GPS NMEA, Failover Circuit upgrade attempt.

[CDRIVE]

I'm going to consider myself very fortunate that all the Chinese components I've purchased off of Ebay have performed admirably. A couple of months ago I bought a bag of 100 2N2222's and was amazed at how closely matched the Beta was. I tested about 20 of them and they all were within a value of 3 (nnn) of each other.

BTW, a month had past and hadn't received them. Contacted the seller and asked them to re-ship them but they refunded my $1.56 instead. A month later (two months total) the darn things arrived!

Chris

 

[chopnhack]

Hi Constantine, I had some more time to look for a vendor that delivers to Greece - aside from RS that handles transactions for small fees, Tayda Electronics comes to mind. They are a major outfit and I have ordered from them several times. The quality seems good. I looked on the net for some reviews, and they seem well received. Best of all, free shipping to Greece

http://www.ebay.com/itm/10-x-MAX232...sh=item3a69170748:g:qgMAAOxyf~hRzKUM#shpCntId

 

[CDRIVE]

Clicked the link and read the page. Pleasantly no Pigeon English!

Looking forward to your results when they arrive.

Chris

 

[HellasTechn]

Oh, well... with the demise of the MAX232, I think this thread is ready to die too.

Maybe i am not sure yet because i still have some things in mind.

Digikey has free shipping to Greece on $100 order or more, I am sure you can do better with European outfits, have a look around Germany, England, etc. for better rates.

Digikey is very expensive. I know of no other in Europe.

Constantine, I use ebay too, but this is a critical piece of your hardware

It is not that critical since there is always the resistor solution !

 

[HellasTechn]

That is weird. the MAX232 and MAX3232EPE have different datasheets.

according to MAXIM datasheet for MAX3232EPE there are no 400KΩ resistors from pins 11 and 10 to pin 16.

just like the open circuit i measured from pins 11 and 10 to pin 16 on the MAX232.

But according to MAXIM datasheet again for MAX232 there are 400KΩ resistors.

I am a little confused here...

 

[CDRIVE]

Is it possible the Chinese sent you a mismarked chip. IE it's actually a MAX3232EPE clone?

Chris

 

[HellasTechn]

Yes most probably.

 

[CDRIVE]

As a side note the data sheet I have for the Max232 is a TI publication. It doesn't cover the internals in detail. Hence no view of those 400K pullup resistors. I just downloaded a Maxim version and sure enough, there they are!

Chris

 

[chopnhack]

I guess to be on the safe side, measure according to the datasheet for your particular chip and see if the resistors are inline with the pin. If not, add them and test out the circuit. I am glad they came! And we just talked about them, LOL. If they fail, go to the link I sent you on post 262 to get it from a better quality provider. Good luck.

 

[HellasTechn]

Thanks !

I have eraised my prev. post because i understood why the diagram on picaxe manual was different. That was because they do not indicate which pin on picaxe signal flows to but which pin the signal flows from.
that way it matches the datasheets. weird but true.

I have tested the MAX3232EPE today using 10uf electrolytic capacitors like suggested in picaxe manual and it seems to work. I have "crash tested" it At least so far it is stable.

I feed it with 5V from my bench PSU and have noticed the presense of ripple on the supply line when connected.

I tried adding decoupling caps between pins 16 and 15 of 10uf, 100uf and also the same values with a 100nf (104) ceramic in parallel and nothing changed. I dont know why.

P.S.

The freq indicated is not stable, it goes from 0 to 250 Khz

 

[chopnhack]

The freq indicated is not stable, it goes from 0 to 250 Khz

I feed it with 5V from my bench PSU and have noticed the presense of ripple on the supply line when connected.

When you say the supply line, do you mean that the ripple occurs at pin 16 of the chip or you see ripple on your bench power supply after connecting it?
Repeat with your supply at same setting of 5v and a simple load resistor like a flashlight bulb and see if the noise is still there. That can rule out the bench top supply as a problem. I assume that the circuit is disconnected from any ship inputs that could contribute any noise...

Next I would feed in a clean 12VDC either from your bench supply or a car battery with a fuse!! - to see if the v. regulator is working properly. Did you ever increase the values of the capacitors for that part of your circuit? Even though the datasheet shows 0.1μF, you will have better smoothing of the ouput voltage - less noise with a higher capacitance value. Maybe do a sim to see the difference or substitute a higher value and check again.

I doubt the picaxe is responsible for the ripple since you have all the pins tied off properly.

