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Help with a simply relay circuit

kgor

Nov 14, 2014
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Question is:
I have a simple SPDT relay circuit but I'm trying to figure out at what Voltage & Current the coil energizes. The reason I really want to know is because when I use 3V it doesn't work; at 6V I can hear a faint attempt, but at 7.5V it works fine.

I'm obviously beginning my experimenting and my theory is basic so correct me if I am wrong but the current through each LED is independent and hence the same. The best Datasheet I could find for the relay used in this book is attached too (HLS-14F3L-DC12V-C…5A 240V/5A 30V).



Resource Cited: "Make: Electronics" by Charles Platt. pg 59.
 

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KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Hi there and welcome to Electronics Point :)

I'd guess it's a 12V coil. Relays generally pull in at around 2/3 of their rated voltage, and are often guaranteed to pull in at 75% or 80% of their rated voltage.

A relay coil is both resistive and inductive, but the resistance determines the operating current when the coil is energised continously. You can measure the coil resistance with a multimeter set to the resistance range, or you can calculate it by applying the rated voltage, measuring the current, and using Ohm's Law rearranged to R = V / I to calculate R.

In that circuit, if both LEDs are the same type, then yes, they will have the same current flowing through them. Only one will be illuminated at a time, and in either case, the 12V supply is applied across one LED and the single 680Ω series resistor. The LED that is not illuminated has no effect on the circuit because the relay contact it's connected to does not connect to anything.

For more on LEDs and series resistors see Steve's tutorial at https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
 

kgor

Nov 14, 2014
3
Joined
Nov 14, 2014
Messages
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Hey!

Awesome info. Thanks a lot for the quick reply you're awesome.

Is there a way to calculate the V/I/R of the coil accurately?

Wht is the best ways to measure current: across the coil or in series? Also, my multimeter is pretty basic and I'm still getting familiar with it but is there one you recommend? I may post this question elsewhere but I figured I'll give it a shot.

Thanks again

....


Hi there and welcome to Electronics Point :)

I'd guess it's a 12V coil. Relays generally pull in at around 2/3 of their rated voltage, and are often guaranteed to pull in at 75% or 80% of their rated voltage.

A relay coil is both resistive and inductive, but the resistance determines the operating current when the coil is energised continously. You can measure the coil resistance with a multimeter set to the resistance range, or you can calculate it by applying the rated voltage, measuring the current, and using Ohm's Law rearranged to R = V / I to calculate R.

In that circuit, if both LEDs are the same type, then yes, they will have the same current flowing through them. Only one will be illuminated at a time, and in either case, the 12V supply is applied across one LED and the single 680Ω series resistor. The LED that is not illuminated has no effect on the circuit because the relay contact it's connected to does not connect to anything.

For more on LEDs and series resistors see Steve's tutorial at https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
Hi there and welcome to Electronics Point :)

I'd guess it's a 12V coil. Relays generally pull in at around 2/3 of their rated voltage, and are often guaranteed to pull in at 75% or 80% of their rated voltage.

A relay coil is both resistive and inductive, but the resistance determines the operating current when the coil is energised continously. You can measure the coil resistance with a multimeter set to the resistance range, or you can calculate it by applying the rated voltage, measuring the current, and using Ohm's Law rearranged to R = V / I to calculate R.

In that circuit, if both LEDs are the same type, then yes, they will have the same current flowing through them. Only one will be illuminated at a time, and in either case, the 12V supply is applied across one LED and the single 680Ω series resistor. The LED that is not illuminated has no effect on the circuit because the relay contact it's connected to does not connect to anything.

For more on LEDs and series resistors see Steve's tutorial at https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Joined
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Is there a way to calculate the V/I/R of the coil accurately?
Not calculate, no. You can measure the resistance directly with a multimeter, or you can apply a voltage, measure the current that flows, and calculate the resistance using R = V / I.
Wht is the best ways to measure current: across the coil or in series?
ALWAYS measure current in series. You need to break the circuit and connect the multimeter across the break. Remember, current is a flow. You need the current to flow through the multimeter as well as the circuit you're measuring. Then you will get a true measurement of the current flow.
Also, my multimeter is pretty basic and I'm still getting familiar with it but is there one you recommend? I may post this question elsewhere but I figured I'll give it a shot.
Have a look at Dave Jones's DMM buyer's guide for beginners.

While you're there, you should subscribe to his eevblog channel and look through the previous episodes. All kinds of interesting and useful information there.
 

kgor

Nov 14, 2014
3
Joined
Nov 14, 2014
Messages
3
Wow thanks!! awesome help!




Not calculate, no. You can measure the resistance directly with a multimeter, or you can apply a voltage, measure the current that flows, and calculate the resistance using R = V / I.

ALWAYS measure current in series. You need to break the circuit and connect the multimeter across the break. Remember, current is a flow. You need the current to flow through the multimeter as well as the circuit you're measuring. Then you will get a true measurement of the current flow.

Have a look at Dave Jones's DMM buyer's guide for beginners.

While you're there, you should subscribe to his eevblog channel and look through the previous episodes. All kinds of interesting and useful information there.
 
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