Relay Coil Hysteresis & Contact Pull

[ElecDesigner]

I'm aware that relays generally have a large amount of Hysteresis. I am currently using quite a large model for High Voltage/Power.
The pickup voltage is 24V +/- 10% but the manufacturer does not gauratee dropout voltage.

I've measure it on one sample (about 1V). I want to save power (and prevent the coiuls heating so much) so I'm thinking of using somthing in between (even 16.9V will half the power consumption) to hold the contacts after initially boosting to 24V for pickup.

My question is will the contacts be held so tightly together at a lessser voltage (I want to make sure I dont get contact damage) or it is simply an on or off.

My knowdege is a bit lacking on this. I.e. could there be a performance penalty if the coil is driven at a lesser voltage and still looks engaged.

 

[crutschow]

I've measure it on one sample (about 1V).

That's very low. Typically it's at least 50%.

Is that a 24Vdc coil?

To reduce coil power, you can connect a resistor in series with the coil after it pulls in.
You can connect a spare relay NC contact across the resistor, to short the resistor when the relay is de-energized, and remove the short when the relay energizes.

If you want to minimize total power and avoid the resistor power loss, you can use a PWM "Hit & Hold" circuit that efficiently reduces the average coil power with a low duty-cycle PWM signal after the relay is energized.
I have a circuit for that if interested.

 

[D.A.(Tony)Stewart]

The force of the coil on the contacts works against the springs that pull them apart during power-off unless there is some stiction as well.

The wear on contacts depends on fast switching to minimize the time during arcs when opening. But power-up surges only occur on capacitance and battery loads. During operation temperature effects of the I^2R losses in the contacts are partially improved with contact forces, but you could certainly use 50% power reduction after contact.

A 1/24V or 4% hold voltage suggests very weak springs against the stiction of the contacts which may suggest a failure. Do you have a datasheet? I know they only rate MUST HOLD voltage for relay turn off and MUST SWITCH for turn on, but this is unusual.

 

[KlausST]

Hi,

I don´t think that 24V +/-10% is the pickup voltage. I rather guess it´s the nominal operating voltage.
Please read the datasheet.

I guess the exact relay type and manufacturer is no secret, so please post a link to the datasheet, so we have full access to the specifications.

16.9V will usually not half the power consumption ... it more will half the dissipated power in the relay ... but if you do the 24V down to 16.9V drop with a linear solution (regulator, resistor, diodes, transistor...) you have additional 21% dissipation there. So the total power consumption will be 71%.

I´d keep the voltage above the pick up voltage, just to ensure the relay does not release even with mechanical vibration.

Klaus

 

[crutschow]

I apparently misunderstood your measurement.
So the relay drops out at 1V?

 

[D.A.(Tony)Stewart]

In general, most Panasonic relays guarantee a maximum pick-up voltage of approximately 75% of the nominal coil voltage and a drop-out voltage with a minimum of 10% of the nominal coil voltage.

Some are 5% min. so 4.2% is not far off.

 

[ElecDesigner]

The part is the RL21 here:

https://www.spselectronic.com/fileadmin/user_upload/relais/datenblaetter/Data_sheet_RL_42-l_RL_42_RL_21.pdf

I queried by email and they stated that the pickup voltage is 24V-10% minimum and that there is no seperate dropup voltage.

Klaus - The voltage used for the coils is from a boost regualtor so I was planning to just 'up the voltage' during pickup, so it will save the power I state (I think). You do have a point about vibration though.
They are thirsty coils and get quite hot (multiple parts in the product) - which was my motvation for trying to minisise dissipation.

--- Updated Nov 7, 2023 ---

The part is the RL21 here:

https://www.spselectronic.com/fileadmin/user_upload/relais/datenblaetter/Data_sheet_RL_42-l_RL_42_RL_21.pdf

I queried by email and they stated that the pickup voltage is 24V-10% minimum and that there is no seperate dropup voltage.

Klaus - The voltage used for the coils is from a boost regualtor so I was planning to just 'up the voltage' during pickup, so it will save the power I state (I think). You do have a point about vibration though.
They are thirsty coils and get quite hot (multiple parts in the product) - which was my motvation for trying to minisise dissipation.

 

[D.A.(Tony)Stewart]

> "they stated that the pickup voltage is 24V-10% minimum and that there is no separate dropout voltage."

This is bad info. (not info from an engineer)

Activate or Pickup or MUST SWITCH should not exceed 75% so that the supply voltage may vary 10% with std. tolerance. It is rated for 5g <= 55Hz and <=11 ms @ 24V

The Release voltage may be 5 to 10%


Getting back to your original question concerning heat.
24V * 0.2A = 5W is too much coil power for a 10A relay.

Consider if the best quality Relay Mfg companies are in Japan, namely Omron and Panasonic (HE series) using only 1,920 mW for similar capability and vibration, shock specs. Reduce your holding voltage to 12V and test with a rubber mallet to the chassis and use accelerometer to verify limits.

HE-S specs "170mW coil holding power for energy saving" is rated for 1,880 mW

So 50% is easy to accomplish if Panasonic can do it with < 10% power, rated for 35A with 3.2 mm gap.
Their 80A relay uses 4.2W in the coil.

5W is too much for a 10A switching relay. (non switch ratings are less relevant)

 

[KlausST]

This is bad info. (not info from an engineer)

As a German I have to fully agree.
They advertise with "German quality", but the datasheet is crappy
and the given information "the pickup voltage is 24V - 10% min" is nonsense.

Hopefully the relay is better quality.

Klaus

 

[crutschow]

Below is the LTspice sim of the efficient PWM hit and hold circuit I mentioned:
The pot adjusts the duty-cycle to just above the minimum to hold in the relay.
If needed, the 5V supply for the circuit can be derived from the 24V with a regulator, such as the LM7805.

 

[KlausST]

Hi

Klaus - The voltage used for the coils is from a boost regualtor so I was planning to just 'up the voltage' during pickup, so it will save the power I state (I think). You do have a point about vibration though.
They are thirsty coils and get quite hot (multiple parts in the product) - which was my motvation for trying to minisise dissipation.

For sure the boost regulator brings down total power consumption to 50%. This surely is the more "advanced" solution. But without the "boost regulator information" I just guessed you used simple series resistor or similar...

I had a similar problem. A unit containing a lot of 24V valves (in a closed box) got too hot.

I solved it by using BD8LA700EFV. It is a 8x low side driver.
The outputs may be controlled by SPI or by INputs.
So basically I had three options to drive the output:
* ON
* OFF
* dedicated INput.
Now I connected the all INx to a microcontroller generated PWM:
Now the options were: ON, OFF, PWM.

The software was like this:
* OFF as long as needed --> ON for about 0.5s --> PWM as long as needed --> OFF
I found out that a single PWM with a duty cycle of constant 50% safely kept the valves OPEN .. but the overall heating was reduced to less than 30%. Before it caused a 60°C increase from 20°C ambient to 80°C inside. After the optimisation it was about a 20°C increase to 40°C)

The hardware effort was low. The software effort in a real time environment also low.
One single PWM (for all 8 valves), that only once needs to be set up. So even processing power was very low.

For sure there are other good solutions, too.

Klaus

 

[dick_freebird]

Peak/hold drivers are a thing, though linear ones from
a single supply won't be much of a power saving solution
(more just relocating the heat).

The single-chopper (at least in cases where nobody is
sniffing your EM emissions) is a cute idea for the "hold"
or "peaking" supply, take your pick (buck vs boost and
how much on-peak duty?)