Solution to dim 12V-100W led strip?

Other Hardware Motors, Mechanics, Power and CNC
1

Hi everybody,

I am building a fully automated greenhouse. It currently works using netatmo devices and IFTTT but reaction times for the heater, watering pump, ventirad, etc are above 5-20 minutes. IFTTT is very limited and....this in no fun :smiley:

I am new with arduino but it is okay, i like to learn new things. I managed to have temp and humidity sensors up and running, RTC etc.

My problem concerns the power supply.

Each of the devices has its own plug and operating U and P:

  • The heater and vapor generator are using 240VAC, so this is easy --->shield relay 5VDC
  • The ventirad is 12VDC but only 0.1Amps, easy enough as well with a transistor or optocoupler + a few components
  • The watering pump power block is certifed 14VDC, 30 VA, but my measurments puts it at 26VDC...i will look into it a bit further.
  • my main issue concerns the light. It is a 12VDC, 8.3A, 100W led strip. I want to use PWM (hence i will not use relays) to dim the light early in the mornig and late in the afternoon. Lights in a greenhouse a swithched on about 16 hours per day.

What would be the best solution?

  1. optocoupler + Mosfet with gate linked to 12v?
  2. TTL or logic mosfet?
  3. transistor + mosfet?
  4. anything else?

I had a good look and googled many things but failed to make a choice. Hey, I even don't now which parameters are the most important. Heat? RdsON? max Power or max A?

Anyway, can anyone please help me on this one?

Thanks

P

2

logic-level MOSFET with low-side switching, use a device with on-resistance of 10 milliohms or less for
easy heat dissipation (only small heatsink needed).

Max amps is an irrelevant spec usually - if you are anywhere even close to this, you will need very large
heatsink! Use the on-resistance as the primary decider.

For PWM you have to consider switching losses too - for a high PWM frequency that is important - MOSFET
gates are highly capacitive so the impedance used to drive them sets the switching speed.

3

Hi MarkT,

Thx for your answer. I was thinking about this PSMN2R7-30PL that seems to do the trick. It has on- resistance between 2.2 and 2.7 milliohms.

I am not sure why you would not consider max amps ? My load will use about 8,3A which is kink of big, no? Still a beginner in this though.

What would you recommend regarding PWM then?

4

Two reasons - one is that the Vds(on) must not get too large without affecting the Vgs needed,
so you need to know the on-resistance to calculate this anyway.

Secondly the on-resistance allows one to calculate the power dissipation, which affects if a heat
sink is needed and how big.

Choosing a 100A MOSFET for a 50A load isn't telling you if you need to raise Vgs due to large
Vds, nor is it informing you of the loss (which might be tens of watts)

For instance: 40A logic-level MOSFET 0.1ohm on-resistance, would have Vds=2V at 20A, so you'd need
to raise Vgs to more like 7V to compensate, and dissipate 40W.

Choosing a 0.01 ohm MOSFET brings the Vds(on) to 0.2V for 20A, negligible, and dissipation is 4W.
Such a device is likely to have a max I of 120A or something like that, but 20A is much more
sensible level to use it at.