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# Relay Required

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#### m_arazik

##### New Member
Hello

I'm sorry i'm not experienced in circuits but i want a very simple circuit but don't know how to do it

this is a car circuit, so only 12 volt components, and no current load will be there, it's only for trigger wire, very low current required

i want a SMD relay number and design to do this

i have a wire which is having constant 12 volts

in certain condition when this wire voltage goes back to zero, i need the relay to supply me with with 12 volts wire and once 12 volts comes back to the original wire so relay cut off and gives 0 volts

thanx

Welcome to ETO!
i have a wire which is having constant 12 volts.

in certain condition when this wire voltage goes back to zero

That's confusing. If the wire voltage is a constant 12V it won't ever be 0V.
Do you mean the voltage is normally 12V?
If I understand your problem correctly then a transistor wired as a simple inverting stage could replace the relay.

this wire containing 12volt is coming out from another relay, when this relay is activated through remote controlled circuit, the voltage output of this wire drop to 0 volts, like ground wire

what i need that when this wire voltage drop to 0 to activate another circuit to output 12 till the wire again switch to 12 volt then this circuit disconnect

Welcome to ETO!

That's confusing. If the wire voltage is a constant 12V it won't ever be 0V.
Do you mean the voltage is normally 12V?
If I understand your problem correctly then a transistor wired as a simple inverting stage could replace the relay.
yes normally 12 Volt (it's one leg of a relay inside a circuit), when this relay get activated and click sound come this leg which was containing 12 volt originally now becomes 0, I need this leg to be trigger for another circuit so when the circuit is receiving 12 Volt so the new circuit output to be 0 volt and when the input is 0 so the output now becomes 12 volt

I know my explanation may not be so good so please forgive me
thanks

How much current does the circuit-to-be triggered require at its trigger terminal?

How much current does the circuit-to-be triggered require at its trigger terminal?
Very low current can be not exceeding 0.5 amp

Very low current can be not exceeding 0.5 amp
In the electronics world 0.5 amp is not consider a "very low current", more like a moderate current.

What you could use is a P-MOSFET with the gate terminal to the "normally 12V", the source terminal to a steady 12V, and the drain terminal being the output "trigger" signal.
Look at an electric supplier in your country for a P-MOSFET with about at least a 50V, 5A rating and the SMD package you want.
A higher rated device is fine.

In the electronics world 0.5 amp is not consider a "very low current", more like a moderate current.

What you could use is a P-MOSFET with the gate terminal to the "normally 12V", the source terminal to a steady 12V, and the drain terminal being the output "trigger" signal.
Look at an electric supplier in your country for a P-MOSFET with about at least a 50V, 5A rating and the SMD package you want.
A higher rated device is fine.

I found this one, can you please check it for me
IRF9Z14PBF 60 Volt, 6.7 A
Thanx

Last edited:
I found this one, can you please check it for me
IRF9Z14PBF 60 Volt, 6.7 A
That should work fine, but it's not in an SMD package as you mentioned wanting.

When you connect the transistor, add a 100k ohm resistor in series with the signal to the gate to minimize the effects of any voltage transients.

Just a small question, how this will work without ground wire ? Should be +ve and -ve wire to work ?

Just a small question, how this will work without ground wire ? Should be +ve and -ve wire to work ?
You only need the plus voltage as long as the input goes between 0V (transistor on) and 12V (transistor off), [see below].

Does this P-Mosfet get heated, so any precautions to be done, also resistor 100k ohm, shall i get thermistor ? To resist any anticipated heat ?

Does this P-Mosfet get heated, so any precautions to be done, also resistor 100k ohm, shall i get thermistor ? To resist any anticipated heat ?
That P-MOSFET has a 0.5 ohm on-resistance so its power dissipation at 0.5A will be 125mw, which is no problem.

The 100k resistor carries no current so its dissipation is zero.

A thermistor is just a resistor that changes value with temperature.
It has nothing to do with being tolerate of heat dissipation.

You can get P-MOSFETs with on-resistance well below 0.5 ohms if the 1/8W of heat is a concern.

You can get P-MOSFETs with on-resistance well below 0.5 ohms if the 1/8W of heat is a concern.
This only depend if the P-MOSFET will be continously running until the power drop to 0 so it will stop, this ofcourse will make it get heated, but if it will be only operating when voltage drop to 0 volt then it's of no issue as this is only 8 to 10 seconds every 2 or 3 hours

So this mosfet is which one ?

You only need the plus voltage as long as the input goes between 0V (transistor on) and 12V (transistor off), [see below].

View attachment 127705
I'm trying to simulate the circuit first but i get this error

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Last edited:
I'm trying to simulate the circuit first but i get this error

I

View attachment 127713
Forgive me , i'm just a beginner, is this circuit different from the one sent above? I tried to run the circuit i posted above after addition of ground and same result 0 volt !!

I

Forgive me , i'm just a beginner, is this circuit different from the one sent above? I tried to run the circuit i posted above after addition of ground and same result 0 volt !!
Your circuit has an error. Disconnect area in red, add my drawing that is in black. You need some type of load to complete the circuit in the simulation so add a 10 ohm to 10k ohm resistor to complete the circuit (circle).

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