12V relay voltage drop

[PhillDubya]

Hey guys,

I was just wondering, what is the voltage drop (about) on a typical 12v relay?

I was doing some calculations for finding a resistor value on the base of a transistor used to switch the relay, and couldn't remember if its 12v dropped across the coils of a relay, or something like 2v or so???

Thanks

 

[Mikebits]

It depends on the coil resistance spec. You should look for that in the data sheet.

 

[IT_Guru]

Yea... r use an Ohmmeter across the winding of the relay and measure it ;-)

 

[PhillDubya]

Well, this is a design problem, and as of right now, the only specs. are that it HAS TO have 250mA operating coil current, and it is a 12Vdc relay. Based on those numbers, that puts the coil resistance at 48 Ohms. Therefore would it be safe to assume that my 12v dc relay, at 250mA, will in fact drop a full 12v across the coil when active?

The reason I ask, is because, if I have a 13v dc power supply to run the relay, and it is a 12v relay @ 250mA, then 13v - 12v = 1v. This would be my base to collector voltage on the NPN transistor used to switch the relay correct?


Or is the entire voltage applied to the relay, regardless, going to drop on the relay when active? (I.E. if I am using a 13vdc supply, the entire 13v will be dropped across it?)

Wow, I think I am making this WAAYYYY over complicated.

Thanks for the quick response Mike.







GO Texas Tech #2 NCAA!!!

 

[Mikebits]

What you need to do is design your transistor circuit so it draws 250ma. This is the current specification of the relay as you have stated. So knowing the relay turn on current, design your transistor circuit with that spec in mind. If you can set your transistor to draw 1/4 amp then you should be good to go

Tale a look here.
**broken link removed**

 

[PhillDubya]

I just used 48Ohms from 12V and 250mA, and the numbers came out fine, based on what you stated. I just calculated the base resistor current from Ib = Ic / hFE, with Ic = 250mA.

Thanks again for the help.

 

[Hero999]

A general rule of thumb is to set the base current to about 1/10 of the colltector current. In this case if your base drive circuit is 12V then use a 470R base resistor, or 180R if it's 5V.