Some unknown interference on my circuit

[RivaultUser]

Okay guys, so I've been working on this project and I found an unknown interference on the circuit.

Here's what happened :

1. I me an astable circuit from 555.
2. Then I connected the pin 3 output to the gate of a mosfet to turn led on and of ( the drain has12v and connected to the leds. The source pin connected to ground

So far so good, the leds turn on and off

3. Then i connected the gate of an scr to the pin3  output of 555.
4. The anode connected to a fan and 12v, the cathode connected to the ground.

Now when I connected the scr, okay thefan turns on constantly because of the scr. But the leds also turned on constantly. And when I checked It with my multimeter, the output ofpin 3 change to a constant 5 v ( before i connected the scr Gate to pin 3, the pin was emitting square wave)

What's happening in here guys??  Help.....

[PA0PBZ]

This will help:

[Andy Watson]

Try a 1k resistor in series with the gate of the mosfet. Also, as PA0PBZ has indicated, you will need another resistor in series with the gate of the SCR to limit the current.

[RivaultUser]

Try a 1k resistor in series with the gate of the mosfet. Also, as PA0PBZ has indicated, you will need another resistor in series with the gate of the SCR to limit the current.

Thx for you reply man, I've tried what you said here's what happened :
1. I use 1k and connected it series with the gate of mosfet and 1k series with the gate of the scr
What happened was the led is blinking and the fan is not working, I measured the current that went to the gate of the scr and the result was 0.1mA ( almost no current go to the gate )

So I tried it with 100 ohm resistor, and what heppened is :
The led won't went on off on off, but instead on-dim-on-dim. And my fan was still not working, but it gave me sound 'tich' 'tich' tich'. I measured the current and it was 2mA

What I use : mosfet irfz44, scr bt151-500r

The funny thing is when I replaced my fan with my LED, the circuit is working the way I want ( the LED strip is blinking and the led that I used to replace the fan is turning on constantly)

What's happening in here I need help.....

[PA0PBZ]

The gate of the SCR is at high potential once it is switched on, and this is controlling your mosfet. Try putting a diode in the lead going to the gate of the SCR:

555 pin 3  --:>|-- gate of SCR.

[RivaultUser]

The gate of the SCR is at high potential once it is switched on, and this is controlling your mosfet. Try putting a diode in the lead going to the gate of the SCR:

555 pin 3  --:>|-- gate of SCR.

Well, it's ALMOST WORKING. The LEDs is 'on off on off'. But my fan instead of giving me the ' tic tic tic' sound, it gave me this :'turned on - turned off - turned on - turned off'. Well, it should be turning on constantly.....

Please helppp.....

[Andy Watson]

But my fan instead of giving me the ' tic tic tic' sound,

Typically fans like this push the rotor around with a series of pulses (hence the tic tic tic). Between the pulses the current consumption of the fan drops to a very low level. Thyristors require a certain amount of "holding" current to remain ON. When the current drops (between pulses) the thyristor turns off. But it also sounds as if there are other problems with your circuit because the fan should run whenever there is sufficient gate current for the thyristor. Any chance you could upload a circuit diagram?

[RivaultUser]

But my fan instead of giving me the ' tic tic tic' sound,

Typically fans like this push the rotor around with a series of pulses (hence the tic tic tic). Between the pulses the current consumption of the fan drops to a very low level. Thyristors require a certain amount of "holding" current to remain ON. When the current drops (between pulses) the thyristor turns off. But it also sounds as if there are other problems with your circuit because the fan should run whenever there is sufficient gate current for the thyristor. Any chance you could upload a circuit diagram?

Sure, here it is.

[kaindub]

Why are you using an SCR?
If you use an SCR in a doc circuit you get to turn the SCR on once and it never turns off.
See the equivalent circuit that was posted in post  2 and you'll see why it just latches on

SCRs are used in AC circuits . The SCR is turned on when the supply cycle forward biases the SCR. Once the SCR becomes reverse biased (actually it's when the SCR stops conducting) the SCR turns off.

What is wrong with using the MOSFET in your circuit?

Robert

[RivaultUser]

Why are you using an SCR?
If you use an SCR in a doc circuit you get to turn the SCR on once and it never turns off.
See the equivalent circuit that was posted in post  2 and you'll see why it just latches on

SCRs are used in AC circuits . The SCR is turned on when the supply cycle forward biases the SCR. Once the SCR becomes reverse biased (actually it's when the SCR stops conducting) the SCR turns off.

What is wrong with using the MOSFET in your circuit?

Robert

If I use a MOSFET for my fan, then the fan will go on off on off because my signal is from 555 timer ( astable mode ). That's why I want to use SCR for my fan, because once the gate gets the signal from 555 then the anode and cathode of the SCR will latch open ( well, at least that's what I hope gonna happen though )