J
John O'Flaherty
- Jan 1, 1970
- 0
Everything except the pinout, which could have been improved had pins 3
& 4 been swapped.
As it is now, you could put it in backwards without causing any
problem.
Everything except the pinout, which could have been improved had pins 3
& 4 been swapped.
Yes, I have.. and still..
John said:Well, I have a 3.3 volt cpu and 5-volt relays here! I'm using some
cute little Fujitsu dpdt telecom relays. They pull in and drop out in
under a millisecond.
Okay, say I have 5V on the base, and 4.4V across the relay coil, and
(say) 10mA is flowing out of the emitter. Steady state. Now I suddenly
drop the base voltage to 0V. Current continues to flow out the
emitter, but the emitter voltage drops to -0.6V (because of coil
inductance). It starts out at 10mA or so, and drops towards zero from
there.
Yeah, mechanical relays are pretty darn good even for currents waaay below
spec. I've grown fond of the Pansonic TN2 series. Which ones are you using?
robert
On Tue, 13 Nov 2007 22:20:07 -0500, Spehro Pefhany
....So long as the base driver supports positive current in that
quadrant. Digital or overdriven linear parts are usually cut off,
making the only source of base current the parasitic diodes to
substrate.
......You might measure a larger negative voltage on the emitter than
is anticipated here.
RL
As it is now, you could put it in backwards without causing any
problem.
Yes, that's the upside, but PCB layout could have been simpler.
Spehro said:Okay, say I have 5V on the base, and 4.4V across the relay coil, and
(say) 10mA is flowing out of the emitter. Steady state. Now I suddenly
drop the base voltage to 0V. Current continues to flow out the
emitter, but the emitter voltage drops to -0.6V (because of coil
inductance). It starts out at 10mA or so, and drops towards zero from
there.
Best regards,
Spehro Pefhany
So, I'll assume after your description that you are pulling the base
low to common. That being the case, with the return potential from the
coil and expecting the emitter to handle that is risky.
I've seen circuits like that many times where it simply destroyed the
transistor gain but past the simple diode test.
Maybe.John said:Why? It's still acting like a normal emitter follower.
What would kill the gain? Nothing's zenering here.
Speff's circuit is fine, if you don't mind losing 0.6 volts of coil
drive. The relay is off if the port is hi-Z before being configured,
it drives fine, it clamps the flyback fine.
John
Maybe.
I'll stick with experience and what's obvious to me. Maybe it's
flawed how ever, it's been working for me so far.
look at these articles. They have Emitter relay driver examples.
You'll notice they insist on diodes.
I wonder why.
look at these articles. They have Emitter relay driver examples.
You'll notice they insist on diodes.
I wonder why.
http://www.uoguelph.ca/~antoon/tutorial/xtor/xtor3/xtor3.html
Because they're idiots. Figs 4 and 5 are dangerous nonsense, for a
number of reasons.
John