Low Voltage Cut Out

[WillyB]

Well would not a simple solution be to simply play with the pulldown resistor values, choosing a value that will no longer provide enough current to fire the reg at let's say ~6.5V yet be fine at 6.5+V?

I guess I don't understand what is needed, or how to figure it without trial and error.

I thought the datasheet calls for 1 mA. If I use Mamu's 10K with Ohm's Law at 6.5V I get .65 mA.

What an I missing here?

What say ye?

 

[Rocketman]

The specs define a known on state and a known off state, and maximum current through the control pin to a ground. In between the on voltage and off voltage is an 'un-defined' logic state. Just like old 74XX gates, they switch in this zone, but maybe not 'all of the time'.

With the OKR, does the switching zone shift with temperature? If you want to operate 'within specs' the voltages for on and off should be followed. Other voltages MAY work.

The 'avoid this voltage for reliable switching' concept should be considered when designing a control circuit.

 

[skipdashu]

Can a 6.2v zener diode be considered functionally open at 6.1v and functionally a short at 6.3v?

In the schematic I posted earlier there is no dropping resistor between the zener and the control pin. Should there be to limit current on the control pin?

Thanx, Skip

 

[Rocketman]

I think the spec sheet (haven't looked at it for a while) gives a maximum voltage for the control pin.
The zener has a sharp turn on conduction curve. Very high resistance (not open) below the zener point, and a dynamic resistance above that point. Find a representative conduction curve via Google.
The specs you would be interested in would be:
Zener voltage, at 1ma, zener voltage at rated power level, the power level (like 0.5 watts @ rated forward voltage).

Reverse voltage for the zener diode should be higher than the expected maximum voltage (like 20 volts reverse rated).

The current/ power consumed by the zener would depend on the pull down resistor on the OKR control pin.
The resistor should be such that the OFF voltage is below the 0.4 volts in the OKR specs. The on voltage should rise to over the spec sheet ON voltage.

There will be a compromise because of the 'undefined zone of operation' at the control pin. 6.4 volts at the positive input, 6 volt zener, 0.4 volts at the control pin (with a pull down resistor) seems like it would cut off. 6.6 volts at the positive and it MAY cutoff, 6.6 volts and it MAY cut off. 7 volts and it will most likely stay on, 7.4 volts at the positive and it will stay on for sure.


Switching in this 'undefined zone' may be different when going from an ON to an OFF state, than when going from an OFF to an ON state.
I don't think we want a 'works most of the time' situation.

 

[skipdashu]

Remote On/Off Control (Note 5)
Positive Logic
[Standard version]
ON = +1.5 V. to +Vin max. or open pin
OFF = –0.3 to +0.4 V. max. or ground pin
1 mA

So it sounds like I should start with something like a 6.2v zener and play with the resistor(R2) that ties the control pin to ground in THIS.

 

[skipdashu]

I posted this in the EZ OKR-T thread also but if any of ya'll could tell me what to check I'd 'preciate it greatly. I'm currently stuck as everything seems to be right... it just don't work right!

OKR w/ Zener

The 5.6v zener does seem to be about the right value based on the voltages measured though.

UPDATE: Disregard, current discussion in the EZ OKR-T thread.

Got the thing working and running down a pair of IMR 14500s to see what the actual cut off is using a 5.6v zener.

 

[skipdashu]

I have most of the OKR VV working on a bread board and ...

Using a 5.6v zener and a 8K dropping resister to ground off of pin 1 -
batts - AW IMR 14500
carto - 1.25ohm dual coil
adjusted to 4v output w/o load

Most of this is on the workbench on a bread board so you can visualize me bending over, my face in a blue LED sticking up and vaping this thing... LOL.

Granted this is a heavy current draw carto but the zener is going to work off of the under load voltage which got down to just under 5.8v. The unloaded battery voltage was 6.3~6.4v when it quit.

