Maximum chip current per PICIAXE

[neiltechspec]

Can anybody recall the max current source & sink for a 20x2.
Other PICAXE's could be useful as well.

I know it's on here somewhere as I've seen it in the past, but I can't find it.

 

[hippy]

https://picaxeforum.co.uk/threads/picaxe-i-o-current-capability.11963

For 20X2 -

Sourced through chip : 200mA
Sunk through chip : 300mA
Sourced by all ports : 70mA
Sunk by all ports : 200mA
Sourced or sunk per pin : 25mA

 

[neiltechspec]

That's the one,
Thanks hippy.

I'm gonna be way over that (sourcing), so looks like p chan MOSFETs may be the way to go.
(2 X 1" 7 seg cc displays).

 

[neiltechspec]

Looks like it may have changed since 2009.

From the Microchip datasheet.
I can pull upto 250mA total & have upto 6.5v supply, if they are industrial rated - how can one tell ?.
I wouldn't get that close though.

 2009-2016 Microchip Technology Inc. DS40001365F-page 319
PIC18(L)F1XK22
26.0 ELECTRICAL SPECIFICATIONS
26.1 Absolute Maximum Ratings (†)
Ambient temperature under bias...................................................................................................... -40°C to +125°C
Storage temperature ........................................................................................................................ -65°C to +150°C
Voltage on pins with respect to VSS
on VDD pin
PIC18F1XK22 ........................................................................................................... -0.3V to +6.5V
PIC18LF1XK22 ......................................................................................................... -0.3V to +4.0V
on MCLR pin ........................................................................................................................... -0.3V to +9.0V
on all other pins ............................................................................................................ -0.3V to (VDD + 0.3V)
Maximum current(1)
on VSS pin
-40°C  TA  +85°C, Industrial ............................................................................................. 250 mA
-40°C  TA  +125°C, Extended ............................................................................................. 85 mA
on VDD pin
-40°C  TA  +85°C,Industrial .............................................................................................. 250 mA
-40°C  TA  +125°C, Extended ............................................................................................. 85 mA
sunk by all ports................................................................................................................................... 250 mA
sourced by all ports ............................................................................................................................. 250 mA
Maximum output current
sunk by any I/O pin.............................................................................................................................. 50 mA
sourced by any I/O pin ....................................................................................................................... 50 mA
Clamp current, I K (VPIN < 0 or VPIN > VDD) ................................................................................................... 20 mA
Total power dissipation(2) ............................................................................................................................... 800 mW

 

[hippy]

I can pull upto 250mA total & have upto 6.5v supply, if they are industrial rated - how can one tell ?.

The change to sourced and sunk current is interesting, particularly the 50mA for each pin. The absolute max voltage has risen as well. As to what we supply, that's always the Industrial version as we spec the one with maximum current capability and that's greatly reduced for the wider temperature range extended versions.

The question is; are those revised ratings for older chips or do they only apply to more recent chips ? I'll have to see what I can find out.

 

[neiltechspec]

Interesting indeed, and thanks for clarifying the 'industrial' spec.

As an experiment, I am direct driving 2 x 1" Green 7 Seg displays (Kingbright SC-21WGA), no current limit resistors.
These displays have two leds per segment (apart from d.p.), datasheet says typical 4.4v for 20mA.

Running at 5.9v gives approx 16mA per segment, so a total of 220mA when displaying '88'. Definately shows the brightness
I was looking for. Left it on overnight, PIC gets warm to the touch, as do the displays, still working ok now 18hrs later.

 

[inglewoodpete]

Using any chip at or near its voltage and current limits is risky. There is very little room for movement, particularly if the chip is not in a controlled temperature environment. Thermal runaway can occur.

Using buffer transistors or chips is a far safer option. Eg ULN2803 lowside and UDN2981 highside with a suitable power source.

 

[neiltechspec]

No room for adding any of that in the casing, so I'll stick with it & blame microchip if it blows.
And I did say warm, not hot.

 

[tmfkam]

I`ve driven the red versions of these displays from 5V using a single 47R resistor in the common (Anode in my case). I did drive them through 74HC595 shift registers as I had four displays and needed to free up some pins on the processor. Not sure even I would use *no* current limiting resistors...

