Relay switching

[Raja Vigneshwaran]

Hi,
I am constructing a Home Automation using ESP8266 Wifi Module and making a relay circuit on my own .
But I am getting problem with the switching process in relay
I used BC107 transistor but it doesnt get required Current to switch ON and
I used IRF840 N Channel MOSFET but it requires Negative voltage to Turn OFF.

The output from my ESP Module is 3.2Volt and 30 Amps .

Could anybody tell me how to solve this issue ?

Regards ,
Raja.

 

[(*steve*)]

Please post the circuit diagrams.

 

[Raja Vigneshwaran]

 

[Raja Vigneshwaran]

please send me the correct circuit diagram , If this is wrong

 

[duke37]

There needs to be a diode across the relay coil to protect the transistor from exceedingly high voltages.
The mosfet does not need a negative voltage, it needs the gate voltage the same as the source voltage.

 

[Raja Vigneshwaran]

ya I will add Diode in Forward Bias in Circuit ,
If MOSFET needs Gate voltage as Source Voltage then I must use a 3.3 volt relay for this cicuit ?
Or is there any other alternate for this problem ?

 

[(*steve*)]

1. the diode must be reverse biased across the relay coil.
2. the relay should have a 12V coil
3. The mosfet will need to be a logic level mosfet (what mosfet have you chosen?)

 

[Raja Vigneshwaran]

I will reverse bias the diode now and relay is a 12volt coil voltage
I am using IRF840 N Channel MOSFET
Can you suggest any Logic Level MOSFET that I could use here < TTL or any other Logic >

 

[davenn]

I will reverse bias the diode now and relay is a 12volt coil voltage
I am using IRF840 N Channel MOSFET
Can you suggest any Logic Level MOSFET that I could use here < TTL or any other Logic >

and you should not have or need 30 amps on the gate

 

[AnalogKid]

The IRF840 is not a logic-level MOSFET, and needs a minimum of 4.0- Vdc from the gate to source to start conduction. The rule of thumb for these parts is a min of 10 V Vgs for "saturation." If your max drive voltage is only 3.2 V, search your vendor websites for "logic level MOSFET" and see what they have in stock. What is the relay coil operating current?

IRLZ14
IRLZ24
IRLZ34
IRLZ44

ak

 

[Raja Vigneshwaran]

I have ordered for FDT439NCT N channel MOSFET , mean while is there any other way that i could make a switch for this issue , i have a suggestion of using ULN2003 for switching purpose .
Do you have any other suggestions ?

 

[(*steve*)]

Try this:



R1 and R2 about 1k, D1 1N4001, Q1 and Q2 BC548 or similar.

Note that this driver (like many simple drivers) will invert the signal from your microcontroller. Program it to operate the output the reverse to how it does now.

 

[AnalogKid]

Depends on the relay coil resistance or nominal wattage rating.

What is it?

ak

 

[Raja Vigneshwaran]

I have a doubt with this below link ,
This guy is using 3.3volt digital output voltage from the ESP8266 module to turn ON the coil of the relay which has a coil voltage of 5V .
How is it possible ?
Any explanation ?

Link below /

https://openhomeautomation.net/control-relay-anywhere-esp8266/

 

[(*steve*)]

3 wires going to that relay board. That means it's using a transistor (or similar) to actually turn the relay on.

 

[Raja Vigneshwaran]

To Steve,
Actually there wasnt any transistor in the module he was using and In the tutorial he said that 3.3 volt will be enough to turn this 5Volt relay ON .

And About your circuit diagram , where should I connect the Relay ?

 

[(*steve*)]

Actually there wasnt any transistor in the module he was using

I doubt that very much

Here is an updated link. It's written in Chinglish, but one of the photos clearly shows the transistor and several other assorted components.

 

[(*steve*)]

And About your circuit diagram , where should I connect the Relay ?

That circuit is designed to drive the gate of the mosfet if 3v3 is insufficient to turn it on. You probably don't have to go to all that effort, but I had the circuit handy.

 

[Raja Vigneshwaran]

Okay if that so , I should connect my MOSFET at the Emitter of the Transistor Q1 ? Am I Right ?

 

[(*steve*)]

yes.