The IRF822 is a poor choice for this project; Rds (on) is rather high even when Vgs=10v, and you won't get half of that. The threshold voltage spec is for when I'd (drain current) is only 250uA, which isn't enough to light even ONE of the LEDs. Basically, IRFxxx devices are specified for Vgs=10v, where IRLxxx are specified for Vgs=4.5v, so the latter are suitable for use with logic-level voltages. I find it very handy to have some N-channel logic level MOSFETs around that have Vdss ratings of 30v, 60v and 120v for various projects. The lower the Vgss, the lower the gate charge and so with all other things equal, the lower Vdss rated will turn on/off faster.

 

Yes, i got a tip about the logic level n mosfets, but i can't find them in my town, so i need to go with one of these options to drive the 300mA LED array. The problem that i am facing is that even when i source 30mA to the base of my BC337 it won't conduct 300mA from collector to emitter, even if i lower the load resistance to a minimum.

 

Yes, i got a tip about the logic level n mosfets, but i can't find them in my town, so i need to go with one of these options to drive the 300mA LED array. The problem that i am facing is that even when i source 30mA to the base of my BC337 it won't conduct 300mA from collector to emitter, even if i lower the load resistance to a minimum.

Post the actual circuit you are using...

 

NOTE: forgot to change the 10k res to 110ohm. I mounted really quick.

The diode has a forward voltage of 3.2v and the R2 simulates 15 resistors of 66ohm in parallel. Also this is just a test, it's not connected to the output of the 555, i'm actually just trying to get 300mA through BC337. I first threw a 143ohm resistor in to the base and got 20mA, lowering to 110 got me 30mA. And the maximum current i am getting through is 200mA, like 12mA per LED, i want those 20mA.

A side question, could you point me some nice n-channel logic level mosfets, i buy things from china in really big packages to save me money and i'm researching good ones to buy it.

 

The final current when the supply is 5.0V is a function of the Vf of the individual LEDs and temperature. If you are not getting 300mA at 5V it is because your Vf is not what you think it is...

 

Yes, in my simulations, i got 300mA, nice and perfect, but then i go to the breadboard, connect everything and bam, 180mA. I measured the Vf of each LED, it goes around 2.8-3.0v

 

Then the transistor model doesn't match the actual transistor you are using. My LTSpice lib has many versions of the BC337, some of which are rated at 200mA max...

 

I have a bc368 here, might be good to try with this one right? I thought the bc337 had only versions rated for that current standard current of 800mA

 

@ErnieM: The Vgs threshold of the IRFZ44N is 4V. Will the output pin of a 555 operated from a Vcc=5V pull-up high enough to turn on the gate of the IRFZ44N without some additional help?

Its been a while since I've actually used 555 but as I remember with no load they go tight to the power rails.

If not, just leave the pull up resistor there. The 555 will pull that to ground easily, and you don't have the .7 volts the diode gives you, and a piece of wire alone is cheaper than the diode.

 

Nothing is working, so the only thing that i have left to test is the use of IRFZ44N. I'll test driving the gate of the mosfet with a 5v source.

 

The shop didnt had IRFZ44N so i bought IRFZ46N, i plugged everything in, but in a test circuit, connecting the gate directly to the 5v and with 15 resistors of 66ohm and it gave me 190mA, so i thought, let's get the resistance lower, tried with 33ohm, got 230mA, then i went to 11ohm, and got 280mA, it is not running as in your simulation Mike, do you know why? I'll try with a 7 ohm resistor at each LED to see how it goes.

 

Have you tested your 5V supply to see how well it holds up its output voltage at >300mA?

 

Not yet, but shall do tonight. But i believe it can hold currents up to 1A, because i am using a 5v cell battery charger with a USB port, that common charger that comes with the smartphones. Also it says 1A in the case. As the charger comes from China, it may.. be true or not. I'll wire five relays and my circuit, each relay consumes 60mA and my circuit 280mA, lets see how it goes.

 

the circuit is like the follow that i downloaded from the web:

The circuit is kind of meaningless in that the transistor does nothing -> you are better off without it.

