HELP - AVR driving 16 5v. 3A lamp

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rene_a133

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Hello.

I need some help, I am new in electronics and MCs, but i am trying my best to get this proyect out. I will tell you about it.

I am trying to drive 16 LED lamps.
These are, 5volts-3amps each led lamp.
A lot of light, each one is 1,800,000 mcd aprox.
I will make them blink in a lot of patterns that include one by one like knightrider and all at the same time.

To do this i have an ATMEGA8 that gives away in the i/o pins 5.0~volts and 20mA.

So the thing is that I have been looking all around the web and find that I can drive the led lamps with transistors or relays. The problem of relays is that the lights will be switching constantly and the life of relays in that way is kind of short compared with the transistors (or such i have read) and the problem of transistors (as far as i know) is that they can´t handle a lot of current.

So the question is this, can anybody help me please to choose between the two options? and recomend a model of the selected one to do the job, something really tough.

And dont think i am just trying to get someone else to do my homework, thats not the point in learning. I am a real newbie in electronics and i still dont have many things water clear, and information on the "www" is a little bit overwhelming when nobody points at you in the right direction.

So i hope you can help me and point me in the right direction.
Thanks for your time.

(Don´t hesitate in correcting me if I am wrong in anything of the just posted thread, remember I am very new to this )
 

The easiest device to use is a logic level power MOSFET transistor. The source terminal of the FET wires to ground, the gate wires to the micro-controllers digital output (high = led on) and the drain of the transistor wires to the LED either by way of a series current limiting resistor or a current driver, (it sounds like your LED's might have a built in current driver?) and finally the opposite end of the LED goes to positive voltage. They come in all current values and 3 amps is not a problem. Just be sure it is a logic level FET because that insures that a digital high (4-5vdc) will turn it on completely, the non-logic level FET requires 10vdc to turn on full.


Lefty


Here is one
 
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Hi.

Thank you so much. I was a little bit lost.

Just another quick question, do you recognize any of the following as logic level, this is because these are the ones in my country´s electronics shop marked as "power fet" but it says nothing about logic level. If they are not logic level i will have to go to ebay.

STP45NE06ST
MTP3055V
NTP18N06
IRF640
MTP2955V
NTP2955

Thanks a lot. Again.
 
I was just exploring the forum and come by ome interesting things that may help me, but something is just nos to clear.

The data sheet of one of the power fets that i found says that Vgs(on) = 3v and that Vds(on) when Id=12A,Vgs=10v = 2.2v

This means that the mosfet will conduct at 3v? or at 2.2v?
or what letters should I look for?
 
Vgs threshold is the wrong data to check if the MOSFET is a logic level one or not.

Check the Vgs which gives the rated current of the MOSFET. That's the real voltage one needs to apply to the MOSFET to make it conducts fully.

In your case above, that means Vgs=10V for Id=12A. So your need 10V Vgs to saturate the MOSFET and so it is definitely not a logic MOSFET.
 

hi,
The best guide is to look at the Vgs gate graph on the datasheet for the MOSFET.
It may start conduction at 'low' Vgs but the Ids current will be low, until the Vgs increases according the graph.

Do you follow Ok.?

EDIT:
Look at this dual FET
 

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Let me see if i got it.

The data sheet says: Vgss -Gate to source voltage- = +-20v

So to be a logic level one the Gate to source voltage needs to be 5v?
Am i right?
 
rene_a133 said:
Let me see if i got it.

The data sheet says: Vgss -Gate to source voltage- = +-20v

So to be a logic level one the Gate to source voltage needs to be 5v?
Am i right?
Click to expand...

Look at my edit with the images.

Although this too low a current for your application, the Vgs curve threshold/conduction curve will be a guide to choosing a suitable FET
 
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"logic level" is a MOSFET selling point and nearly all manufacturers would place that in BOLD characters right at the front page of their datasheet.

So if the datasheet said "logic level", then it is a logic level one. Now check the graph for details.

Beware there are two common logic levels, one is 5V and the other one is 3.3V. A 5V logic level MOSFET may or may not work satisfactorily at 3.3V.

If it is not explicitly mentioned in the datasheet that it is a logic level one, then chances are it is NOT.
 
Ok.

I think i am understanding.
Here in the document i attached in page 3/9 in figure 2 it shows a similar graph.

If for my proyect i just need 3 amps, and have a current of 5v to drive the mosfet this mosfet will be ok, or it will present some kind of problem?

Hope this one works because where i am there are not a lot of electronic shops.
 

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hi,
This high side driver is TTL controlled.
 

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Hi Eric,

Your proposed part has a min. VCC voltage of 5.5V and typical 13V. OP cannot use it on 5V circuit.

@rene_a133:

Did you see "logic Level" mentioned on the front page of the datasheet?

Yes, it will do nicely. I have marked the piece of information on the datasheet that one need to check if it is OK for one's application. The part is NOT suitable if one wants 10A or more but it is good for 5A or less so no problem in your application.

 

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Thanks to you all. Most of all for you patience and sharing your knowledge.

Finally i get it and i hope i can design a circuit diagram and next translate it to a PCB.

I hope you dont mind if I keep asking more questions on this same proyect as they come out.

Thanks a lot, really.
 
Hi again.

I just noticed that the voltage regulator for the led modules its not going to be strong enough.

Well more on the proyect. Its powered by a car battery (12v).
I still intend to use a 7805 to input the voltage to the microcontroller. And for the led modules i was expecting to build each led module with a voltage regulator that could go from 12v to 5v, also thinking on 7805 until i realize that it gives up to 1.5 amps max and i am looking for 3 amps.

Been looking around but i couldnt found a piece that fits this. All are 1 to 1.5 amps max.

Added: "Just found a LM138 voltage regulator, gives up to 5A"


Any ideas of which part could perform better than this, or any other idea on how to control the voltage on the proyect.

Thanks.
 
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hi,
Ideally to get from 12v to 5v at 3A you should use a switched mode psu, the efficiency will be higher.
 
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