Darlingotm or Mosfet?

[seyed]

Hi
I want to drive a gigvac relay with rely current 75 m amp from DSP output pin . I am confused between two options.
The first option is using a mosfet to drive it. But the problem is that the DSP output unlike to pic micro is 3.3 v which is not enough to directly drive the mosfet. I can add another stage using bjt to drive the mosfet. I do not feel good adding a new stage because if I am adding one more bjt to drive the mosfet I need to add resistive division to supply the bjt with 12 volts (actually I do not have 12 volt line on my board and the gigavac relay works with 24 volts) .
I started looking for very low Vgs(th) power mosfet then I may be able to use only mosfet being drived directly form 3.3 volt of DSP pin . Something like IRL510SPBF-ND which has Vgs(max)=2 volt. But I am not sure if 3.3. is high enough to perform well on the mosfet even with 2 volt Vgs(th).


So what do you think about using a darlington to drive the relay without any need for mosfet and another bjt. I got he feeling that mosfet are much more popular than darlingon for drive. i know they are more lossy. But is it the only downside or there are more disadvantages that I am not aware of ?


Any comment?

 

[crutschow]

Darlingtons are slower than MOSFETs and require a small amount of base current. In your application they should work fine.

 

[electronicsbasecom]

What is the frequency that you will turn on/off relay? If not to high (more than 250MHz actually), I suggest you digital transistor such as DTC114YKA. You can see that high logic level iz minimum 1.4V and that is good for your application. You can notice that digital transistor has embedded resistor so you save place and money.
Remember to put freewheel diode since ralay is inductive load to transistor.

 

[Grossel]

Hi.

Be aware that Vbc for a NPN/NPN darlington pair (as seen as one transistor) is around 0,7V even if fully saturated. But you don't mention the relay's source voltage.