Using a 12VDC coil relay with 19V

[floppy32]

Hello,

For some reason after a supply change I need to use this 12V relay with 19VDC.
I cannot find a 19V or even 18V version of this relay, so seems I'm stuck with using the 12VDC relay with 19V supply.
Somehow I need to match the coil voltage down to 12V form 19V.

In this case, would using a series 160 Ohm 1W resistor be fine?

 

[danadakk]

Use a zener diode, make sure you adhere to its power rating and thermals.

The resistor R needs to be computed R = E/I, I the required relay current, E,
( 19 - 12 ) = 7 Volts

 

[betwixt]

The coil operating current is 12/275 Amps = 44mA.
So you can use a resistor to drop the extra 7 volts: R = 7/.044 = 160 Ohms, 0.5W or a 6.8V Zener diode rated at 0.5W.
A resistor might slow the rise and fall times of the current, making the relay operation slightly slower. Whichever you use, still add a diode across the relay coil, not across the coil and series component.

Brian.

 

[dick_freebird]

"12V" is a nominal rating, there should be a coil
resistance and a max coil current or voltage to
let you determine whether you need any change
or can "suck it up" (like if you see min coil R and
max coil I rolling up to your 19V, or better).

 

[barry]

Use a zener diode, make sure you adhere to its power rating and thermals.

The resistor R needs to be computed R = E/I, I the required relay current, E,
( 19 - 12 ) = 7 Volts

Relays are current devices, not voltage devices. The voltage specification infers a current based on the coil resistance. Unless you’re really concerned with speed, as Brian mentioned, a series resistor should be fine.

 

[KlausST]

A resistor might slow the rise and fall times of the current

I think it´s the other way round.
The coil acts as an inductor at the beginning, thus it is higher impedance, the voltage across the coil is higher than with a zener, thus the current rises faster.

it´s an RL .. an RC would cause a delay.

Klaus

 

[barry]

I think it´s the other way round.
The coil acts as an inductor at the beginning, thus it is higher impedance, the voltage across the coil is higher than with a zener, thus the current rises faster.

it´s an RL .. an RC would cause a delay.

Klaus

I dont think so. The higher impedance infers a lower current.

 

[FvM]

Relay operation speed is determined by electrical (L/R time constant) and mechanical delay. As for L/R time constant, it obviously decreases by adding a series resistor to given coil L and R. For the discussed 10 A relay, I'd expect operation speed dominated by mechanical delay.

 

[barry]

Relay operation speed is determined by electrical (L/R time constant) and mechanical delay. As for L/R time constant, it obviously decreases by adding a series resistor to given coil L and R. For the discussed 10 A relay, I'd expect operation speed dominated by mechanical delay.

This is Electronics 101. If you put a resistor in series with an inductor, the current will rise SLOWER with larger resistance. L/R is the VOLTAGE time-constant, not current.

 

[KlausST]

Hi,

Your simulation does not show the situation in question.

do the test like in the application:
* relay connected to 12V without resistor (normal timing)
* relay connected to 19V with series resistor (faster -- according my understanding)
* relay connected to 19V with series zener (normal timing)

Klaus

 

[FvM]

@barry: Please reconsider

The simulation compares overloaded relay with normal power situation. Not surprizing that di/dt is higher in overloaded case. To see time constant, you'd need to run the simulation long enough.

 

[D.A.(Tony)Stewart]

Hello,

For some reason after a supply change I need to use this 12V relay with 19VDC.
I cannot find a 19V or even 18V version of this relay, so seems I'm stuck with using the 12VDC relay with 19V supply.
Somehow I need to match the coil voltage down to 12V form 19V.

In this case, would using a series 160 Ohm 1W resistor be fine?

That supplies the 1/2 W needed for the coil at 12 V with 275 ohms and dissipates only 30% of 1W and only 30% of the max 125 ‘C temperature rise in an open environment.

The current rise time dI/dt is identical with the same current using a resistor from a higher voltage.

Normally relays are designed with an L/R=T much less than the mechanical time to endstop. E.g. 1 ms vs 5ms

There is no advantage to limiting the voltage with a zener.


Sim

 

[danadakk]

R seems to turn on faster, but Zener faster turnoff....

 

[D.A.(Tony)Stewart]

Here is a better version of my simulator to demonstrate 160 Ohms from 19 V into 275 ohm 200mH coil with a 30 mA make current but using the specified 45 mA for a faster response of 5 ms.

I made a Schmitt Oscillator and showed the effect in slow motion.

https://tinyurl.com/2yf7u2x5

Note the relay response is identical and using a Zener offers no benefit.

 

[KlausST]

R seems to turn on faster, but Zener faster turnoff....

It depends how the relay is driven.

Your simulation shows 19V - low impedance and 0V low impedance.

In most cases it is some kind of open collector driver: 19V low impedance .. OFF = OPEN = high impedance.
And in case of high impedance it would turn OFF very fast .. by causing a high voltage spike.
To avoid the spike one usually connects a free wheeling diode. With the free wheeling diode the turn OFF time should be similar to turn ON time.

In case of added series devices ... one could connect the free wheeling diode across the relay only or across the relay+serial_device. Which may lead to faster turn OFF.

Klaus

 

[Easy peasy]

A resistor to limit the current to nominal, and a cap across the resistor to give a little extra impulse at turn on - this is a tried and true way.

 

[dick_freebird]

Still have not seen what the figuring of worst case
coil current vs actual (not nominal) spec range has
to say. Especially automotive grade, might hold
a lot of margin in pocket.

 

[D.A.(Tony)Stewart]

A resistor to limit the current to nominal, and a cap across the resistor to give a little extra impulse at turn on - this is a tried and true way.

Although this is true and useful when using less than rated DC, there is no practical speed benefit using rated voltage as a 5 ms switching time example would only decrease < 10% with 100 uF across 160 Ohms ( which I verified in my simulation) DC also controls the vibration immunity which drops rapidly when current is near the "may switch" threshold.

@dick_freebird Coil current much like stepper motors allows a much higher pulse current as long as the average coil power and temperature rise do not exceed the max operating temp. with rapid pulse current changes..

 

[Easy peasy]

We normally use 220-470uF, there is a slight resonant effect, and an effect from the L going up as the relay swings to close,

As you say this same ckt can be used to run the relay at say 80% of nominal current - to increase the life of the relay coil.