# Mosfet and transistor election for 3v3

#### Luis itoo

Joined May 20, 2021
5
Hello, I want to create a circuit to drive an esp8266-03 and simple 12v led stripe. After a lot of reading I understood that I need a TTL level, N-channel mosfet or place a transistor between it and the micro controller, also found an schematic with some theory in it, but I can not find where I live that transistor, nor even most components I seen on tutorials.

What do i need to check in a datasheet of a transistor to know if it is suitable for my usage?
What about a smaller mosfet driving something like an irf540 or 6n60? (stuff I have on hand)

This is the schematic i mentioned before, found here.
Q1= 2N3415
Q2= irfz44n

#### dl324

Joined Mar 30, 2015
12,871
Welcome to AAC!

Any general purpose NPN transistor can be used. You can't drive the gate of an IRF5r40 directly because it's Vgs(th) is 2-4V. Vgs(th) is the voltage where the MOSFET just starts turning on

How much current does the LED strip require?

#### Luis itoo

Joined May 20, 2021
5
Welcome to AAC!

Any general purpose NPN transistor can be used. You can't drive the gate of an IRF5r40 directly because it's Vgs(th) is 2-4V. Vgs(th) is the voltage where the MOSFET just starts turning on

How much current does the LED strip require?
Thanks!
I'll look into the irf540 again, I did some research "choosing a 3v mosfet" and discarded it because i found this:

This lists the ‘on-resistance’ under the condition that Vgs=10V (and Id=25A). If there is no rating for Rds(on) when Vgs=5V (or 4.5V), then it is not a logic-level MOSFET. A logic level MOSFET will have Rds(on) specified for Vgs=5V or 4.5V. If its only specified for Vgs=10V, its not logic-level.

Another thing to beware of in datasheets is Vthresh (threshhold voltage). This is not the gate voltage to turn the device on, its the gate voltage at which it switches fully off (less than a few uA of current, typically). If Vthresh is given as 2..4V range, it cannot be a logic level MOSFET (Vthresh is usually 0.5 to 1V for logic-level MOSFETs).
Source: https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/

The leds are taking almost 3A.

#### dl324

Joined Mar 30, 2015
12,871
I'll look into the irf540 again,
If I wasn't clear, you can't use IRF540 because the threshold voltage is too high. If you get a worst case device it won't even start to turn on with 3.3V.

#### Luis itoo

Joined May 20, 2021
5
oh, I misread that. Thank you very much!

#### BobTPH

Joined Jun 5, 2013
3,316
Except that his circuit us using 7V on the gate, not 3.3.

Bob

#### Luis itoo

Joined May 20, 2021
5
Except that his circuit us using 7V on the gate, not 3.3.

Bob
In my case it would be 12V.
Whats the issue? What am I not seeing?

#### BobTPH

Joined Jun 5, 2013
3,316
Then why does your schematic show 7V?

The issue is whether you need a logic level MOSFET or not. If the schematic is correct, other than the 7V, then you are placing 12V on the gate, plenty to fully turn on a regular MOSFET.

Bob

#### Luis itoo

Joined May 20, 2021
5
Then why does your schematic show 7V?
In my first post I quoted that image mentioning that it was another option I found to drive non ttl mosfets, but since I could not find the one mentioned there I tough to ask on what do I need to know to find a suitable one.

#### peterdeco

Joined Oct 8, 2019
357
You can do away with the transistor completely if you use an IRLZ44N.

#### Audioguru again

Joined Oct 21, 2019
3,199
The article that had its 7V schematic copied, wrongly used an IRFZ44 Mosfet because the author does not know that the words "activation or threshold voltage" Vgs does not fully turn on the Mosfet, instead those low voltages actually barely turn on some of those Mosfets with a drain-source current of only 0.25mA. All IRFZ44 Mosfets and IRF510. IRF420 and IRF540 Mosfets fully turn on with a 10V Vgs. With a Vgs of only 7V some of them do not fully turn on.

Use an IRLZ44, IRL510, IRL520 or IRL540 Mosfet instead because all of them fully turn on when the Vgs is 5V.
Some IRLxxx Mosfets do not fully turn on when the Vgs is only 3.3V.

#### dl324

Joined Mar 30, 2015
12,871
If you use the IRLZ44 that was suggested in #10, you can eliminate the BJT if you reverse the polarity of the control signal.

Turn on voltage is 1-2V and a typical device will conduct over 30A with a gate voltage of 3.3V.

#### BobaMosfet

Joined Jul 1, 2009
1,776
Hello, I want to create a circuit to drive an esp8266-03 and simple 12v led stripe. After a lot of reading I understood that I need a TTL level, N-channel mosfet or place a transistor between it and the micro controller, also found an schematic with some theory in it, but I can not find where I live that transistor, nor even most components I seen on tutorials.

What do i need to check in a datasheet of a transistor to know if it is suitable for my usage?
What about a smaller mosfet driving something like an irf540 or 6n60? (stuff I have on hand)

This is the schematic i mentioned before, found here.
Q1= 2N3415
Q2= irfz44n
View attachment 239146
Welcome to Electronics-

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### Audioguru again

Joined Oct 21, 2019
3,199
If you use the IRLZ44 that was suggested in #10, you can eliminate the BJT if you reverse the polarity of the control signal.

Turn on voltage is 1-2V and a typical device will conduct over 30A with a gate voltage of 3.3V.
View attachment 239203
View attachment 239204
You show the "threshold voltage" for an IRLZ44 that has a Vgs of 1V to 2V. Then it conducts a max current of only 0.25mA so it is almost turned off. You show the graph of curves that are only for a "typical specs" one. But you cannot buy a typical one, many that you buy have minimum and maximum printed specs. It says 5V or 10V so use 5V or 10V.

#### peterdeco

Joined Oct 8, 2019
357
We have been using the IRLZ44 for many years with a Vgs of less than 3V without any problem with a drain current powering devices up to 2A.