Reverse voltage protection

Hi

In my search for a low cost reverse voltage protection, I made some experiments with various circuits including bipolar transistor, diode connected in serial mode, crow bar diode.
It came out that the circuit combining a cheap price and a low voltage drop (0V) is a crow bar circuit. I added a resettable fuse (Polyswitch) to protect the batteries.



But, there is a major drawback, although I never had any problem with the 20M (16F677) : in the case of a reverse voltage, the chip is exposed to a negative voltage (-1V) between VDD and VSS.
The Microchip specs is giving a -0.3 as the maximum reverse voltage.

The question is : what about the long time survival of the chips in a regular day to day use where this situation may occur. Any prediction? Any return on experiment?

I thought using a Schottky diode, but it is more expensive and they do have a quite high reverse leakage current.

Technoman

Why not use a low-voltage dropout regulator that has built-in protection against reverse input voltage?

That way your PICAXE should never be exposed to any reverse voltage, and the forward voltage drop can be small AND you get some over-voltage protection.

I'm thinking of doing that for something I'm going to make and send to some non-technical users!

Rgds

Damon

Technoman said:

I thought using a Schottky diode, but it is more expensive and they do have a quite high reverse leakage current.

Click to expand...

You have a power on LED and you're worried about the leakage current of a Schottky diode?? (many orders of magnitude difference)
If using regular batteries (3*AA), then most PICAXEs will survive reverse voltage for breif periods despite getting too hot to hold. They are very tough.
I'm very confident that a polyfuse in series with the supply and a schottky diode crowbar will protect any PICAXE.
Alternatively, put the diode in series and suffer a minimal volt drop. If the rail voltage is important, then an LDO plus series diode plus suitably over-volt supply is your best bet. That will also protect against high voltage up to the limit of the regulator which could be protected by polyfuse + zener.

If you want to go fancy "ideal diodes" from Linear.com - http://cds.linear.com/docs/en/product-selector-card/2PB_idealdiodesf.pdf

Thanks.

I am comforted for reliability in Picaxe.

Used in a school robot where battery-packs (4*AA) can be removed for recharge, but preferably not too frequently, I intended to reduce the power consumption to a minimal value.

Th use of a LDO (ie LM 2940CT5.0) would be elegant, but it has a quiescent current of nearly 15 mA. Per comparison, the low consumption LED is drawing 2 mA... Not mentioning the price...
For the Schottky diode (ie 1N5822), a closer look suggest a lower leakage current for the current drawn but is still 3 times more expensive than a 1N5404.

The PSU is supplying current to the Picaxe and an L293. I am not sure, too, of the capability of this last chip to handle some reverse voltage. No problem occurs on this side too.

Use polarised connectors on the battery packs ?

If you are using rechargeables then do NOT use a reverse crowbar unless you have a polyfuse or similar.
Rechargeable AA cells can deliver 10's of amps which will instantly melt the battery holder and vapourise any PCB tracks.
The LDO regulator that Rev-Ed sell (MCP????) has VERY low quiescent current.

I like srnet's idea. Avoid the problem by good choice of connectors.

There are other LDO regulators that have a much lower quiescent current such as the LM2931 @ 1mA or the LM2936 @ 15uA.

Simple series diode?

For the 20M, 4.5 to 5V is recommended. The minimum voltage is it as in Microchip specs (16F677) as low as 2V? What about the 20M2?

I thought I don't really need a stable supply, but an reverse polarity protection because the battery pack is using a 9V battery clip as connector. Short duration reverse voltage situation while the students are attempting to connect the battery pack are not excluded.
Checking further, the L293D has to be reverse voltage protected at least because of the free-wheel diodes on its outputs.

The board is powered by 4 AA rechargeable batteries. I am using 3V motors. Because of the voltage drop in the L293D, to get the 3V on the motors, I needed a 5V supply.
Any voltage drop (series diode, etc.) will have an impact on the available voltage on motors.

The "ideal diode" as suggested by Premelec would be an elegant solution. Unfortunately, this component is not easily available (Farnell, ...). Also, I try to get standard components from local dealers ; not, of course, an up to date technology.