 

@dannyf i kinda need that transistor :/

@MikeML I think you discovered the problem, something i should have tested right before initiate the tests... I connected two relays giving 100mA to one and 50mA to other, and my circuit that has giving 280mA, when i first connected the power, the multimeter accused 280mA tight and i noticed the brightness of the leds were so weak, so i disconnected the relays and bam, still 280mA, but the leds were glowing. I think that is were all my problems lies. I suspected that from the beginning, but even with all my senses saying to test another power supply, i kept doing tests. Tomorrow i'll go to downtown and find another one. A picture of the simple usb charger psu(my enemy):


I don't know how i believe this small beach could handle 1.0A.

 

Then just use the circuit of post #14, with 15 resistors, and 15 LEDs, and a proper 5V supply.

 

Yes, thank you very much @MikeML !

 

Hello, i having some troubles with a few BJT i am using, it may be a mislearned thing or two that i picked while reading about BJT, so i came to you guys, the circuit is like the follow that i downloaded from the web:

But instead of one LED, it has 15 LEDS, and the Vcc is 5V, but the circuit is basically the same. I'll guide through my process of calculating the dc bias resistors: First i pick the transistor, for the first time i chose BC337, i took a look at it's datasheet and followed the same process i did last time when i was passing 100mA through it's collector and emitter from a last project. I first choose the typ value of LED forward voltage, 3.2v, so after the LED, 1.8v remains to be used, then i say that collector voltage will be 1v, so it's a 0.8 drop to there, applying ohm's law:

0.8 = R * 0.02
R = 40

Then after that i check the gate current that i need to pass 300mA(15 LEDS), using this datasheet:
http://pdf.datasheetcatalog.com/datasheet2/3/06i96k727i3oloo2gis1ozg0jlyy.pdf
The gate current is about 2.1mA when Vce is 1.0v, so i make the gate voltage about 0.7v, so that the drop from base to emitter can happen and still maintain a Vce of 1.0v. Using ohm's law: 4.3 = R * 0.0021 ; R = 2047

And the emitter is tied at 0v, so it doesn't need resistors.

Then i plug these values into PROTEUS 8.3 and simulate it, and got a perfect result with the BC337AP model of bc337, so i went to test it and for my surprise the maximum collector current that i managed to achieve with this values was 180mA, giving 12mA for each LED. That was really a surprise, because i used this method 3 times, in three different projects and it worked everytime, it maaaay be a really nice coincidence of values, but i want to know where is my mistake.

I thought at first that was my transistor, so i went and bought BC638, TIP31C and a mosfet IRF822. I tested with TIP31C using the same method and i got the same result. One thing that is also surprising, is that in PROTEUS there is a BC337 model that is different from the BC337AP and this model is working more closely with my results, but it's still different. Can anyone point my errors ? Thanks in advance.

More info, please. What are the LEDs like? What is the capability of the power source?

 

@hp1729 After a long walk, i discovered that my psu can only supply 280mA, that may be the cause of the weird behavior. I'll test with another one, but thanks for the interest anyway. You're very kind to come and help.

 

After a long search, i discovered some cheap transistor arrays, it was sooooo cheap that i had to give it a try, so i went and bought 3 CA3081 NPN Transistor Array, and then i thought, how about regulating each single LED? And it worked, new PSU(Actually i used an original USB CHARGER,a motorola one), new transistors.



Good 20mA. And the PSU when drove really hard gave me 1.2A, the difference is huge between an genuine motorola made charger and a china one, just imensely huge. I opened a china one and it had a half wave rectifier, A HALF WAVE!

I discovered that when i tested the LEDS without a large base resistor, the resistance of the trails of the breadboard was playing a very large role, because 15 LEDS each one with 66 ohm resistors, would create a very small parallel resistance. The trails depending on how good is the resistor attached to it had a resistance, it could reach up to 4ohms, that was in series with the 15 resistors creating very weird current situations.

I just confirmed what that thread about driving leds said, it's not a good practice to run too much LEDS in parallel.