As a note, KingBright have discontinued many of these displays. I'm currently designing some drop in replacements using discrete LEDs.
Forge do some 1" white seven segment displays that are REALLY bright. They need 7.6V across them so I spent yesterday modifying a PCB to add a simple series regulator to swap the 5V supply to 12V and reduce it to nearer 7.6V. At 12V (and 470R limiting resistors) they were fine, but when all the display elements were off, the decimal points lit up. I presume this was down to the 595s still being powered from 5V, there was enough difference between 5V and the supply of 12V to illuminate the single LED for the decimal point. Reducing the 12V with the regulator cured this. I had individual 100R limiting resistors fitted to be on the safe side...

The SMD version of the 595 could fit directly behind or even under the displays if pushed for space?

 

[Buzby]

You could try using PWM to drive the displays. ( Just set the display value alternately to digit or blank )

It is quite surprising how bright modern LEDs can be, even when driven with short duration pulses.

 

[neiltechspec]

Have actually looked at the UDN2981, a bit expensive as I need 2, just not sure how I can fit them in the case,
unless I make a deeper case to take an additional pcb, but that is still under consideration..

Also, it will never display '88' anyway with a 70mph speed limit & a 51 year old classic.

 

[papaof2]

However, "68" is just one segment shy of "88".

 

[neiltechspec]

Correct, I think I could work that out thanks !!. But it is 16mA less.

 

[hippy]

Using any chip at or near its voltage and current limits is risky.

Absolutely, and a no risk approach will usually be recommended; stick within the advised operating specification. But one has to allow people to take an informed decision on what level of risk they are prepared to take, what potentially adverse consequences they are prepared to accept or suffer - Though that should be "none at all" when there is a risk to life or limb, or may have undesirable consequences for third-parties, etc.

Unfortunately any risk is rarely quantified and the only way it can be quantified is by those walking that path reporting their results so one has to be grateful to those who do. It doesn't mean others should walk the same path, but it's useful to know what happened to others if they choose to.

 

[neiltechspec]

You lot don't seem to understand - there is simply no room for all that guff.

It's metal cased apart from display & fed through a Bourns MF-R090 1A PTC Resettable Fuse (1A approx).
So there isn't any risk.

 

[Buzby]

I still think a rough-and-ready PWM system will give adequate brightness with a reduced effective current.

On the 20X2 it's easy to use internal interrupts to create a timebase. ( You might already be using this for your speedometer timing. )
In either case, just make the interrupt fast enough, count a few cycles, swap nulls for digits,

Easy to try this, especially if the hardware is already built.

 

[neiltechspec]

Maybe.
But I decided to go with UDN2981's. So no current concerns with the PIC now.
Had to increase the depth of the case to 50mm to add an extra PCB, but I am limited to
56mm internal diameter of the casing (60mm outer diameter to fit in a 60mm hole) and can't go any deeper.
So room is very tight.

As I said, it's metal cased (machined alloy) with a glass front & PTC Fuse, so no risk as far as I am concerned.

At some point I may play about with PWM, but not for now. As it stands I didn't need to use interupts, pulse speed
is fairly quick, 106hz at 60 MPH and a simple count for 1 second is adequate, checked accuracy with analogue speedo & gps
and it's way more accurate than the analogue speedo at different speeds.

 

[papaof2]

I've had one vehicle that was comparable to GPS speed accuracy across much of its speed range - a 1995 Toyota Camry. My 2008 Tacoma speedo is acceptably accurate (analog dial, electronically driven) but not as good as the speedo in the '95 Camry. Possibly a fluke where all the tolerances worked together?

 

[neiltechspec]

Just one thing I should mention.

I have a version of this that uses Com Anode OPD-S10020LY-BW displays.
Driving direct (again with a 20x2), running off 4.4v to limit segment current (under 10mA), this has been in use since mid 2017
without failure, nearly 6 years now. These are yellow displays with a orange glass filter (photo filter) & metal cased of course.

Changed all instrument lighting & alarm OLED display to green, hence the need for green displays now.
It's just the green displays need more current to achieve an acceptable brightness.