why this occure

You may have a bad switching device. Not knowing what is switching to provide power to the coil. Look to that device for bad contacts. Carbon build up on the tips of contacts can in a no load situation show full voltage at the device it is operating. Under load may heat up dropping the voltage. Maybe even create an open.

You connect the coil at no load and measure the voltage.Then we can say this reason

You say your instrument can drive loads up to 100 mA. and your coil resistance is 250 ohms 12x12 divided by 250 thats 0.576A 576mA . you are choking your power supply.

You wrote 12x12 divided by 250 thats 0.576A

This is incorrect. 12/250 = .048A well within his power supply spec. Your formula calculates the watts (V²/R=Watts). However your statement regarding choking the supply still sounds like the logical answer.

To the POSTER: I would try a purely resistive load (around 150 ohms) and proove the power supply can source 80% load. The inrush to the relay coil (inductive load) could be triggering the short circuit (crowbar) protection of the flow device power supply, meaning you can not use the flow device to drive the relay directly.

The 2mv looks like your instrument was only good for a few milliamps, or perhaps you have a 12vdc coil with a diode as a suppressor around it and you have the polarity backwards. Follow suggestion of using a known purely resistive load to verify drive capability of instrument.

The diode would generate approx 0.6 V.

Only if he has some drive capability. I think his instrument is saddly lacking for the load he is attaching.

He should be able to drive 100mA and still giving its nominal power.

It looks to me that there is a short somewhere.

What they can do is take a power supply module and try to attract the coil from the leads that are connected to the instrument. measureing the voltage at the power supply, the coil and measuring the current.

How long is the connection between the coil and the instrument, what is the section of the connection cable.

We are in full agreement.

There is something fishy, and it needs to be systematically troubleshot.

1) Instrument is nfg?

2) short?

3) load is not what it appears?

4) (long shot) power supply for instrument is nfg?

it looks like you have a Short circuit on the relay coil, may be caused by dirt or moisture, you need to check the relay coil again to confirm the correct resistance of the coil.

Is this instrument an analog device, doing flow measurement, or a switching device? The query arises from the mix of "measure flow" and "coil not energise" in the posted question.

What is the maker and part number of the device, and what is the measurement technique used to convert the flow to an electrical signal? Moving vane? Pelton wheel? Variable area? Electromagnetic? Ultrasonic? Mass flow? Vortex shedding?

What do you mean by: ************ Quote ************* we have 12 v dc come from instrument ... .. instrument can drive load at 100 mA ************ Un-Quote ************* 1. I assume that battery of your flow-meter is connected directly from its terminals to coil. 2. you are measuring voltage across coil terminals when connected to the battery of your flow- meter & it is 2 mV on-load. Then it is clear that the battery is either of low capacity to handle load more than say 5 ma[can just work with flow-meter] or if it is at least 1AH capacity it has gone bad. What I could make out from your post that You have: 1. a Flow-Meter whose internal battery is 12V & can deliver 100ma. To which load? [12/100]*1000 =120 ohms It means your battery is capable to deliver 100 ma to 120 ohms load. How you calculate this? 2. The coil you are testing is [coil resistance =250 ohm]. But the current of the coil @ which it energizes is not mentioned by you. 3. What is the Type & capacity and how long you have used it?

It seems there a large voltage drop acros the cable from the instrument to the relay coil.There maust be approx 12 V drop across the cable. Or simply you should go for 120 or 100 Ohm resitance coil so that akes for 12 V.

he given coil is 250 x 100 MA= 25 Volt sonot menat for a instrument of 12 V .

Any takers