Relay Specification...

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# Relay Specification Question

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(@donpk)
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In the image of the coil specifications for a 3VDC relay:

1. In the column "Coil resistance at 20C" what do the four resistances for four different wattages mean? Doesn't the coil have a fixed resistance at 20C and at 3VDC it has a single wattage?
2. What does the spec "Max Operate Voltage VDC" mean? Should that actually be "Minimum Operating Voltage VDC", in other words, the minimum coil voltage at which the relay contacts close?

(@will)
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Anything seems possible when you don't know what you're talking about.

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(@donpk)
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Thanks, Will, that's useful.

I'm guessing "Max pickup voltage" is what the DigiKey article calls "Pickup Voltage" which I would understand as being the minimum voltage required to activate the contacts.

I still don't understand the "Coil resistance at 20C" section. It shows the resistances at 3VDC for four different wattages.

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(@will)
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Anything seems possible when you don't know what you're talking about.

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(@donpk)
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Thanks, that article (and others it led to) help explain the issue of coil resistance change with temperature.

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(@davee)
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Hi @donpk,

The table with 4 coil resistances, taken out of context looks a little strange, but I don't think temperature has anything to do with it having 4 sub-sections ... That section is headed with 20 degC for all 4 ratings.

(Of course the coil resistance will also change with temperature, but that applies to all 4 sub-sections.)

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I think it is implying the relay can be ordered in 4 different variants, each with a different coil winding.

The coil winding determines the resistance ... say 60 Ohms for the leftmost one, and hence the power dissipation in the coil when the relay is activated.

If 3 Volts is applied to a 60 Ohm resistor (coil), then the power dissipation is given by P = V * V / R

i.e. P (Watts) = 3 (V) * 3 (V) / 60 (Ohms) = 0.15 Watts

Similarly for 20 Ohms.. P = 3 * 3 / 20 = 0.45 Watts

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Exactly why nominally the same relay should be made in 4 versions is less clear .. perhaps it is for different sets or types of contacts, which correspondingly have different spring and magnetic force requirements.

Optimistically, this will be explained by other information on the data sheet, though I will not be astonished if the explanation is missing, or at least difficult to spot!

If you can't see why, you might try copying the whole sheet or provide a URL reference for further comment.

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With @will 's help, you have probably figured out the different voltage specs, but just in case:

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Max Operate ... if you apply a slowly increasing voltage to the coil, starting from zero, the relay will have switched by the time the voltage reaches this value ...  2.5V

That is the relay might switch at a value below 2.5V --- say 2.3V, but it might wait for the full 2.5V

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Min Release ... if the voltage of say 3V is applied to the relay coil, then it will be in the switched state .. if that voltage is slowly reduced, it may have to get down to as low as 0.3V, before it releases.

Note, it is normal for the voltage to definately activate (2.5V) the relay to be much higher than the voltage to definitely release (0.3V).

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Max Applicate ...  is an unusual term, perhaps the result of translation from another language, but I think it means exceeding this voltage (3.9V) may result in it overheating and failing.

Hope this helps a bit. Best wishes, Dave

(@donpk)
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Topic starter

Yes, the coil/resistance/temperature spec seems strange. Other relay specifications show this coil specification as a graph of coil resistance versus temperature.

(@davee)
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Joined: 3 years ago
Posts: 1798

Hi @donpk,

Since you have only provided the small snippet of a table, I can't be sure, but I strongly suspect the table is describing 4 different relay types from the same 'family'. All 3 Volt coils, but probably with different types of contacts.

With parts like relays (and multipole connectors), it is common for the data sheet to describe a family of different part numbers, and it is necessary to carefully examine the whole sheet, to figure out the exact part number to order with the required options.

Increasing the number of contacts, and/or increasing the current rating of the contacts, means the spring and magnetic force must be greater, and the coil will need to have a higher power rating, which means lower resistance, since the voltage is 'fixed' at 3V.

So which of the four sub-columns is appropriate, depends upon exactly which relay you order.

Of course, if the table is from an advert from a reseller, it is possible they have snipped the table from a data sheet, not showing the full information set, in which case, what you get will be pot luck!

If you already have the relay, then measuring the coil resistance with meter, should enable you to find which one applies to your actual part. (Allow for at least 10% variation between the measured value and the table values.)

Best wishes, Dave

(@donpk)
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Joined: 2 years ago
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Topic starter

Here is more of the coil specification. Since even at a fixed ambient temperature (20°C) the coil resistance can change due to self-heating, perhaps the chart is attempting to show the current/wattage changes.

Don

(@davee)
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Joined: 3 years ago
Posts: 1798

Hi Don @donpk,

Sadly, my fears of a confusing data sheet appear to be justified, but I still think it is describing three or four different versions of a 3V coil relay.

I don't know which company's products you are looking at, but as an example of the different relay types with the same voltage rating I found :

From https://docs.rs-online.com/4250/0900766b81663c47.pdf     the following snips:

(click each picture to see enlarged image)

In this case, the different coil resistances for the 3V coil part, correspond to different functionality, 1 'conventional' operation, the other two with latching operations.

You can see that each type has a 3V version, but with different coil resistances and hence power dissipations.

Of course, I am not saying that the table you have matches these parts ... I am merely suggesting that a '3V relay' can be manufactured in many different coil resistance versions, according to its function, number and rating of contacts, etc..

You have only posted section 3.x of the sheet for your parts .. I would hope you also have access to the other sections, and that 1 or more of the other sections has further information to enable you to decide which part you require.

Best wishes, Dave