lossless reverse portection

Could be stressing for a few milliseconds. If in doubt put a chunky

Fred, maybe you should rethink your solution for reverse polarity protection. What about a low threshold P-MOSFET? Source to protected load, Gate thru resistor to ground and drain to input source. Harry

Yes, this is good simple solution I didn't though of. The diode would help the transistor to charge the cap. During the ramp up, as the transistor begins to saturate, Vce may "try" to go lower than 0.7V. But this is only 0.7V to catch up so it is surely a relief for the collector-emitter junction.

Harry ( in the next reply) suggested a MOSFET. I don't know here if a MOSFET can accept a reverse gate-source voltage. I should find a non-diode or non-zener protected gate MOSFET to do so. Otherwise it would be even bette because the bipolar version do its job at the expense of a few milliamps (4.3mA in my case).

Joer

In article , Joer Breton wrote: [....]

Most of the MOSFETs on the market don't have the Zener on the gate. Only having 5V to turn it on is the greaterr problem.

You have to wire the MOSFET in a unexpected way for the circuit to work. The drain connects to the input power and the source goes to the load. When normal power is applied, the body diode initially takes all the current so you have to check the specs on it.

On the PNP circuit, you could make the base drive a little slow to start up using another transistor:

PNP In --------+----------- -------------- Load ! \\ / \\ ----- / ! \\ ! ! !/ c +-----------! NPN ! !\\ e --- ! --- \\ ! / GND \\ ! GND

But will the MOSFET go to saturation? Once the source is up to 5V, the gate won't have anymore voltage.

Joer

In article , Joer Breton wrote: [....]

We are speaking of a P channel MOSFET. You tie its gate to ground.

Body diode ->!-- ! ! Vin ------------+ +---------------- ! ! --------- --------- ! GND

Go it! then, in your previous message, you probably meant the input power is connected to the source of the MOSFET and the load to its drain.

Regards! Joer

"Joer Breton" schrieb im Newsbeitrag news:8a%yg.2$ snipped-for-privacy@wagner.videotron.net...

connected to the source of the MOSFET and the load to its

Hello Joer,

The drain-pin of the PMOS-transistor has to be connected to Vin as Ken already wrote in his previous message.

You can see this too in the applications of the LTC4412.

Best regards, Helmut

My was response was not offered as a "solution for reverse polarity protection" because the OP was not asking for one. The original question pertained to the durability of his PNP low power transistor circuit:"Will the transistor survive in the long run?" That is the only question in the post, everything else was context declaration, he did not ask for any commentary about his general approach, using the PNP was the starting point.

The little PNP + diode ruggedizer is the exact bipolar equivalent to the PMOSFET high side reverse polarity configuration. I assumed the OP knew what he wanted. Now it turns out he didn't, as is typical with USENET. From what I surmise of the situation, the major issues here are live insertion, current drain, and obtaining parts. Why don't you take it from here since you're obviously in need of major improvement of your reading comprehension faculties.

Hey Fred, with all that BS you should have been a lawyer. Harry

Like I said, you have reading comprehension impairment, and a bad memory too, I have posted more than one schematic in the past showing N- and P- MOSFET reverse protection.

No, although that is the normal way to have current flow in a P-MOSFET, you have to flip it around so that the body diode ends up as shown.

Hey Robert, been riding your bike without your hat?

Oi M surry; doent no hau 2 reed; no perfessors hear.

Really weird way to wire the FET! I'll have to dig into this stuff.

In the meantime, about my original post and following suggestions, I added a diode accross the emitter-collector junction. Should be ok. I'll see in the long run. I may gather further informations during that long run.