# Selecting a Relay for 48V DC application.

#### Scott Reimers

Joined Jul 16, 2019
6
I am new to playing with relays...

I have a current which will range from 36v to 54v at 1-3a

I would like to use a relay to switch off the current to a sub circuit once the main circuit voltage crosses 50v.

When I look-up 48v relays I see MUCH higher switching voltage. The one below shows 300V DC @ 16A and sounds like massive overkill to me.

https://www.mouser.com/ProductDetai...2RL-1A-E-ASI-DC48?qs=zOfyGkaCkAVubB8YPacbPQ==

It sounds to me like I should use relays with much lower switching voltage limits (~60v@~3a) and use resistors to make sure that when the circuit voltage crosses the 50v it hits the pickup voltage level of the relay (I'm seeing numbers as low as 3-12v).

Unfortunately we don't know what we don't know. From what I'm expressing does it sound like I have a reasonable idea of how relays work and how to apply them into real world applications? If so, does this plan in particular sound reasonable or is there something else you would advise I look into?

Thank You,
Scott

Joined Jul 18, 2013
28,669
The switching (contact) voltage is rated the maximum, this does not mean you have to go this high.
Max.

#### Scott Reimers

Joined Jul 16, 2019
6
Thank you. I guessed that, but it looks to me like relay price comes down as the switching voltage/amperage does.

Am I correct that I could use a much smaller relay that the one I linked above by using resistors to trigger the relay at the goal voltages?

Joined Jul 18, 2013
28,669
You don't need to exceed your present switching requirements, just allow a little leeway is the norm.
Max.

Joined Jan 15, 2015
7,512
I would like to use a relay to switch off the current to a sub circuit once the main circuit voltage crosses 50v.

When I look-up 48v relays I see MUCH higher switching voltage. The one below shows 300V DC @ 16A and sounds like massive overkill to me.
The relay you linked to has a 48 VDC coil which draws 8.96 mA of current at its rated voltage. The data sheet also shows the Must Operate and Must Release voltages in addition to the voltages and current the relay contacts are rated for. I assume you have a circuit that will switch the relay on and off at the 50 volt threshold you mention? The relay will not pull in and drop out at its rated coil voltage.

The rated load for the contacts is:
12 A at 250 VAC
12 A at 24 VDC

Those contact ratings are for a resistive load being switched. You are not depending on a relay coil where you mentioned "to switch off the current to a sub circuit once the main circuit voltage crosses 50v" are you?

Ron

#### KeithWalker

Joined Jul 10, 2017
3,089
I am new to playing with relays...

I have a current which will range from 36v to 54v at 1-3a

I would like to use a relay to switch off the current to a sub circuit once the main circuit voltage crosses 50v.

When I look-up 48v relays I see MUCH higher switching voltage. The one below shows 300V DC @ 16A and sounds like massive overkill to me.

https://www.mouser.com/ProductDetail/Omron-Electronics/G2RL-1A-E-ASI-DC48?qs=zOfyGkaCkAVubB8YPacbPQ==

It sounds to me like I should use relays with much lower switching voltage limits (~60v@~3a) and use resistors to make sure that when the circuit voltage crosses the 50v it hits the pickup voltage level of the relay (I'm seeing numbers as low as 3-12v).

Unfortunately we don't know what we don't know. From what I'm expressing does it sound like I have a reasonable idea of how relays work and how to apply them into real world applications? If so, does this plan in particular sound reasonable or is there something else you would advise I look into?

Thank You,
Scott
You can not rely on the "must pull in" voltage to reliably control your circuit. The actual pull in voltage will be less than this but will vary with temperature, magnetic environment and between relays of the same type. You will need an electronic circuit to accurately detect the voltage threshold and operate the relay.

#### crutschow

Joined Mar 14, 2008
34,392
How accurately do you want the 50V switching voltage to be?

Joined Jul 18, 2013
28,669
If talking the relay hysteresis, it is very simple to use a variable voltage to record this.
I did one recently to record the hysteresis of relays using unfiltered DC using a Variac, to dispel the myth of relay/solenoid buzz when using this type of DC supply.
A Variac and simple DC bridge was used.
A similar method can be used to determine the hysteresis using a smooth supply.
Max.

#### crutschow

Joined Mar 14, 2008
34,392
I did one recently to record the hysteresis of relays using unfiltered DC using a Variac, to dispel the myth of relay/solenoid buzz when using this type of DC supply.
And did you dispel the myth?

Joined Jul 18, 2013
28,669
And did you dispel the myth?
Yes, I ran a Variac with unfilterd F.W. DC up from zero up to the relays rated voltage through the pick up and drop out hysteresis levels and the relays did not exhibit any detectable buzzing of any kind.
The relays were several different makes models and sizes.
Max.

#### panic mode

Joined Oct 10, 2011
2,733
solid state?

