Transistor Ic... Derate for long life?

[James W]

I'm using a 2N3906 to drive an automotive relay. The relay coil current
is ~150mA, the 3906 is rated at 200mA continuous. So, I'm inside the
spec. However, I want to install this circuit and forget it, so is
running for hours at a time at 75% of max overdoing it? Should I opt for
something like a 2N2907A (rated 600mA), or will the 3906 survive

- jim

p.s. YES, I have a diode to handle the cemf when the transistor opens

 

[John Popelish]

James W wrote:

I'm using a 2N3906 to drive an automotive relay. The relay coil current
is ~150mA, the 3906 is rated at 200mA continuous. So, I'm inside the
spec. However, I want to install this circuit and forget it, so is
running for hours at a time at 75% of max overdoing it? Should I opt for
something like a 2N2907A (rated 600mA), or will the 3906 survive

- jim

p.s. YES, I have a diode to handle the cemf when the transistor opens

Just based on current gain (and thus, base drive requirements) I
almost never select a transistor such that it is carrying more than
half of its rated current in a switch application. You will get a
lower voltage drop (cooler operation and higher relay coil voltage)
and might be able to drive with less base current with a bigger
device.

I would probably use nothing smaller than a 2N4403:
http://www.fairchildsemi.com/ds/2N/2N4403.pdf

This is rated at 600 ma, but look at the current gain curve and
saturation voltage specs on page 3. It is starting to grunt at 150
ma.

--
John Popelish

 

[CBarn24050]

However, I want to install this circuit and forget it, so is
running for hours at a time at 75% of max overdoing it? Should I opt for
something like a 2N2907A (rated 600mA), or will the 3906 survive

Once you get over 70% of Ic max the gain starts falling for a typical
transistor. So long as you have enough base drive to keep the transistor
saturated you will be ok.