Voltage Relay Sensor and DC Power Supply

This doesn't sound like a difficult problem to solve. But you don't give us a schematic, any of the components specified or even pertinent measurements like initial relay turn on voltage, turn off voltage, or even relay current levels. With this information we might be able to help, without it I certainly cannot help you.

the value of my resistor is 1.5k ohms. my relay coil resistance is ranging from 720-725 ohms. by applying KVL the relay is partially energized about 34.5 percent of soure voltage of 24v. as the generator start the dc supply decreases so the relay was de-energized, in running state of the generator dc supply stabilized (24-26v) but the relay doesn't re-energize. is the value of my resistor correct? or is there any component needed to be added to satisfy the relay to energize

Cool, I love it when somebody asks for technical help and demonstrates that they're willing to do the analysis work. But I get that the voltage across your coil is about 32.5% instead of 34.5%. I'll count that as a typo.

For some reason, relay considerations are rarely taught in school today. These seemingly simple parts have many nuances that get easily overlooked. So let me give you a good reference from Panasonic who purchased the Aromat relay line that this reference comes from. This reference does not precisely identify what I think your difficulty is but it gives me a common point to discuss from.

Now with the circuit you've described, with 24 volts the voltage drop across the relay coil itself there will be only 7.8V across the coil. But relays can be considered little tiny motors that have to move and hold just one thing, the armature that move the contacts. The force available to move this armature is proportional to the current found in the solenoid, not the voltage. But people tend to take relay operational values by measuring the voltage across the coil instead of the current. Also the force required to move the armature is not the same force required to hold this armature. This is why a good relay manufacturer like this DS series of signal relays, will specify not just the nominal coil voltage but also the pick up and drop out voltages (pg. 3). Also the first required pick up voltage will likely be less than the manufacturers specified value. This is because no residual magnetism is present from a previous cycle, but the second and following cycles will likely be close to the manufacturers numbers.

Now in your generator detection by a relay, you've added a series resistance to drop the voltage across the relay. Your voltage source will not change quickly since the rotor will not quickly come up or drop in speed. So voltage changes at the coil will also be gradual.

Without knowing which relay you are using, I cannot help any further. I hope this helped.

im using idec relay 24vdc 5 amp contact, sometimes ston relay, is there anything i can add to the circuit to solve my problem? the starter of the generator takes to much current ithink that drop the voltage of the dc source (battery) upon startup so my relay was de energized,, that thing can be justified but my real problem is my relay doesnt energized even the battery already charge the alternator(dc supply built up to 24v). i also tried using capacitor to delay the discharge of my relay but it doesnt work

Now we're getting somewhere. Which specific model IDEC relay do you have? Is it part of the RY series or another part in their catalog. Now in the first link you will notice that IDEC specifies their pickup and dropout voltages instead by percentages of 80% and 10% respectively. So, as we discussed in the earlier page the resistor you have in series with the relay coil places only 7.8 volts across a 24 volt relay. With a maximum pickup voltage being 24V*0.8=19.2V. This is why your relay won't turn ON. Now I'm going to make a wild guess, you added this resistor to reduce the current required while the relay is ON. So this can still be done by taking one of the normally closed relay contacts (NC) and paralleling the resistor. This way the relay sees full voltage to turn ON but reduced voltage to retain being ON. You might occasionally oscillate with just this circuit, so putting in parallel with the resistor and NC contacts will be a 47 μF capacitor.

Hello, redfred.

"...this can still be done by taking one of the normally closed relay contacts (NC) and paralleling the resistor. [Assume half of a DPDT is available] This way the relay sees full voltage to turn ON but reduced voltage to retain being ON."

This is a great trick. I have had similar problems, and have simply reduced resistance until it would fire, and hope for no burnout in the coil. Thanks, and a GA!

post script to Original Poster: I agree completely with redfred; not enough pull in voltage available. I did not have a solution to offer, and now I do.

thats great...but im just thinking if i would your advise...im bypassing the design particularly the resistor....what if i encountered real low dc voltage my relay may turn ON/OFF. im reelly taking hard time about this bec. its already existing and installed...and im just finding a way to correct it if its possible.....this is the operation...im taking a signal for plc from this relay normally close contact for low battery fault. so every time the relay deenergized because of voltage drop out upon generator startup it gives signal to the plc of low battery....we already tried putting delay to the plc program...but the relay doesnt re-energized so the low battery signal is not reset....if paralleling of capacitor to the resistor without the nc contact is will do.. thanks

Since you are trying to sense the voltage from a generator the voltage will change slowly with the generator trying to get up to speed. So a capacitor across the resistor will not work. Now if you were turning ON the relay with a switch, then you can get away with paralleling the resistor with a capacitor. An old trick that people use to use that did not require an unused pair of contacts was to substitute a light bulb for the resistor. When the filament is cold, the resistance is very low but as it starts to glow from warming up the resistance increases. But this makes your circuit considerably less reliable for filaments fail much sooner from heat cycling and vibration than relay coils.

Now you should have noticed when you did your voltage drop analysis, the transition from ON/OFF occurs at a higher voltage than the original voltage threshold. So adding the switched resistor permits controlling at what voltage the relay turns OFF along with reducing the holding current.

Now if your relay encounters a real low dc voltage my relay may turn ON/OFF. Well thats how you turn OFF a relay. Isn't this what you want? If what you want is a perpetual GOOD generator status, regardless of generator status then you can bypass. But now whatever process this relay controls will be defeated.

yes im trying to sense the real status...where there, whre my relay deenergized bec. it really senses a low voltage...but why the relay doesnt reenergized even when the voltage is already built up? i have read about this quenching diode parallel to relay coil reverse bias, is this ca help?

All mechanical relays have what is known as hysteresis. Let me pilfer a diagram from an EDN article that uses this hysteresis characteristic in another circuit.

Now you'll notice the X axis is the coil voltage that can vary to any value. The Y axis has only two states, ON/OFF. The line segments that do not have an arrow on them means that at those coil voltages there can only be one state for the relay , ON/OFF. In the voltages between the two thresholds the relay can be in either state. What determines which state the relay will be in is what state it was in previously in. In other words, starting from 0V lets walk the path of increasing the coil voltage. The relay naturally starts as OFF as the voltage increases. When the voltage reaches the OFF threshold we follow the arrow path going to the right, so the relay remains OFF. Only once the coil voltages reaches the ON threshold does the relay turn ON. Continuing to increase the voltage to the nominal coil voltage the relay remains ON. Now a slow decrease in coil voltage with the relay still ON we reach the same voltage that was the ON threshold but now the relay stays ON until reaching the OFF threshold.

When your generator is producing 24V the resistor and coil resistance values put the coil voltage at only about 7.8V. This voltage is less than the turn ON threshold for the IDEC relays I located. (I still don't know which specific IDEC relay you are using.) So your relay will not turn ON solely by this voltage. Now if a mechanical vibration happens to bump your relay just the right way while the coil has 7.8V you might get it to turn ON. Once this relay is ON 7.8 volts is enough of a voltage to keep it ON.

Now that I've explained what is happening with this circuit to a depth that probably annoys many people following this thread, you probably want to know how to get this relay to work with this circuit. Well technically you can't. The circuit functions precisely to the laws of electronics. The circuit must be changed in one fashion or another. I've already proposed one method that uses some unused contacts of a relay that granted me a GA. You might consider permanently bypassing or reducing your series resistance. You could also use a lower voltage relay, but the coil resistance will also drop in value so likely you will still have to change your resistor value. But without knowing precisely which relay you have, and how all of the relay's contacts will be used I cannot help you any further.