The 232 chip does have a charge pump feature to boost the signal strengths from what I understand and that probably operates at a higher frequency, maybe that is the source of your noise. You may want to increase the value of your cap from Vcc to GND to 10μF instead of 1μF to handle that. Best I can do Constantine, hopefully others will be by that can help you resolve the problem!
-Cheers

 

[CDRIVE]

The 232 chip does have a charge pump feature to boost the signal strengths from what I understand and that probably operates at a higher frequency, maybe that is the source of your noise. You may want to increase the value of your cap from Vcc to GND to 10μF instead of 1μF to handle that. Best I can do Constantine, hopefully others will be by that can help you resolve the problem!
-Cheers

Ditto on that being a good suspect but it looks like hash noise with only the rising or falling edges being coupled to the supply rail. I would think that a .1uF across the 1uF might clean it up. On the other hand If this circuit is being prototyped on a solderless breadboard all bets are off. They're notorious for being next to impossible to properly decouple noise.

Chris

 

[HellasTechn]

When you say the supply line, do you mean that the ripple occurs at pin 16 of the chip or you see ripple on your bench power supply after connecting it?

Both pin 16 and PSU +V rail.

Repeat with your supply at same setting of 5v and a simple load resistor like a flashlight bulb and see if the noise is still there. That can rule out the bench top supply as a problem. I assume that the circuit is disconnected from any ship inputs that could contribute any noise...

It is not onboard the ship but at my lab. The psu is fine since it is clean when not connected on the circuit.

Next I would feed in a clean 12VDC either from your bench supply or a car battery with a fuse!! - to see if the v. regulator is working properly. Did you ever increase the values of the capacitors for that part of your circuit?

I get the same noise when using the regulator also.

I doubt the picaxe is responsible for the ripple since you have all the pins tied off properly.

Correct. Picaxe is fine.

The 232 chip does have a charge pump feature to boost the signal strengths from what I understand and that probably operates at a higher frequency, maybe that is the source of your noise. You may want to increase the value of your cap from Vcc to GND to 10μF instead of 1μF to handle that. Best I can do Constantine, hopefully others will be by that can help you resolve the problem!

I tried adding decoupling caps between pins 16 and 15 of 10uf, 100uf and also the same values with a 100nf (104) ceramic in parallel and nothing changed. I dont know why.

As you can see i have tried that and didnt help.

a .1uF across the 1uF might clean it up.

Same here !

hey're notorious for being next to impossible to properly decouple noise.

I guess so.

 

[HellasTechn]

I have managed to heat up one of the new chips after putting alot efford but it didn't burn.
I will add a self resetting 25ma fuse on in series with pin 16 (supply rail) so that if something go wrong it will not have enought time to overheat and burn and also that way i will affectively protect the Vreg from overheating plus when normal operation is restored i will not have to replace the fuse

 

[chopnhack]

protect the Vreg from overheating plus when normal operation is restored i will not have to replace the fuse

That's a good idea! The V.reg is internally protected from thermal issues, so no worries there.

Sorry I missed the part about your efforts with the caps - somehow though that stray signal is still being coupled with your signal!

When you respond to Chris with I guess so, does this mean you are working off of a breadboard or is the circuit you're testing the etched pbc?

 

[CDRIVE]

Scenario:
Instructor assigns a class project to design a circuit with topography that would represent unsuitability for the following:

(1 ) High Frequency
(2) Fast Rise and Fall Times
(3) High Input Impedance Amplifier

I would simply hand in a just about any circuit of that description on a solderless breadboard. While I love them for their convenience they're a rats (Matrix) nest of stray capacitance and inductance.

Chris

 

[HellasTechn]

When you respond to Chris with I guess so, does this mean you are working off of a breadboard or is the circuit you're testing the etched pbc?

On the breadboard.
I am have not tested it on the PCB.

 

[chopnhack]

On the breadboard.
I am have not tested it on the PCB.

Ah! Well, there you have it If you can spare the board, go ahead and solder it up. This is your first prototype, breadboarding gets you to the prototype phase. Things that show up on the breadboard might not on the pcb and vice verse because they are two very different animals!

 

[CDRIVE]

BTW, you're not alone. I Googled "Max232 Overheating" and you have lots of company out there!

Chris

 

[HellasTechn]

I have already created a PCB for this project.
What you say is true. The noise is not present on the PCB.

Ah! Well, there you have it If you can spare the board, go ahead and solder it up. This is your first prototype, breadboarding gets you to the prototype phase. Things that show up on the breadboard might not on the pcb and vice verse because they are two very different animals!

Yes indeed. I have build a PCB and all are mouted there but the MAX chip. I just wanted to test it alone before putting it together with the rest of the circuit. Now mounted on the PCB i do not see that noise anywhere so yes it must have been the breadboard but it was worth looking into it.

BTW, you're not alone. I Googled "Max232 Overheating" and you have lots of company out there!

Chris

Yes that is right. Just like on my post #242. What can i say other that chances are i am not going to use it on the circuit after all.