Over-voltage protection

[richard]

the red led will never fly with base of tr shorted to gnd

try


red should light at 5.5v

[Demon]

the red led will never fly with base of tr shorted to gnd

try
Attachment 9386

red should light at 5.5v

But why reduce Zener to 4.7V? Out of all the PICs I'm looking at, the one with the lowest max VDD was 6.0V; hence why I chose 5.6V Zener as "fuse".

Is this because of influence of 1K resistor below it?

And how did you arrive at 4.7V? Math? or a spec in some doc?

In this particular case, I was doing this for the 18F14K22 which has absolute max VDD rated at 6.0V.


EDIT: I ask how, because I will have the same over-voltage circuit at other voltage levels(for other purposes).


EDIT SOME MORE: These are the specs for the 1N5232BTR that I was planning on using.

[richard]

But why reduce Zener to 4.7V?

i saw

the 5V model supplied with the unit

and assumed an over 5v alarm was indicated

Out of all the PICs I'm looking at, the one with the lowest max VDD was 6.0V; hence why I chose 5.6V Zener as "fuse".

if you use a 5.6v zenner in my cct then the red alarm led will be lit at 6.5 v

if you want the zenner to be a crowbar and blow a fuse, there are better ways.

https://www.ebay.com.au/itm/203643422252

i use these things , 3-15 v in adj to 3.3 or 5v out or whatever @up to 5w , cheap with no problems

[Demon]

... if you want the zenner to be a crowbar and blow a fuse, there are better ways.

I'm seeing this design a lot.

https://theorycircuit.com/crowbar-ov...ection-module/

It pops that fuse and puts an end to the shenanigans until the user corrects the situation.



I'd like to stay away from 3rd-party components as much as possible, and come up with a design as simple as possible.

Robert

[Demon]

... if you want the zenner to be a crowbar and blow a fuse, there are better ways.

How about this modification to that link I posted above?

- Green LED remains ON while voltage remains under 5.6V.

- Red LED turns ON when voltage reaches 5.6V, and Green LED turns OFF.

- SCR shorts out Fuse and protects everything to the right of this circuit.



This way I get a light on the unit that gives an indication as to what went wrong.

I absolutely hate products that DIE, and you're left clueless what went wrong.

Robert


EDIT: I'm just not sure about the need for that 1N4007 for my application. It will cost me a voltage drop to add that in.

[richard]

- Green LED remains ON while voltage remains under 5.6V.

Green LED remains ON , it will not extinguish

- Red LED turns ON when voltage reaches 5.6V, and Green LED turns OFF.

Red LED turns ON when voltage reaches 5.6V + at least 0.6 v , so more like
when voltage exceeds 6.3v Red LED turns ON and Green LED remains on.

- SCR shorts out Fuse and protects everything to the right of this circuit.

in practice :- just after every semiconductor device is destroyed SCR shorts out Fuse

[Demon]

Green LED is driven by a PNP. Won't that shut off once the Zener breaks?

[richard]

no, there is no mechanism there to make vbe for the pnp tr ever be below the 0.6 v required to switch the tr on

[Demon]

... in practice :- just after every semiconductor device is destroyed SCR shorts out Fuse

What destroys ICs downstream after the SCR shorts to VSS?

Is it a current surge?

Or reverse voltage?


Robert



EDIT: Added NTC for current surge protection at output, and extra TVS up front (we get a lot of power surges here, hydro lines are not the most stable with so many trees).

EDIT SOME MORE: In keeping with Tim The Toolman's philosophy of "cause we can", I added a Varistor on output.

[richard]

Crowbar protection can be ineffective due to the time taken for it to activate. Everything of value is damaged in the milliseconds before comes in.

[Demon]

... if you want the zenner to be a crowbar and blow a fuse, there are better ways.

https://www.ebay.com.au/itm/203643422252

Ok, so I found Alice on ebay Canada. Turns out to be $1.79 each when I buy 10.
https://www.ebay.ca/itm/313748820419


I guess nothing stops me from adding a zener/crowbar/fuse setup in front to make me feel like I'm doing something.


I can even afford a one-way diode in case someone uses a wall-adapter with a negative center (I think all the ones I have are positive center).


I hope you get a freebie for making her a sale.

Robert

[Demon]

So I started looking closer at Alice, she's using a B6289C IC; I can't even find a datasheet for that.

But I did find a relatively similar circuit using a LM2596 for $.99:
https://www.ebay.ca/itm/333463165949


Is this as good as Alice?


I can even order a batch from Amazon and avoid import/duty fees, $2 each:
https://www.amazon.ca/Yizhet-Convert...dp/B0B5GQTS64/


Robert


EDIT: just the LM2596 costs over $4 here:

https://www.digikey.ca/en/products/d...TVADJG/2003293

[richard]

i very much doubt "Alice" is an individual.
the sm ps units i linked are good, o/p v adjustment is fiddly. i cannot vouch for every-thing else
there are multitudes of cheap sm power modules on ebay,amazon,aliexpreess,....
i pick ones to suit project re i/p o/p power.
if the price is too good to be true , it probably is.
i use R-78E-0.5 for pic mcu mission critical jobs , not cheap but super reliable and tough

[richard]

Is this as good as Alice?

its a different animal , the ones i linked are buck boost in 3 to 15 , out 1 to 15

ie 3v-15v in 5v out those others are buck only ie 7.2v-35v in 5v out

[Demon]

its a different animal , ... those others are buck only ie 7.2v-35v in 5v out

That's ok. I haven't used weaker wall-adapters than required at output yet. I'm always using exact ratings, like a 5V wall-adapter for a 5V circuit, or a stronger wall-adapter and using voltage regulators to bring it down.

I ordered 10 and I'll see what the railure rate is. Some comments say they suck, but you never know how they treated the units.

Found the Alice design is on Amazon (same IC), but they're advertising it as boost only:
https://www.amazon.com/Eiechip-Volta.../dp/B07RNBJK5F