Hi Joan,joan wrote:Of the order of 50K ohm from memory. Not sure where it's specified.
However it is not recommended to take more than about 50mA from the 3V3 pin so marginal at best.JohnBeardmore wrote:OK, consistent with Joans figure. Thanks.techpaul wrote:The internal pull-up resistor is fairly high I BELIEVE of the order of 30 to 100k.True, but with a 3V3 supply, 330R only sources 10mA. Even if all six inputs are pulled low at once this is only 60mA.techpaul wrote:So suscepatble to interfrerence. normally 10k or slightly lower is sufficient. The lower the resistance the more current used
Depends if the line sits low most the time then the average is 9mA, but in reality you need to worry about peak as so near available 3V3 limit determined more by the onboard regulator and its existing on-board load.The inputs get a 100ms pulse every few seconds typically, so if I pull up with a 330R resistor, the average consumption is probably well 1mA per channel - only a tiny fraction of the Pis consumption.
Might be better with 1k.I'd like to keep the average energy consumption low, but the noise immunity high, and if the kit is used in an industrial environment, to some degree I'd like to reduce the risk of damage due to mild EMP from welding kit, switching reactive loads etc.
But runs off a psu with much more spare capacityI have a little experience of this sort of thing. We have a multi channel phase angle control light dimmer with TTL inputs connected to switches round the house for dimming or brightening lights. This has the inputs to the TTL 74LS150 mux connected to long wires and 220R pull-up resistors, and it's been running 24/7 for 19 years without problem.
For one input no but with limited 3V3 available and many inputs, your are risking other effects on board.JohnBeardmore wrote:I can't see any penalty except power consumption ?techpaul wrote:and I would not go below 1k for switch or similar pullups.
When the open-collector is off you are more than likely feeding a small amount of current to the Pi, instead of when on taking small current from Pi and mainly the resistor. Current flow direction changes.As long as the open collector thing you are connected to can sink the current your input is sourcing of course.
Not familiar with them myself.We're using the Elseter A100C meters with SO Pulse output (IEC 62053-31). I assume these can sink 10mA. I'm sure I checked at some point, though now I go looking for it, I can't find any documentation to that effect.
Almost any value much higher than 10k and its impedance makes it a weak force, below 1k and multiple lines then you start taking too much current a lot of the time from 3V3 and could exceed safe operation.JohnBeardmore wrote:Surely pretty much any value will do ? In general I'd agree, but I'm using pull ups on the lines I'm using, and the other GPIO pins aren't connected to external wiring.techpaul wrote:Better to tie unconnected signal inputs to pullups of 10k to 3V3.
http://elinux.org/Rpi_Datasheet_202_Rasperry_Pi_IO in Power sectionJohnBeardmore wrote:>John Wrote:
>>...with a 3V3 supply, 330R only sources 10mA. Even if all six inputs are pulled low at once this is
>However it is not recommended to take more than about 50mA from the 3V3 pin so marginal at best.
Where is this stated ? See my calcs below.
Yes there is very little spare capacity on those pins.>>I can't see any penalty except power consumption ?
>For one input no but with limited 3V3 available and many inputs, your are risking other effects on board.
Let's try and quantify this. Looking at the Pi schematic, there doesn't seem to be anything between the 3V3 regulator and the GPIO 3V3 connector. The track might be a bit thin, but I can't imagine any track struggling with 60mA. The 60mA 'limit' strikes me as a bit arbitrary.
I guess the issues are 1) does the peak current exceed the regulators rated output current (which could kill
it quickly), and 2) with the additional load of the pull ups during pulses, will the regulator overheat with the
heat sink capacity available (which could kill the regulator slowly).
For heatsink the regulator will needI suspect that given that the Pi isn't plugged in to anything else, we probably won't exceed the rated output, though it's interesting that the schematic notes by the 3V3 regulator, "Place on Mh 400sq mm Copper Area for pd 800mW".
Only with suitable heatsink and a) the square pad is not enough for 1A power dissipation and b) at that level no doubt the polyfuse will blow. People are using all sorts of power supplies often because they are not doing tests as the PSUs are often chargers not PSUs, they have lossy cables and sometimes really bad PSUs.If the voltage regulator is dropping 1V7, it dissipates 800mW at 470mA. And here we all are buying 1500mA powers supplies... Anyway, it looks as if the NCP1117 itself is rated at 1A.
But some of its internal logic runs from 3V3 and internal regulators.Web pages like http://www.raspberrypi.org/phpBB3/viewt ... f=2&t=6050 seem to indicate typical current consumptions on the order of 200mA. Even if we allow 300mA to give some headroom, I don't think it looks as if my drawing pulses of up to 60mA for 100 ms will either exceed the max rated regulator current, or the maximum recommended dissipation on the 'heat sink' provided.
USB outputs run at 5V, so hopefully the USB wi-fi doesn't further stress the 3V3 regulator.
I think the 3V3 rail on some Pi models can supply up to 1 amp (based on the chip used)..rpdom wrote: ↑Thu Jan 18, 2018 6:47 amI'm pretty certain that the 50mA recommendation was based on the old 3v3 linear regulator on the original Pi. The newer ones (B+ and later) use a more efficient switching regulator and can supply much more current. I haven't seen an actual recommended maximum, but I believe it can supply over 100mA for external circuits.
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