4n35 collector wired to TIP31c base?

[kheston]

Noob question here:

I found a circuit while Googling that has me connecting a 4n35 collector to a TIP31c base. If I'm reading the data sheets correctly, 4n35 Ic has a max rating of 100mA and TIP31c IB can draw up to 1 amp. There should be a 2N2222 or something between, correct?

The fact that I haven't seen smoke yet means I'm lucky, right?

[haklesup]

just because the TIP31 can go to 1A doesn't mean it is. And the 4n35 won't exactly fuse open at 101mA. So you have some room for things to work out. As long as the TIP31 is biased for lower current (<150mA) and the other is not too severely over driven, it will last a while. Marginal over-current on the 4n35 may shorten its lifetime but not necessarily all the way to minutes or seconds.

[kheston]

I'm using four of them to drive a stepper.

The stepper coils are 2.5 ohms. Since my PS is 13.5v, it appears that I might be exceeding Ic (5A). Does that mean (theoretically) that the base of the TIP31 is drawing the full 1A?

[Bob Scott]

The stepper coils are 2.5 ohms. Since my PS is 13.5v, it appears that I might be exceeding Ic (5A). Does that mean (theoretically) that the base of the TIP31 is drawing the full 1A?

This does not right. Are you sure of the coil voltage rating? I have a couple of 12V steppers and the windings have 40 Ohms, center tapped at 2 X 20 ohms. If it is rated at a lower voltage than 12, don't run it at 13.5V

However if they do indeed draw 5 Amps, you should use the TIP41. They are rated at 6A. The 31 is only rated at 3A. The gain of the TIP41 is as low as 15, the TIP31 as low as 10. Limit the base current to 0.5A with a resistor in series with the base.
It looks like you need another stage or a driver capable of delivering 500mA to the power transistor's base. See the Jameco catalog. There are a few darlington transistors in the TIPXX lineup that you might use

Recheck the motor specs first.

[sofaspud]

There may have been some "behind-the-scenes" current-limiting going on. The 4N35 may be unable to provide sufficient
drive to the TIP31 so the transistor was dissipating more than its share of power. I second Bob's advice: check the
motor specs to find what you need from the TIP31s. Divide that Ic by the transistor gain to find what you need to provide
for Ib. Odds are you'll need a base drive stage. Or moving up to TIP120 Darlington, etc. may be the most effective fix.

[dyarker]

kheston,

What is link to that circuit?

Collector of 4N35 transistor to base of TIP-31 does not make any sense because both those transistors are NPN. For anything at all to happen there would also have to be a resistor from that junction to positive supply. And that would make for unusual logic; the LED in the 4N35 would have to be ON in order to turn OFF the TIP-31. A good way to burn up parts.

On the other hand, if the power transistor is TIP-32, a PNP, it does make sense. Positive supply to TIP-32 emitter, collector to motor, other motor lead to negative supply (often, but not always common). TIP base to 4N35 collector, 4N35 emitter to negative supply. When LED in 4N35 is ON, 4N35 transistor is on. That allows TIP base current, TIP turns ON, and current flows through motor. (I would expect a series resistor between 4N35 collector and TIP base to limit current.(But enough current saturate TIP)) This is "high side driver".

If this is H-bridge for bi-directional motor, then there will be two high side PNPs and two low side NPNs.

Looking at schematic will clear up any guess work.

Cheers,

[kheston]

Dale,

Here's what I'm up to:

Schematic

I'm pretty sure an ON led is what starts current flowing, no?

[dyarker]

Now it makes sense ... NOT 4N35 collector; those are emitters connected to TIP-31 bases. The configuration is darlinton.

Looks to me like transistors in 4N35s will burn out. I put a series resistor between each 4N35 collector and positive supply.

Cheers,