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#### crutschow

Joined Mar 14, 2008
34,392
Yes, I ran a Variac with unfilterd F.W. DC up from zero up to the relays rated voltage through the pick up and drop out hysteresis levels and the relays did not exhibit any detectable buzzing of any kind.
That is the result I would suspect.
Unrectified AC can cause buzzing/chatter because the coil current goes through zero at twice the line frequency.
Due to the coil inductance, full-wave rectified AC generates a coil current than never goes to zero, thus no buzz.

#### panic mode

Joined Oct 10, 2011
2,733
yup... i would expect buzzing to be result of solenoid wire or lamination oscillating due current changes.
when fully rectified, changes are smaller (not big deal) but more importantly field does not change direction so forces on wire are biased.
fluctuation (of pulsed DC) causes forces to change in magnitude but not direction and copper wire is not elastic enough to spring back between peaks. any audible effect would be dramatically reduced.

Joined Jul 18, 2013
28,669
I did this a while back to attempt to refute many claims that smoothing capacitor required, as I had never experienced relay armature 'buzz' before, I decided to carry out the test.
Similar to DC solenoids where often the DC used is of the same nature.
Max.

#### Scott Reimers

Joined Jul 16, 2019
6
I think I heard from the replies that the activation voltage will vary slightly from relay to relay as well as other environmental factors.

KeithWalker mentioned that I would need: "an electronic circuit to accurately detect the voltage threshold and operate the relay."

Someone asked me how accurate I need it to be, and I think someone else mentioned using a variable resistor/potentiometer to customize the circuit to get close to the specific goal voltage at relay activation...

I would like to be +/-1 volts around 50.

I admit I was hoping that I could use the resisted circuit which activates the relay to BE the detection and operation circuit. I'm a bit confused because I think I heard someone imply it could be done with a margin of error and Keith say it wouldn't work?

#### crutschow

Joined Mar 14, 2008
34,392
I would like to be +/-1 volts around 50.

I admit I was hoping that I could use the resisted circuit which activates the relay to BE the detection and operation circuit.
For that accuracy you likely need some electronic circuitry.
Is that acceptable, and are you able to assemble such a circuit on a perfboard with perhaps a dozen or so parts?

Joined Jan 15, 2015
7,512
I would like to be +/-1 volts around 50.

I admit I was hoping that I could use the resisted circuit which activates the relay to BE the detection and operation circuit. I'm a bit confused because I think I heard someone imply it could be done with a margin of error and Keith say it wouldn't work?
I had a feeling that is where this was going back in post #5. No, using the relay rated coil voltage won't work. You will need as was mentioned, some additional circuitry. What you are looking to do is not all that difficult and there are several ways to go about it.

A simple approach would be to make a divider network to get your voltage down to a usable level which is proportional to your measured voltage. You need to know the range of input voltage possible. Just for example 0 to 60 Volts with a 15:1 divider network becomes 0 to 5 Volts. Your 50 Volt limit is now 3.33 Volts. Then you take that voltage and feed it into a Comparator Circuit. Then you want to apply something which has been mentioned which is "Hysteresis". The idea here is when the input voltage is right around your set voltage you do not want your circuit turning a relay On/Off and chattering away. Just for example at 50 Volts the comparator toggles output but will not toggle again till the monitored voltage drops to maybe 48 or 49 Volts.

Another option is the use of a small uC (Micro-Controller). You program a threshold and the controller will toggle out states much like a basic comparator.

What needs known is in detail exactly what your goal is. Voltage range monitored, the tolerances and as much information as you can provide. Anyway, no, you can't rely on a relay coil to do what you want to do.

Ron

#### crutschow

Joined Mar 14, 2008
34,392
Edit: Changed BJT to cheaper MOSFET--
Below is the LTspice simulation of a fairly simple circuit (8 components plus relay) using a TL431 programmable reference as a comparator to control a grounded-gate N-MOSFET which turns on a relay at 50V nominal.
(M1 isolates the TL431 from the high voltage since its Vmax is 36V).
The trigger point is determined by the TL431 2.5V internal reference voltage, which is very stable.
It switches when the Ref voltage reaches 2.5V, as determined by the resistive divider R8-R9.

The nominal values gives a simulated relay turn-on (yellow trace) at 50.1V (blue trace) and turn-off at 49.5V (0.6V hysteresis provided by R2).

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#### Scott Reimers

Joined Jul 16, 2019
6
Wow... you guys are awesome. Thank you for all the effort to explain.

I'm thinking I miscommunicated with my +/- 1v statement.

I just meant I wanted relay turn-on to be between 49v-51v. I wasn't worried about relay turn-off voltage because I planned to manually reset it.

I think need to study some more circuit design basics. Every time I think I have a grasp of what I need to achieve a goal, I learn about new dynamics I wasn't aware of.