R Value for Base of NPN 2N3904

[Turbo Boss]

Hello Ron, thank you for the Mosfet recomendation. Just curious, do you know if there is a ECG or NTE replacement for the Zetex ZVN4424A ?

I live in Central America, and usually ECG and NTE are the brands I know the electronic shop carry.

I also was looking on my 1996 ECG Master Replacemet Guide and dont find it :roll:

Or maybe you can recomend me a ECG MOSFET that can do th same job.

THX for your time and help

 

[mattg2k4]

Ron, could you tell me what the purpose of R1 is? I've never used a resistor on a gate of a mosfet.

Also, I think you should use a pull-down resistor of 10k or so on the gate.

 

[Roff]

Ron, could you tell me what the purpose of R1 is? I've never used a resistor on a gate of a mosfet.

Also, I think you should use a pull-down resistor of 10k or so on the gate.

The resistor may not be necessary, but I put it in because I have no knowledge of how long the gate drive trace (wire) is in Turbo Boss's circuit. See this app note from Fairchild. I used a larger value resistor than the 4R7 that Fairchild recommends, because I picked a small MOSFET with much lower gate capacitance than the ones they are dealing with.
A pulldown resistor serves no purpose if the gate is always connected to the driver. If there is a connector on series, then you're right, a pulldown on the same circuit board as the MOSFET should be used..

 

[Roff]

Hello Ron, thank you for the Mosfet recomendation. Just curious, do you know if there is a ECG or NTE replacement for the Zetex ZVN4424A ?

I live in Central America, and usually ECG and NTE are the brands I know the electronic shop carry.

I also was looking on my 1996 ECG Master Replacemet Guide and dont find it :roll:

Or maybe you can recomend me a ECG MOSFET that can do th same job.

THX for your time and help

Boss, I think I searched NTE with no success. What you need to look for is an N-channel MOSFET with at least 20v BVdss and BVgss (drain-source and gate-source breakdown voltages) of at least 20v (as a safety margin), and Rds (drain-source resistance) of 5 ohms or less at 3v or less Vgs. I would personally try to avoid large MOSFETs with Rds<0.5 ohms, because they are generally in larger packages, and your driver resistance may be too high to drive their gate capacitance with acceptable risetimes. You may be able to use one of these higher power MOSFETs if your driver can handle it.

 

[mechie]

Just passing

I note no-one has suggested an Emitter Follower yet,
the FET may be the simpler answer but just to give options ...
The 2N3904 has a DC gain of about 60 at 50mA IC
so 3mA should be plenty of base current (taken from 12v rail)
A BC107 has a hfe of 200, for 3v to saturate it (1.6v across its base resistor) I calculate 220k is ample.

Any thoughts?

 

[Roff]

Mechie, that's a good idea. My only problem is with your base resistor value.
Ib=1.6v/220k=7.3ua.
With beta=200, we only get Ie of about 1.5ma, which is half the desired base drive for the 2N3904. Minimum beta for BC107 is 110 at Vce=5v. It will be less for Vce<1v.
Why not just use another 2N3904 for the emitter follower, assume you need Ic/Ib=20 minimum (for guaranteed saturation with 3v drive), and use a 10k base resistor? Then, with 3v drive, we'll get 150ua base drive in the emitter follower, and 3ma base drive in the relay driver. With 12v drive, we'll get about 1ma of base current in the emitter follower, and about 4ma of base current in the relay driver.
It's not good engineering practice to design to typical specs. If you're building a one-off, you might get away with it, but it will be disaster for a production design (which this may be).