I wanted to control a 84V LED strip using a ATtiny13a microcontroller with minimal power wasted. I just want to turn it on and off, don't need any PWM dimming. The strip uses 270mA current and is about 22W power. MCU will be getting around 5V and 10-15mA supply and MCU uses only about 3-4mA max.

I have 3 components for this but I don't know which will waste least power as heat.I have IRL540N Logic level Mosfet , TIP122 Darlington power transistor and BT169D Thryistor/SCR. I'm avoiding sugar cube relays because I only have minimal space in the case/box for this project, which is also why I need the component which wastes the least power since I couldn't fit a heat sink.

I have programmed the MCUs eeprom to turn the circuit on and off alternatively each time the setup is powered on and off so the latching feature of the thyristor/SCR is not a problem here. I also have opto-isolators to protect the MCU from high voltage.

So which one of those components is ideal for my project? Also if there are any other components which will do the job better please let me know.

Thanks in advance.

  • 4
    \$\begingroup\$ Latching relays are worth investigating. No power consumption except when switching. Various sizes are available. \$\endgroup\$
    – KalleMP
    Commented Mar 1, 2018 at 18:47
  • 1
    \$\begingroup\$ I agree - a latching relay is a better option, if the goal is to minimize wasted power. \$\endgroup\$
    – Norm
    Commented Mar 1, 2018 at 20:28
  • \$\begingroup\$ @Norm yes, especially if switching is relatively infrequent. With frequent switching, the power used by the relay coil might become significant. \$\endgroup\$
    – Ale
    Commented Mar 2, 2018 at 0:04
  • \$\begingroup\$ Does it meet the size criteria? is there a latching relay small size as a mosfet? \$\endgroup\$
    – Kokachi
    Commented Mar 2, 2018 at 4:10

2 Answers 2


Use the MOSFET. Its use case is switching a DC load. If the LED strip has a lot of inductance, put a flyback diode across it.

If you drive the gate with 5 V relative to source, then the RDS(on) is specced as 0.077 ohms from the linked data sheet. The power dissipated will thus be 0.27 * 0.27 * 0.077 = 0.006 W. No need for a heatsink.


The IRL540N mosfet will most likely be the best in all areas even in terms of power dissipation. With a listed Rds(on) of 0.077 ohms (@5Vgs) it will dissipate only about 6mW at 270mA. Required gate drive from the control circuit is virtually zero. You may just want to ground the gate to ground with a high value resistor to insure it is off when required, that will also reduce potential static discharge events that could damage the mosfet.

The TIP122 and SCR will have higher on voltages due to Vce(sat) and Vt/Vf ratings, thus a higher dissipation at 270mA, perhaps as high as 0.5W, not exceedingly hot by any means but likely noticeably warm.

The opto-isolators you mentioned are also a very good way to keep the low voltage side safe.


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