reverse polarity help :$

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reaperarch

New Member
hi all,
i tend to build projects quite quickly, and sometimes dont think things through...
i would just like to know if connecting a ds18b20 temp sensor
with reverse polarity will kill it?
reason i ask... is 'cause i did, and i cant test it.
thanks all.
 
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Michael 2727

Senior Member
How much voltage were you using at the time ?
And for how long ? in reverse.
You can't test it (not setup etc) or it does nothing when you test it ?

I don't know about reverse polarity, but I know from experience that
they can survive 10V to 11V, with a faint smoke trail comming
off them (couple of secs) and still work properely.
I don't recommend you try this at all.
 
R

reaperarch

New Member
heya, thanks for the reply.
was using 5v for 30 secs
in the datasheet i see it has an "AC charicteristics" section, so i think its ok
and... dont laugh.. but i cant test it because i blew the only picaxe i had..
well, kinda blew it... some things still work.
thanks again.
 
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marcos.placona

Senior Member
reaperarch said:
heya, thanks for the reply.
was using 5v for 30 secs
in the datasheet i see it has an "AC charicteristics" section, so i think its ok
and... dont laugh.. but i cant test it because i blew the only picaxe i had..
well, kinda blew it... some things still work.
thanks again.
Click to expand...
Heh that's what happens when you tend to do things quickly. I'm just puzzled about how can a picaxe kinda work?
 
hippy

hippy

Technical Support
Staff member
It's luck of the draw really, sometimes you get lucky and a component survives, sometimes not, and it's always hard to tell whether a component which has survived is working 100% or for how long. You also never know what's going to happen if you connect a damaged component to a working system, that could simply cause more damage.

The only way you're going to know if it's working or not is to test it. Do so on a breadboard with a cheap PICAXE you can throwaway if it damages that. Use a lab PSU with current limiting to help prevent damage.

Sometimes it's best to just throw a suspected damaged device away, or clearly label it and put it to one side and use another until you can properly test it.

Mark it up as a learing experience.
 
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Michael 2727

Senior Member
The AC Electrical characteristics are only for the DATA Pin.
It has to go Hi, LO during communications.
The Supply pins are a different story.
 
D

Dippy

Moderator
The AC Characteristics does NOT mean it can be powered from AC. In this case it refers to ony-offy digital stuff.

As to whether it has survived I really don't know.
Obv you'll be buying a new PICAXE, suggest you simply buy a new ds18b20 at the same time to save further delays. If the old one is OK then you have a spare for another project, or when you break it...

There is a moral here... but you know that now :)
 
hippy

hippy

Technical Support
Staff member
marcos.placona said:
I'm just puzzled about how can a picaxe kinda work?
Click to expand...
Me too when I achieved that !

I connected an LCD up and wired the contrast pot wrong. I got all sorts of weird behaviour as I adjusted contrast and downloaded. Eventually downloads stopped working which is when I'd realised I'd wired the pot straight across an active output pin and 0V and was decreasing the resistance.

It still ran the program last downloaded but wouldn't download or respond to identifying Firmware.

I zapped it in a PICmicro programmer and turned it into a vanilla 16F88 and it worked fine again, just not as a PICAXE. I thought I'd burned out an I/O line so hoped it would all work except that but the damage must have been more esoteric and repairable. No idea what.
 
B

BeanieBots

Moderator
The "AC charicteristics" section refers to how it's data lines work, not its ability to cope with abuse of its DC characteristics!
A lot will depend on what you were using as a power supply. Most devices (I don't know about the DS18B20) look like a conducting diode when reverse polarity is applied. The PICAXE is such a device. As long as your power supply cannot provide enough current to melt the "diode", then you will usually get away with it. That's why it so frequently mentioned on this forum to use a quality current limited supply.
Personally, I would take Hippy's advice and throw it away just incase.
 
M

marcos.placona

Senior Member
hippy said:
Me too when I achieved that !

I connected an LCD up and wired the contrast pot wrong. I got all sorts of weird behaviour as I adjusted contrast and downloaded. Eventually downloads stopped working which is when I'd realised I'd wired the pot straight across an active output pin and 0V and was decreasing the resistance.

It still ran the program last downloaded but wouldn't download or respond to identifying Firmware.

