reset circuit

Did you allow for the voltage drop across the transistor in your Reset Supervisor selection?

Just asking...

John :-#)#

Sure. In normal operation it's inverted-state saturated, so the MAX part sees all the +5.

If the +12 dips to about +10, the transistor base is +5, and the emitter is 4.4, and the MAX says reset.

That transistor has an insane beta and a pretty hunky inverted beta.

A saturation voltage drop of maximum 0.25V CE for the BCX70, BE saturation is max. 0.85V...

Assume a lightly loaded emitter follower with +5 on the collector. Drive the base from some variable supply Vb. Ramp Vb up from zero.

Initially, the emitter will track Vb with about 0.6 volts drop.

It gets interesting when Vb exceeds +5. The emitter keeps following the base until the B-C junction forward biases, around +5.6. Then the transistor saturates in inverse mode. As you tease Vb, the C-E saturation voltage can be made exactly zero, or you can make the emitter go above +5 with a little more base drive. Backwards saturation.

I made a bunch of 16-bit DACS that way once, all discrete parts.

The Fairchild BCX70 has beta about 500 and inverse beta about 16, both measured at 2 mA. I like that part, but AoE rates it dead worst for Rbb at 760 ohms. Hurt my feelings.

Thanks for the explanation, I've never been that great with transistors

- too much magic smoke! Too easily released...

I'm better with tubes, EM, and 8-bit vintage TTL for some reason.

So complain to Winfield!

John :-#)#

I started with tubes. My first job interview, I told the guy that I preferred tubes because transistors were too easy to blow up. He didn't hire me. The second guy laughed and hired me.

I got tubes free from old TVs, or dirt cheap from Fair Radio Sales, surplus. Silicon Valley use to be full of cool surplus stores, but the real estate got too valuable, and ebay took over.

The beauty of Spice is that you don't really have to understand things, you only have to make them work. I plan to Spice that powerup reset thing later this morning for the guy that needs it on his board. I hope LT Spice models this upside-down saturation thing right.

Here it is. Seems to work.

Don't forget that simulated voltage sources will sink current as well as source, real-world ones often don't.

You will get significant current passing from the 12v to the 5V through the BC junction of the transistor and holding up the 5V supply when it is not being supplied with power.

The 5v supply comes from an LDO from +12.

The path from +12 through the BC junction to +5 is pretty wussy... +12 divided down 2:1 with 500 ohms impedance.

We have dump resistors on the +12 and +5 supplies to discharge them too. Ditto the 200 volt supplies.

We're not sure what our box does. The customers won't say, except that they really don't want any untriggered outputs, ever.