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Reverse Polarity and Smoke? — Parallax Forums

Reverse Polarity and Smoke?

SunflowerSunflower Posts: 48
edited 2005-05-01 02:50 in BASIC Stamp
Reverse Polarity and Smoke?

What would happen to the Stamp and other chips if power were accidentally connected in reverse? To prevent this possibility would it be appropriate to hook up a diode across 12 volt power supply to blow a fuse instead of smoking the hardware? Would a 2 amp diode blow a three amp fuse?

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2005-04-30 21:56
    1. Approach number one -- never, never, never hook the power up backwards. I think this is a disaster scenario.

    2. Fuses do take a finite time to heat up and blow. And it is current through them that does this. I don't think the BS2 would be very tolerant of the extra current during the time needed to blow the fuse.

    3. There are many alternate solutions, each of which require knowing how much current a reverse-plugged BS2 would take before it dies. I don't think you really want to know that.

    4. It sounds like you want to put a normally reverse-biased diode from Vin to Vss, so that if some lunatic (or newbie) should succeed in reversing the voltages, the diode would conduct the current needed to blow the fuse. Clever. While this was going on, the diode would clamp the voltage to 0.6 volts. Also clever, I think. I'm not sure how useful it is, though.
  • steve_bsteve_b Posts: 1,563
    edited 2005-04-30 22:03
    wouldn't a bridge diode give polarity protection....of course there's the drop across 2 diodes at this point!

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    Steve
    http://ca.geocities.com/steve.brady@rogers.com/index.html
    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • SunflowerSunflower Posts: 48
    edited 2005-04-30 22:15
    A single inline diode on Vin may protect against reverse polarity but I did not want the permanent voltage drop. Would quick 0.6 reverse volts damage the stamp or voltage regulator?
  • steve_bsteve_b Posts: 1,563
    edited 2005-04-30 23:49
    It still depends on the current the stamp sees. A regulator won't huff right away....but it depends!
    I had a brainfart this past weekand put in an electrolytic cap backwards (couldn't remember if the line on the cap indicated negative or the bar on the schematic symbol lol I know I know....)....anyhow...the regulator circuit didn't blow right away....but it also didn't work right!
    Eventually the cap blew! (my first!!...by my own hand) Everything else seemed to be fine! Put the cap back in the right way lol and no other probs!

    Are you asking a quesiton because you did something? If the regulator is busted, you won't have the regulated voltage coming out of it. If your caps are bubbled then they might be done and not letting the regulator 'regulate' properly.
    There is a regulator on the stamp itself....I've toasted one in my time AHEM AHEM lol.
    Can you get away with it....sure....buy lottery tickets while you're at it....will you always get lucky....I Wish, but no!

    Would the inline diode work....sure....so long as your voltage going in is still 2volts higher than the rated regulator while including the diode drop...then yer gonna be alright!

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    Steve
    http://ca.geocities.com/steve.brady@rogers.com/index.html
    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • SunflowerSunflower Posts: 48
    edited 2005-05-01 00:16
    I am driving 12 volt motors and valves, and TTL devices. The whole system is outdoors without management. Could be thousands of units all over the map. I am seeking a fail-safe connection to power without parasitic loss.

    Perhaps Parallax could conduct an industrial bench top test of a normally reverse-biased diode from Vin to Vss and a fast 3 amp fuse. I don't have the spare Stamps for such risky behavior.
  • steve_bsteve_b Posts: 1,563
    edited 2005-05-01 01:12
    or just be really careful not to cross your wires!

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    Steve
    http://ca.geocities.com/steve.brady@rogers.com/index.html
    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2005-05-01 01:24
    Both the LM2936 (the voltage regulator on the original BASIC Stamp) and the LT1121 (on all of the multibank Stamps) have a property callled "reverse input protection". That is a feature for battery powered systems, in case you or some user down the line happens to connect a battery backwards to the Vin terminal, or in case a negative transient ("automotive load dump" from inductive sources) happens to appear on the input. No more than a few tenths of a volt will appear at the output, so the BASIC Stamp is safe, and also the regulator itself is unaffected and will start to operate normally as soon as the batttery is connected correctly. There are of course limits. 120 volt AC or a major load dump will certainly destroy the device. The reverse protection on the LM2936 extends to -50 volts, and on the LT1121 to -30 volts. In other words, there is probably no need for you to add a diode protection circuit there. An extra capacitor and a transorb might still be a good idea in a system with motors and such. Please read the data sheets and application notes for the regulators, National LM2936 and Linear Technology LT1121. They are quite informative.

    Another property of these regulators is that you can apply a voltage from an outside source to the output of the regulator, and current will not flow back to the input of the regulator, even if the input of the regulator is shorted to ground or is open circuit, or is connected to a battery connected either correctly or backwards. That is really quite remarkable engineering! And that is what allows the BASIC Stamps to be powered from a beefy external 5 volt power supply connected to Vdd. Usually in that case you leave Vin disconnected. But the regulator on the Stamp just sits there and acts like it isn't even there.

    However, here is the warning, do not connect a voltage source backwards to Vdd. While the Vin terminal is protected against reverse polarity, the Vout terminal on the regulator is _not_ protected. If you reverse a power supply to Vdd, it will probably blow out the output circuit of the regulator (most likely to open circuit), and also it will blow out the BASIC Stamp CPU and the memory chip. Okay. There is your reverse polarity and smoke. If a system needs to be protected from reverse wiring at Vdd, that is where you need a fuse and a diode connected with cathode to (+) across Vdd. I recommend using a polymer self-resetting fuse, which are available from Raychem, Littlefuse, and others.

    Be sure to protect exposed i/o pins at least with a resistor, like 220 to 22000 ohms, depending on the purpose of the pin. That will limit fault currents.

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    Tracy Allen
    www.emesystems.com
  • SunflowerSunflower Posts: 48
    edited 2005-05-01 01:53
    Thanks Tracy for the engineering. I have just one more question about lightning then I'm good to go...
  • BeanBean Posts: 8,129
    edited 2005-05-01 02:50
    If you can stand the 0.6V loss, just put a forward diode in series with the Vin line.
    If you connect power backwards no workie and fuse to replace.
    Bean.

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