[insert] I would expect the gap between loaded / unloaded to widen using regular Li-Ions and narrow using higher ohm atty/carto. Since the zener is really working with loaded voltages this will slightly change what your done battery voltages is unloaded.[insert off]

Right before it quit I had .44v on pin 1 with switch open and about 1.04v with the switch closed and load on it. So it seems to me what the zener does is DROP 5.5v (from say 6.5 in leaving 1v on pin 1. It had been up over +2v when the batteries were fresh. I don't really think this is quite it and I don't think it's this linear. I'll have to go read the zener wiki.

It didn't shut off quite like I expected. It DID quit vaping but the LED would still light up very weakly. The reg output voltage didn't drop to zero but dropped to something well under 2v (like 1.13v). I was watching the battery voltage at the time and instead of dropping from 6.35v to 5.83v under load all the sudden it quit vaping and the voltage drop on the batteries was only about .2v. So it essentially did shut down the regulator and it did leave the batts at about 3.15~3.2v each.


Mouser electronics does have this

Mouser #: 771-NZX5V6B,133
that should allow them to drop another .10v to .15v getting them down to right on 3v.

and this

Mouser #: 78-TZX5V6E
that would should allow you to cut them off .15v higher or at about 3.3v if that's the target.

I'm gonna swap out my 8K dropping resistor today with a 10K so this might change just a little bit.

Which BTW, brings this up... This is the T/6 and the values for this dropping resistor seem to be drastically lower than with the values reported earlier for the T/3 (10K vs 100K). It would not shut off with 12K, does shut off with 8K, gonna try the 10K Mamu was using today.

 

[Rocketman]

Cut off/turn on sharpness may improve with a little more current through the zener. Possibly a little lower pull down resistor, 5K or 6K would put the zener into the sharper area of the conduction curve. Too low a current and the zener may still be in the 'knee' zone with a small amount of current flowing below the zener knee voltage. Look at the curve for your zener.
You will still probably have cut off at loaded voltage, then the cells recovering a little, turn on again, but that would be OK I guess.

 

[skipdashu]

Cut off/turn on sharpness may improve with a little more current through the zener. Possibly a little lower pull down resistor, 5K or 6K would put the zener into the sharper area of the conduction curve. Too low a current and the zener may still be in the 'knee' zone with a small amount of current flowing below the zener knee voltage. Look at the curve for your zener.
You will still probably have cut off at loaded voltage, then the cells recovering a little, turn on again, but that would be OK I guess.

Well then I'll put two of the 10Ks I just picked up in parallel and see what that does vs the 10K by itself vs what I reported above with the hacked 8K.

 

[skipdashu]

10K works fine. Retested cut off and it cut off at was about the same to .1v lower (~6.2v unloaded, 5.69v loaded).

LED on the output side still fires off even though it will not even light up my voltage reader screwed into the 510 connection with a carty also screwed in. Unscrew the carto and it reads the set voltage again (4.1v). Either need to also zener it or up the value of the dropping resistor to get it to cut-off about the same time. Adding a little diode I had laying there in series with the LED and dropping resistor was 'nuff to shut it when the regulator shut off.

 

[skipdashu]

I moved my LED over to the trigger/control pin circuit (per mamu design) with another 5.6v zener trying to get the LED to turn off when the regulator turned off but it didn't pan out for me and I didn't have any lower voltage zeners around (or at Rat Shack). I ended up moving the LED and dropping resistor back over to the output side and used a 1K dropping resistor instead of the 220~470 ones I'd been using. Then I tried a couple different LEDs. My big (5mm) white ones I took out of a broken flashlight work the best and since this will go on the inside of a translucent blue box it'll be fine. By the time the zener on the control circuit cuts the regulator off (there's still ~0.95v on the output, don't know why yet? Willy?) which makes the LED so dim it you have to really look at it dead on to see that's even on. Good 'nuff.

Were it stands now.

Off to add the meter and meter switch.

 

[skipdashu]

Done (see HERE) and the voltage cut out works well. Vapes at 6.9v and the high end of 6.8v. Doesn't at 6.7v.

Thanx Ya'll.