I am new to Arduino (do I really need to tell? ) and I'm trying to build a sunrise alarm clock. This project will be built of a 5 metres LED strip (working at 12V and a maximum of 2A), a buzzer and probably of an LCD I'm willing to buy soon.
For what I have read, in order to dim the LED strip I might use any n-Channel MOSFET (as long as it is capable of handling that much voltage and current my strip requires).
I have plenty of transistors available from an old PC motherboard, so after checking the datasheet of each one, I decided to use a random one, as all of them surprisingly meet the criteria.
But I found a problem: the LED strip did not turn off completely, even being the MOSFET turned fully off!
So I googled and found that I had to use a MOSFET whose Vgs would be 5.0V (so that Arduino can fully turn it on and off, am I right?)
The thing is, how do I know this!?
Having this in mind, I searched for the transistor of my PCB having the less Vgs possible. I found a 9916H model http://www.datasheet4u.com/download.php?id=705636
In the first page it is written:
Gate-Source Voltage ± 12V
but there were also some numbers about "Vgs @ 4.5V"
So I though it may work correctly, i.e., it would totally turn off the LED strip and totally turn it on.
And it worked! Could anyone explain me why? Could anyone explain me why the other transistors having Vgs ± 20V cannot turn off completely the strip, and this one can? And why should IRF540N work?
I know this may be a dumb question, but I need to understand why some transistors work, and some others don't
Regards!
PS: about the installation: I do have a 12K (I don't have a 10K one available) resistor between the Arduino pin controlling the PWM and Ground. I don't know why, but I do have it
PS2: this is the LCD I'd like to buy. What do you think about it? Recommendations For You - DealeXtreme
A "standard" N-channel MOSFET needs 10V Vgs to turn on. A "logic level" N-channel MOSFET needs anywhere from 1.8V to 5V to turn on (you can find many different voltage levels). The key is finding logic level device that will turn the MOSFET on at more than 250uS at the logic level.
For example, AOI516 from Digikey shows Rds with Vgs at 4.5V right on page 1:
RDS(ON) (at VGS = 4.5V) < 10m?
and if you look at the table page 2:
RDS(ON) Static Drain-Source On-Resistance RVGS=4.5V, ID=20A 7.1mOhm to 10mOhm MAX.
So this device, along with AOI518, would be a very good part.
IRF540
can work with Vgs = 4.5V, but the datasheet only shows that for 20uS pulses, I wouldn't use it for continuous on activity.
Numbers like Vgs = 12V or 20V are generally the MAX that can be applied before the device breaks.
For 9916H, this line in the table on page 2:
aklau:
But I found a problem: the LED strip did not turn off completely, even being the MOSFET turned fully off!
So I googled and found that I had to use a MOSFET whose Vgs would be 5.0V (so that Arduino can fully turn it on and off, am I right?)
The thing is, how do I know this!?
For example, lets check the datasheet of a MOSFET that lots of people recommend, the IRF540N...
Does the MOSFET switch on/off properly when you connect the gate to 5V and 0V respectively?
If you look at figure 1 of the IRF540N datasheet you will see the performance of the FET at different Vgs. The bottom curve shows the current vs. voltage drop Vds at Vgs = 4.5V. From the linear part of the Vgs = 4.5V curve the effective resistance of this FET is 66 mOhm. As you increase Vgs the effective resistance of the FET drop to the stated 44 mOhm. You can also see from figure 1 that the maximum current you can get through the FET is lower at Vgs=4.5V (about 10A) than at say Vgs=5V (about 23A). You ideally want to use logic level FETs with Arduino output voltages, see e.g. Logic-Level Power FETs you'd like to see/use?? - Motors, Mechanics, Power and CNC - Arduino Forum
Thanks CrossRoads and Christo!
Now I understand waaay better the MOSFETs!
Does the MOSFET switch on/off properly when you connect the gate to 5V and 0V respectively?
Now they do.
At first, I though that the erratic behavior was related to Vgs, but then I realized I was damaging the transistors in the desoldering process (I damaged 4 of them! )
So, what I learned is:
MOSFETs need to be handled with care
Be careful with amount of heat added
Be quick with the soldering iron!
Aaaaand I have got another question:
I'd like to plug a battery backup (either a 9V one, some AA/AAA or even a 2032, I can use any of them).
I have seen complicated schematics even for charging these batteries, but I don't need any of these. I just want a battery backup that will work eventually for small amounts of time
I plan to use the DC adapter plug (it will provide the necessary 12V for the LED strip and also feed the Arduino) and I have read that I can simply plug the battery to the USB port (with some kind of USB-battery DIY cable). So that, as Arduino chooses automatically the higher voltage available, it will run on 12V. But once these 12V are lost due to an electrical failure, it will automatically switch to the -say 5V- available through USB port.
Is this correct?
Will Arduino be quick enough to switch between sources without being actually reseted?
) and I'm trying to build a sunrise alarm clock. This project will be built of a 5 metres LED strip (working at 12V and a maximum of 2A), a buzzer and probably of an LCD I'm willing to buy soon.
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