Can 9 Volts Trigger a 12V Relay?

is 9 volts enough to activate a 12 volt relay?

thanks

It can be, but it depends on the relay.
Typical 12V automotive relays have a "pull in" voltage around 8V and a drop out voltage somewhat lower. Those voltages may vary with temperature.

Be aware that for power circuits, a lower coil voltage means less contact pressure which can reduce contact resistance (less current, more heat etc).
And in any circuit (signalling or power), the contacts will bounce easier, hence perhaps risky vehicles etc.


It may be easier to ground-switch the relay (#85) whose coil (#86) is powered from a 12V source (eg, from #30).
Otherwise a transistor could be added to switch a suitable voltage.

If it does pull it will also pull more current, so make sure you have a decent power source.

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Kevin Pierson

Bosch/Tycho automotive "cube" types pull from 7.5 volts but as the man says, more power needed across the coil.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

so it may or may not, depends on the specific relay. I was just kicking arround ideas with various devices and how to incorporate them into my car alarm system so if I actually have one good enough to try I will test on different relays.

Oldspark, could you explain more about this ground switching and is it still a possibility if the 9v device is on a seperate sytem without common ground to the 12v relay I might try to operate?


thanks guys

Unless ohm's law has been deemed a theory it will draw less current with lower voltage.  It will latch and hold easily with 9 volts.  The only problem with using only 9 volts is the fact that it may not pull the contacts together as tightly as it would with 12 volts.  A bosch/tyco relay may only handle 30 amps instead of the normal 40 amps.  It may or may not be affected by the lower voltage, but if there is any drawback, that would be the only one.

i am an idiot wrote:

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Unless ohm's law has been deemed a theory it will draw less current with lower voltage. It will latch and hold easily with 9 volts. The only problem with using only 9 volts is the fact that it may not pull the contacts together as tightly as it would with 12 volts. A bosch/tyco relay may only handle 30 amps instead of the normal 40 amps. It may or may not be affected by the lower voltage, but if there is any drawback, that would be the only one.

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thanks IAAI, this seems to suggest that increased wear on the contact points is a possibility also since less contact pressure might increase initial arc.. probably not enough to be a factor with limited uses.

I was going through older threads on this section of the forum and found a great post you made here :
https://www.the12volt.com/installbay/forum_posts.asp~TID~128698~PN~2~TPN~2#638830

i am an idiot wrote:

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Actually you can use a 12 volt relay. LED positive wire to a 1,000 ohm resistor. Other end of that resistor to the base of a TIP3055 transistor. Radio Shack part number 276-2020 collector of the transistor to terminal 85 of the relay. Emitter of the transistor is connected to ground.    You must insulate the tab of the transisitor somehow. You should not need a heatsink. But you should monitor it closely at first to see what kind of heat buildup you get.

Left leg of transistor = base. Center leg = collector. right leg = emitter.

apply power to terminal 86 of the relay.

If you already purchased the 5 volt relay wire it up as follows. LED positive to a 100 ohm resistor. Other side of the resistor to the base of the transistor. Collector of the transistor to 12v. Emitter of the transistor to the coil of the 5 volt relay. Ground the other coil connection of the relay.

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this helps me tremendously! because of You, I now have more options to explore if/when less than 9v is all that is available. Thanks

Craig, lower current = less voltage? Really I thought it was the other way round.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

i am an idiot wrote:

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Unless ohm's law has been deemed a theory it will draw less current with lower voltage.

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So you're sure a solenoid behaves like a linear impedance?


two12 - ground switching is a very common output for devices whether CPUs or alarms etc, and certainly anything that interfaces to other voltages.

Because it "closes a connection" (switch) to a common GND, it doesn't matter what voltage the other end of the load or relay coil is, EXCEPT that it must be able to withstand the load's voltage.
IE, a 9V GND switching or Open Collector might ground the +12V relay coil, but when the output opens, the relay coil pulls it up to its +12V.
If that output "switch" is a transistor and the other supply is only 12V, 24V, 48V etc then it's usually no problem because they can handle many tens of volts, ie, a transistor's max Vce (Collector to Emitter voltage) is typically 50V, 100V etc.
[ FYI - a small current into a transistor's Base allows a larger current to flow from its Collector to its Emitter. For open collector outputs, the Emitter is to GND and the Collector only connects to the external load. If "on", the load - or coil in this case - is connected to GND. If "off", the Collector is "open", ie, floating, so the load is NOT grounded and is off. Similarly for MOSFETs, though their terminals have different names - ie, Gate, Source & Drain. ]


Some circuits and alarms may have selectable or programmable output levels, ie, +9V for on or GND for on.

oldspark wrote:

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i am an idiot wrote:

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Unless ohm's law has been deemed a theory it will draw less current with lower voltage.

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So you're sure a solenoid behaves like a linear impedance?


two12 - ground switching is a very common output for devices whether CPUs or alarms etc, and certainly anything that interfaces to other voltages.

Because it "closes a connection" (switch) to a common GND, it doesn't matter what voltage the other end of the load or relay coil is, EXCEPT that it must be able to withstand the load's voltage.
IE, a 9V GND switching or Open Collector might ground the +12V relay coil, but when the output opens, the relay coil pulls it up to its +12V.
If that output "switch" is a transistor and the other supply is only 12V, 24V, 48V etc then it's usually no problem because they can handle many tens of volts, ie, a transistor's max Vce (Collector to Emitter voltage) is typically 50V, 100V etc.
[ FYI - a small current into a transistor's Base allows a larger current to flow from its Collector to its Emitter. For open collector outputs, the Emitter is to GND and the Collector only connects to the external load. If "on", the load - or coil in this case - is connected to GND. If "off", the Collector is "open", ie, floating, so the load is NOT grounded and is off. Similarly for MOSFETs, though their terminals have different names - ie, Gate, Source & Drain. ]


Some circuits and alarms may have selectable or programmable output levels, ie, +9V for on or GND for on.

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I apreciate your explination but its very confusing for me Oldspark, do you know of an example diagram or photo of such a circuit available to look at?

thanks