I zapped it in a PICmicro programmer and turned it into a vanilla 16F88 and it worked fine again, just not as a PICAXE. I thought I'd burned out an I/O line so hoped it would all work except that but the damage must have been more esoteric and repairable. No idea what.
Click to expand...
I've had something almost similar with a 08m I had hanging here that simply stopped working no matter what I changed. I even tried another 08m, and it was working fine. Change PSU, change resistors, tested with multimeter and nothing.

I was gonna put it on the bin, but decided not to for the time being. 2 months later I tested the same 08m just for the sake of curiosity, and guess what...

... Still didn't work

end of story...(???)

No, after another couple of months I decided to give it another try, and it worked straight away. I've got it actively working on an external thermometer for a few weeks and it works like a charm.

I guess it just needed some time for hibernation :D
 
M

Michael 2727

Senior Member
Marcos, it's not the same 08M chip, you must have had an 18 and a 18X
in the drawer and they bred.
Your original 08M is prolly legs in the air, like a dead Wombat, in the
corner of the drawer up the back,,, ;)
 
D

Dippy

Moderator
"a quality current limited supply"

- ABSOLUTELY!!!!!!

You'll be able to get one on Ebay I'm sure, maybe even a good one.

So reap, to be sure, buy a new one of each, learn a lesson, watch what you're doing and start again :)

And sure as eggs is eggs, you'll do it again one day. We all have!
- THIS is why the old lags have quality bench PSUs with current limit controls.
I use Thurlby-Thandar (TTi). Save up your pocket money and buy one.
 
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Ralpht

New Member
Irrespective of what PSU is used, in any circuit, breadboard, stripboard or whatever, alway put a diode in series with the positive supply line.

You'll lose approx 0.6V but it will help prevent reverse polarity issues.
If you apply power the wrong way around at worst the cct won't work because it's getting no power, at best you've saved yourself some heartache. For the cost of 0.6 volts loss.

Only requirement is to ensure the diode can handle the current requirements of the entire circuit. For general picaxe stuff an IN4001 will do the trick. It can handle about an amp.
 
W

westaust55

Moderator
Absolute ratings state:

ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground -0.5V to +6.0V

So the VDD pin should NEVER be more than 0.5V below the VSS/GND pin.

Suggests time for a new IC. There could be damage that is not presently obvious but the DS18B20 could start playing up (malfunction) later
 
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moxhamj

New Member
This is another reason manuka recommends using three ordinary batteries to power picaxe. Batteries have a feature called "internal resistance" which is like a resistor in series with the battery, and it limits the current supplied if you short the battery. So if you short them out or connect a picaxe in reverse, you get milliamps flowing, not amps. For experimenter work - grab a battery holder for 3 batteries and get some batteries.

Down the track when the reverse polarity seems to be happening less - you can change over to regulated power supplies. Start saving now as dippy says. A regulated power supply is a huge asset on a bench, especially one with current limiting where you can dial up 5V at a max of 30mA.

As for picaxes breeding - I wish they would as it would save money. Mind you, the wire hangers in the wardrobe are breeding like rabbits at the moment.
 
slimplynth

slimplynth

Senior Member
worse of all, it wasn't even a lesson learned because I'm not even sure how it happened, Que Sera, Sera... could have been worse, like the television set I melted after three hours out of the box.. and i wonder why i'm poor :confused:

I wonder if Maltik or whoever it was still wants to return any of those parts :)
 
M

marcos.placona

Senior Member
Oops.. sorry man, I really confused you with maltik when saying those things about not checking datasheets and etc :(

I do apologize for that.

But... ok... how did you connect your srf05? Are you sure it's really dead?
 
slimplynth

slimplynth

Senior Member
silver linings

marcos.placona said:
Great man, you finally made it :D. I was sure you'd eventually get to this point after seeing you were doing lots of things without even checking on the manual
Click to expand...
Getting the AXE090 working was a pain and completely down to my below par soldering? But I did have the SRF005 working, transmitting on a 433Mhz unit I got from Sparkfun.

But I'd say 95% of my picaxe time is spent reading the manuals and posts (at work :D). At least with all my gimmicky sensors gone I'll now have time to look at the IR stuff again.
 
slimplynth

slimplynth

Senior Member
haha no worries Marcos... thanks, but i think it is dead now, the red flashing light has gone and connecting the 0V up prevents debugging... unless I've goosed the 08M... in any case it's just how i like my chicken, KFC

The problem may have been my lazy attempts at cleaning off excess flux.. has been soldered onto a perf board for about 6 weeks and hasn't movedfor about 5 weeks. It was a novelyy at first but no great loss,
 
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