28X1 Current Draw

[pilko]

Hi,
I am using a 28X1 to control my heat pump, and it is ,and has been working great for a year or so.
I am making up a new power supply for it, so the other day I measured the current draw to the AXE. I was surprised to see 140mA, even with all of the outputs disconnected. The programme size is 1100 bytes.
Is ths "normal" or should I be getting concerned?

Regards

pilko

 

[Dippy]

You should be getting concerned.

With just the bare minimum it should only be a few mA. It will vary with clock speed.
Check your config against the PIC Data Sheet .

Is PICAXE getting hot?
Is your PSU OK? All in the right places?
Does your measurement accidentally include the PSU circuit?
Are your capacitors round the right way?
If not then you have something else connected or connected wrong or your meter is mullered.

Test your meter with a known voltage and some know resistances.

 

[pilko]

Thanks for the responce Dippy.
Since you say I should be concerned I am doing some trouble shooting.
-- Nothing getting hot
-- Meter OK --using Fluke, and second Fluke agrees.
Does an electrolitic pass high current if reversed? --no heat though
Will report back after more testing.

pilko

 

[Dippy]

"Since you say I should be concerned I am doing some trouble shooting."
- good


"Does an electrolytic pass high current if reversed?"
- yes, due to nature of the goo inside.

Depending on the conditions it will pass a current of whatever then, after it gets bored, it will fail.

If you are making a new PSU you should be able to spot an error like that easily.
I take it you have chosen quality/branded components from a good reputable retailer?

 

[pilko]

Caps are OK.
When I remove the ULN, the current remains at around 140 mA, but when I remove the 28X, the current drops to about 15 mA.
Schematic attached.

 

[hippy]

Are the two 0V legs of the 28X1 connected together or are you running the ULN 0V through the 28X1 ? Try connecting both 0V together and seeing if that reduces current.

 

[pilko]

Hippy, the two 28X legs are connected together.
Updated schematic with meter position and supply shown.

 

[Dippy]

In your schematic you show a pile of resistors and things.

Have you tried checking all your components, values , resistor legs etc. ... nothing touching in the wrong places?

Pads, tracks, unetched copper, solder bridges...?

Is the PICAXE in the right way round?

Magnifying glass and Ohm-meter....

I would suggest putting the PICAXE in a minimal breadboard circuit (or project board) and check it. If the PICAXE is taking 100mA then say hello and wave goodbye. In fact if it was taking >10mA @4MHz I'd be worried.

 

[pilko]

Update.
The 28X is slightly warm.
The big question is:
Why does the current drop dramatically when the 28X is unplugged?

pilko

 

[pilko]

dippy
Everthing runs as it should, and has for over a year.
I will do some more testing as per your recommendation.

pilko

 

[Dippy]

"Why does the current drop dramatically when the 28X is unplugged?"
- because the 28X1 device is doing something, obviously.

But WE can't see your 'build' from here.

Have you tried a consumption test after programming PICAXE to set all outputs Low as well as High?
i.e. no complex code just all Low or all High.


Are any things touching?
Is there some I.C. socket damage , or solder under the socket?

Have you tried another 28X1?

Have you done any continuity or partial-short testing with a meter?

Have you done as I suggested and put the PICAXE in a minimal circuit and measured? (breadboard or dev board).

Keep at it. You'll have a Eureka moment sooner or later. Good luck.

 

[fernando_g]

Something that I noticed..it may or may not be the cause.

The capacitors being fed from the external 24VAC signals will charge to 24*1.4 = 33.8VDC.
The 100k/12k resistor divider will make the voltage to the PICAXE's input pins 3.6 VDC.
Not exactly a full "logic high".

Some inputs start behaving weirdly when their input voltage is not 5 volts plus or minus a small amount.

You may want to check this out. Again, it may or may not be the cause.

 

[pilko]

Update.
After a couple of hours of desoldering, trouble shooting and re-soldering without avail (decided to wear one).
Replaced the 28X and the current dropped to 22mA.
I have no idea what is bad about the AXE, or when it went bad as it continued operating normally.

Fernando,
Thanks for your response. --- the 24V signals are a nominal 24V. they are actually 28V The filtered inputs are around 4V with a little ripple and are not a problem.

pilko

 

[fernando_g]

Pilko, glad that you solved your problem. It is indeed an odd defect, an IC drawing that much current and yet still operating properly....Some section of the die, which your program is not touching at all, must be "leaking".

I would still tweak the resistor divider to get the input voltages closer to 5V. Just for good measure.

 

[Dippy]

I would still test that PICAXE in a minimal circuit at some stage.
For now, mark it with a bit "?".
If , after testing, it confirms heavy consumption then use with care or send it to Silicon Heaven.
You certainly won't be able to trust it in important circuits.

I would recommend spending 20 minutes downloading the monster PIC Data Sheets from Microchip.
They will give you a head start on most electrical and register related queries.

 

[pilko]

@ Fernando g
"I would still tweak the resistor divider to get the input voltages closer to 5V. Just for good measure."
Thanks, -- good advise.

pilko

 

[Peter Graat]

For safety-reasons you can always include a schottky-diode to your input. The anode to your input and the kathode to +5V. This prevents your input.

 

[pilko]

"For safety-reasons you can always include a schottky-diode to your input. The anode to your input and the kathode to +5V. This prevents your input."

Inputs 0,1,2,3 & 7 are 24V AC and are rectified, smoothed and pot devided and have never been a problem. I would prefer to leave well alone.

Thanks for your response.

pilko

 

[Dippy]

... And PortC has internal clamp diodes which will give protection. As you have big val resistors and caps I can't see a major problem either.

Though, as an aside, I would agree with Peter for analogue inputs where other precautions haven't been taken. Or if you need faster clamping.

 

[pilko]

@ Peter & Dippy.

Thanks for the input, and duly noted.

pilko