# How to drive relay

#### qwwe

Joined Jul 18, 2017
31
Hello
I want to drive the relay in the following circuit.
Here the ground is not directly connected to the circuit.
It is connected to the ground through the lamp and the lamp is connected to the relay output.
But the circuit has a problem.And the irregular relay is disconnected and connected.
How should I drive this relay?
Are circuits that are not directly connected to the earth have a special name?

Joined Jul 18, 2013
22,063
Personally I would use a Mosfet driver, Is there any reason you are not switching the lamp from the positive rail,
You need to design with the GND on the Multivibrator at battery NEG.
Max.

#### be80be

Joined Jul 5, 2008
2,049
I think the TS has this drawled wrong

Should be this

#### crutschow

Joined Mar 14, 2008
26,074
Are circuits that are not directly connected to the earth have a special name?
They are called "floating".

What is the frequency and duty cycle of the timer output?

Why can't you connect the timer circuit to ground and solve your problem as be80be shows?

If not, the you could add a diode and large capacitor to store enough energy to keep the circuit operating when the relay is closed.
To minimize the size of the cap and the current through the lamp when charging the cap, you could use a MOSFET instead of a relay to drive the lamp.

#### qwwe

Joined Jul 18, 2017
31
They are called "floating".

What is the frequency and duty cycle of the timer output?

Why can't you connect the timer circuit to ground and solve your problem as be80be shows?

If not, the you could add a diode and large capacitor to store enough energy to keep the circuit operating when the relay is closed.
To minimize the size of the cap and the current through the lamp when charging the cap, you could use a MOSFET instead of a relay to drive the lamp.
The circuit should be as I draw.
The circuit cannot be connected directly to the ground.
Where to connect the capacitor and diode?

#### qwwe

Joined Jul 18, 2017
31

#### crutschow

Joined Mar 14, 2008
26,074
Where to connect the capacitor and diode?
Below is the LTspice simulation of a representative circuit using a 555 astable multivibrator.
Diode D1 and capacitor C3 form a bootstrap circuit to provide power to the timer when the lamp is on.
It uses an N-MOSFET to power the lamp, eliminating the relay..
I used an LM317 regulator to provide the 12V for the timer.

You didn't give any details about the frequency/ duty-cycle of the timer as I requested, or the lamp current, so those shown component values are just arbitrary.
The MOSFET must be large enough to readily handle the lamp large turn-on surge current as well as its operating current.

The timer current is provided by C3 when the lamp is on, so its capacitance may have to be adjusted for whatever time that is.
You can see the ripple in the capacitor voltage (yellow trace) for the selected value of capacitance and on-time for the lamp.

The blue trace shows the lamp voltage going from 0V to 24V.

#### qwwe

Joined Jul 18, 2017
31
When using a relay, the capacitor and diode still have the same problem.

#### MisterBill2

Joined Jan 23, 2018
7,780
The circuit in post#1 will not work,which is verified. The reason that other connections to ground are not available is unclear, perhaps the TS can explain so that other options can be explored. The circuits in both post 3 and 7 should work, but both require aground connection. So the problem is not yet solved. Probably a current driven circuit will be the solution.
However, if the ground for the regulator resistor R4 and capacitor C4 are moved to the "LMP" line, the negative terminal of C3, the circuit may work, depending on if the relay current can be sourced from the capacitors for the desired on time.
This circuit looks a lot like a substitute for a regular turn signal flasher. If that is the case, there are such devices already available that simply require adding an external ground connection someplace. I have done that formyself once, it was very simple.

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#### shortbus

Joined Sep 30, 2009
8,208
And the irregular relay is disconnected and connected.
The connect and disconnect is because of your "multi vibrator". By definition that is what a "muti vibrator" does and you driving the coil of a relay with that signal, so your relay is on "irregular".

#### qwwe

Joined Jul 18, 2017
31
There is a astable multi-vibrator with transistor.
The circuit in post#1 will not work,which is verified. The reason that other connections to ground are not available is unclear, perhaps the TS can explain so that other options can be explored. The circuits in both post 3 and 7 should work, but both require aground connection. So the problem is not yet solved. Probably a current driven circuit will be the solution.
However, if the ground for the regulator resistor R4 and capacitor C4 are moved to the "LMP" line, the negative terminal of C3, the circuit may work, depending on if the relay current can be sourced from the capacitors for the desired on time.
This circuit looks a lot like a substitute for a regular turn signal flasher. If that is the case, there are such devices already available that simply require adding an external ground connection someplace. I have done that formyself once, it was very simple.
Yes this is a flasher.
There is a astable multi-vibrator with transistor.
It is not possible to provide direct ground, and that is the problem.

#### crutschow

Joined Mar 14, 2008
26,074
When using a relay, the capacitor and diode still have the same problem.
Why do you have to use a relay?
The high current required by the relay coil makes it problematic to supply from the bootstrap capacitor.

What's the frequency and duty-cycle of the multivbrator output?

#### qwwe

Joined Jul 18, 2017
31
Why do you have to use a relay?
The high current required by the relay coil makes it problematic to supply from the bootstrap capacitor.

What's the frequency and duty-cycle of the multivbrator output?
Because the relay has a lower price and does not get hot.
Approx. 1 Hz frequency and 50% duty-cycle.

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#### crutschow

Joined Mar 14, 2008
26,074
Because the relay has a lower price and does not get hot.
The MOSFET won't get hot or cost more than a relay unless the lamp takes a lot of current.
And the relay is less reliable, especially for continual operation at a 1 Hz rate.

How much current does the lamp take?

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#### MisterBill2

Joined Jan 23, 2018
7,780
I am trying to imagine an automotive system where the ground side is not accessible, and not finding any such situation. The ground connection may be inconvenient but it is there.The ground connection might not be wired, but it would still be available. OR this may not be a vehicle application.

#### qwwe

Joined Jul 18, 2017
31
The MOSFET won't get hot or cost more than a relay unless the lamp takes a lot of current.
And the relay is less reliable, especially for continual operation at a 1 Hz rate.

How much current does the lamp take?
Current max =5amp

#### qwwe

Joined Jul 18, 2017
31
I am trying to imagine an automotive system where the ground side is not accessible, and not finding any such situation. The ground connection may be inconvenient but it is there.The ground connection might not be wired, but it would still be available. OR this may not be a vehicle application.
It is used in cars

#### crutschow

Joined Mar 14, 2008
26,074
Current max =5amp
There are many inexpensive MOSFETs that will carry that current without getting warm.
For example this 120A N-MOSFET has a 30V rating with a 1.9 milliohm on-resistance and sells for U\$0.82 in unit quantities.
It will carry 5A when fully-on while dissipating only 5*5*1.9m = 40mW, which will raise the device case temperature only about 2.5°C above ambient.
That's probably less then the relay rise in temperature due to the coil dissipation.

#### be80be

Joined Jul 5, 2008
2,049
I guess the TS is putting this in the fuse and relay block and that's why he or her thinks there no way to get to the ground

#### MisterBill2

Joined Jan 23, 2018
7,780
I guess the TS is putting this in the fuse and relay block and that's why he or her thinks there no way to get to the ground
When I installed the one that I used it was far to big for that block, since it was originally for a vehicle with lots of lights. So I had to use extension wires that plugged ito the block, and I mounted it a foot away. The ground connection went all the way to a grounded screw connected to body metal, almost a foot away.