes the 5 volt relay K1.

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FYI

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Zephalon:
Hi guys,

i have some experience with electronics, but i'm a little bit lost right now. This is the first time i want to turn on a load i can not run directly from the digital output.

I have a battery pack with a charging curcuit which can provide up to around 4-5 Watts. I want to run a LED strip with a maximum draw of about 3 Watts. All the MOSFETs i found are overkill in regards of voltage and/or current (i think they won't even work) but i'm not sure which a transistor can handle the load.

I hope you can either point me to a fitting tutorial or just give me a part number of a component i can use with a supply voltage of 3.7V and max. 1 Ampere current.

Thanks!

ZTX851 NPN transistor. Use a 150 ohm base resistor.

Or any logic-level n-MOSFET with an on resistance of 0.1 ohms or less.

Hi,

I went on ebay and searched for Logic Level MOSFET. The first two hits were:

  1. IRLB8743PBF 5 for $7.30
  2. IRLZ44N 5 for $6.35

The IRLZ44N will have ~ 0.035 Ohms with a gate voltage of 4.5V

So why do I suggest these are your best approach?

  1. They are low cost.

  2. They have a very low loss. If you current is ~ 1/4 amp the loss is in the order of 2 mw

  3. The MOSFET gate takes virtually NO current.

  4. A transistor will be hard pressed to have a Vce sat as low as the MOSFET's IR loss.

  5. A transistor takes real power into the base (not much but more than the MOSFET)

  6. The relay takes real power for the coil.

As for overkill, perhaps but the abundant use of power MOSFETs makes them extremely cost effective.

Hope this helps.

JohnRob:
Hi,

I went on ebay and searched for Logic Level MOSFET. The first two hits were:

  1. IRLB8743PBF 5 for $7.30
  2. IRLZ44N 5 for $6.35

The IRLZ44N will have ~ 0.035 Ohms with a gate voltage of 4.5V

So why do I suggest these are your best approach?

  1. They are low cost.

  2. They have a very low loss. If you current is ~ 1/4 amp the loss is in the order of 2 mw

  3. The MOSFET gate takes virtually NO current.

  4. A transistor will be hard pressed to have a Vce sat as low as the MOSFET's IR loss.

  5. A transistor takes real power into the base (not much but more than the MOSFET)

  6. The relay takes real power for the coil.

As for overkill, perhaps but the abundant use of power MOSFETs makes them extremely cost effective.

Hope this helps.

Hi,

that helped a lot and also thank you for the explanation! I'll try to realise this project with a MOSFET then since that seems to be the best and cheapest solution. I only hope a gate voltage as low as 3.5 V (about my cut off voltage to keep the Arduino running) is still enough. I want to run the circuit directly from a lithium ion cell.

Btw. 5 IRLZ44N are about 2 Dollars from China to Germany, thats the cheapest option if you have a few weeks time :slight_smile:

Also thanks to all the other guys, i really appreciated your comments.

Look for logic level mosfets with Vgs(th) or Vth threshold voltage 0.8-1.5V.

I know, the stuff from China is both cheap :slight_smile: and slow :frowning: . I'm often tempted to purchase general lab items just to have.

For what you are doing you might consider purchasing a resistor kit as well. They are cheap enough and you get a number of values for future use.

Good luck.