Custom Probe - combining 2 channels

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Custom Probe - combining 2 channels

Postby FioranoCars » Tue Jan 05, 2016 1:13 pm

It does not appear possible to create a Custom Probe from the input of 2 channels ...

The goal being to create a "Software" Differential Probe (ie for fuse based volt drop), while I know this can be done in a Maths channel, you can't then use a lookup table in the Maths Channel (one for the wish list? please!) and while I don't need a lookup table for the volt drop, while looking at options for this issue, the missing feature hit home!

While using a Maths channel to achieve this works, there are several issues (like screen update) that would make having this ability in Custom Probes very handy.

Thanks
Richard
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Re: Custom Probe - combining 2 channels

Postby Steve Smith » Wed Jan 06, 2016 9:12 am

Hello Richard and thank you for the post and once again the suggestion.

The Custom Probe feature is based upon the input from one channel only which does not help here with the development of a software differential probe. (2 x inputs required)

We can however measure the voltage drop differentialy with one channel using a 4425 or 4225 PicoScope

Here we can then use the lookup table in the Custom Probe menu to display the input from your differential measurement (Volt Drop) as a current value in mA

I hope this helps, take care........Steve
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Re: Custom Probe - combining 2 channels

Postby FioranoCars » Wed Jan 06, 2016 12:52 pm

Thanks again Steve
Sadly already a convert to the new 4x25 series, just failed to get Santa to drop one my way! Still lots of us out there with the older 3 and previous 4 series who would get a great deal of value from this if it was possible ... :-)

However, I do understand there might be a need to keep "Maths" and "Probes" separate, but the ability to use a few more of the maths features, as we were planning on creating probes for each fuse type, giving the technician an easier route to select the correct option when setting up the channel, rather than a secondary task of adding a Maths Channel, which gets complicated quickly ...

The issue comes from needing 3 sets of Math Channels for each fuse (Mini=10, ATO=13, Maxi=9, Midi=12, Mega ... then 3 of each ... over 150!), assuming that you have ChA for you common base supply of fuse, then ChB-D for the loads being tested. 3 Maths for each would be A-B, then A-C and finally A-D (with fuse resistance multiplier) to give the Amps/mAmps ...

I've just realised that it would also take 3 custom probes! Doh! :idea:

As much as I'd say you could split them by fuse type as "embeded" maths channels, most fuse boxes are a mixed Mini/ATO with Maxi, so maybe the ability to create sub menus under "custom" to organise your pressure sensors, verse fuses, with ATO, Mini etc in Sub-sub menu's!? Wading in treacle now :(

I'm going to be ordering a new 4425 shortly anyway, just need to decide foam tray or case, decisions decisions! :D

Would still like to get a manageable solution for the old scope as we'll carry on with that day to day, only roll out the new one when it's needed, so the question is still valid for us ...

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Re: Custom Probe - combining 2 channels

Postby Steve Smith » Fri Jan 08, 2016 3:09 pm

Hello Richard thanks for the feedback and one that has got me thinking for sure.

Forgive me here if I have got lost in theories and maths but looking at your fuse volt drop scenario I like the idea of using 4 channels to measure the voltage drop across 3 fuses.

You mentioned Channel A to the common fuse power supply and then Channels B, C and D to the load side of which ever fuse you choose

So this is what I imagine and hope this is correct.

Channel A 12.600 V Power to all fuses
Channel B to ATO, ATC 10 amp fuse (load side) and measures 12.599 V

A-B returns a voltage drop across the fuse of 0.001 (1 mV)

From what I have worked out from the ATO ATC fuse volt drop chart, a 10 amp fuse has a resistance of 0.00769 ohms

Therefore when connecting to the load side of a 10 Amp ATO, ATC fuse with Channel B, a maths channel A-B/0.00769 will display the current. A-B = 0.001 mV / 0.00769 = 130 mA

This then leaves you to create numerous maths channels for each fuse type and each fuse rating!!

Have I got lost here in translation or off on a mini mission of my own?

Take care.......Steve
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Re: Custom Probe - combining 2 channels

Postby FioranoCars » Fri Jan 08, 2016 4:18 pm

As ever Steve you are on the money!

