Some unknown interference on my circuit - Page 1
Forum section: electronics / projects

author: RivaultUser
author_rank: Contributor
timestamp: February 08, 2014, 05:09:04 pm
content: 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 groundSo far so good, the leds turn on and off3. 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.....

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 08, 2014, 05:29:25 pm
content: This will help:

author: Andy Watson
author_rank: Super Contributor
timestamp: February 08, 2014, 05:39:56 pm
content: 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.

author: RivaultUser
author_rank: Contributor
timestamp: February 09, 2014, 05:08:33 pm
content: Quote from: Andy Watson on February 08, 2014, 05:39:56 pmThx 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 scrWhat 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 2mAWhat I use : mosfet irfz44, scr bt151-500rThe 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 hereI need help.....

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 09, 2014, 08:11:07 pm
content: 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.

author: RivaultUser
author_rank: Contributor
timestamp: February 11, 2014, 03:29:05 pm
content: Quote from: PA0PBZ on February 09, 2014, 08:11:07 pmWell, 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.....

author: Andy Watson
author_rank: Super Contributor
timestamp: February 11, 2014, 03:46:23 pm
content: Quote from: RivaultUser on February 11, 2014, 03:29:05 pmTypically 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?

author: RivaultUser
author_rank: Contributor
timestamp: February 11, 2014, 04:34:19 pm
content: Quote from: Andy Watson on February 11, 2014, 03:46:23 pmSure, here it is.

author: kaindub
author_rank: Regular Contributor
timestamp: February 12, 2014, 08:56:09 am
content: 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 onSCRs 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

author: RivaultUser
author_rank: Contributor
timestamp: February 13, 2014, 03:54:22 pm
content: Quote from: kaindub on February 12, 2014, 08:56:09 amIf 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 )