With a new 4 series you only need one probe for each fuse, but organising the custom probes into sub menus for fuse type, and pressures sensors (OEM rather than pico), etc etc is still useful, more so with older scopes as you can have a sub menu for channel b, channel c and channel d, helping avoid total chaos!

We have the resistances for all fuses from mini to maxi to mega and even CAL, and have probes already written, although I need to test them all against a know load, to prove no typos slipped by! Will do this when the new scope arrives in next few weeks, as then I can use the differential ability :D

As an aside I was looking at an ardunio board with amplifier for maybe 6 or 8 fuses and 4mm banana take offs to both give a straight ma to mv or just the differential ... But lots of projects and worse, lots of work, so must prioritise!

The 4-8 is to allow testing across several and then zero in on the 1 or 2 you want to plot against other things....

What were your thoughts?

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Re: Custom Probe - combining 2 channels

Postby FioranoCars » Sat Jan 09, 2016 2:46 pm

Therefore when connecting to the load side of a 10 Amp ATO, ATC fuse with Channel B, a maths channel A-B/0.00769 will display the current. A-B = 0.001 mV / 0.00769 = 130 mA


For anyone reading this and wanting to have a play, to avoid mistakes, the maths requires some essential brackets!

Code: Select all
("Supply Side Voltage" - "Load Side Voltage") / "Fuse Resistance"

"Supply Side Voltage" being the side of the fuse that would be battery voltage if the fuse was removed
"Load Side Voltage" being the side that has no voltage if the fuse was blown/removed!

for those with older scopes ... ADC-212 / PicoScope 3223 / 3423 / 4223 / 4423 this means losing 1 channel (unless you have a differential probe, then see below) to provide the "Supply Side Voltage" which as long as you are monitoring fuses all on the same fuseboard should suffice, as there is unlikely to be any volt drop on the supply across all the fuses, especially if only doing parasitic drain measurements, if the fuses are on separate fuse boards, or you have a really old car, or are reading high amp loads this may not be entirely true!

So placing Channel A into the role of "Supply Side Voltage" and Channel B "Load Side Voltage" then the maths channel would look like this:
Code: Select all
(A - B) / "Fuse Resistance"

Fuse Resistance (technically "Cold Resistance" in the manufacturer specs) being different based on fuse type (even Mini is different from LowProfile Mini), voltage rating (32v verse 58v) and fuse rating (5amp verse 7.5amp etc), so for an ATO 10amp we use 0.00770 (no not rounded from Steve's, this is the manufacturer of our fuses supplied data "LittleFuse state 7.70 mOhm", and different manufacturers may have subtle differences in manufacturing tolerances, or indeed have rounded data they publish.

So If you have channel B on the "Load Side" of the 10amp ATO fuse, then the formula would be
Code: Select all
(A-B)/0.0077

see how it looks in the software, formulae entered at (1):
Image
Sadly for us older scope model users, this means having 3 Maths channels, one for B and also C and D for each fuse type
So Channel C's maths formula for a ATO 10amp fuse would be
Code: Select all
(A-C)/0.0077

and so on for channel D

For those lucky enough to have a differential probe or a PicoScope 4225 / 4425 then you can save yourself the hassle and just go straight to the custom probe library! Create a new probe ... oh, but wait we don't have any maths in the custom probes ... well you do have scaling! No Offset needed ... but heck, there's no divided by?! Well we can just convert our resistance to a multiplier, using this

Code: Select all
"Multiplier" = 1 / "Fuse Resistance"


so for our 10amp ATO Fuse (0.0077 resistance, so 1/0.0077 = 129.8701299) therefore the custom probe would be:
Code: Select all
129.8701299 = Gradient with 0 = Offset


See Gradient at (1) and Offset at (2)
Image

The Theory is one thing, in the field you'll find fuses that don't allow access in the top (fully sealed with no "probe hole"), not knowing who made the fuse, and cheap fuses both can have massively different resistances from the defined specification - both messing up our calculations but also in terms of performance in protecting the circuit, ie blowing at 20amps not 7.5amps. So sometimes the relative values rather than absolute values are the key. We've tested our fuses (not each one!) against a know load to be sure of the quality and readings, so we only buy good fuses of known manufacture.

So why are we bothering at all? Well as VW and several other major VM's have found, tracing Parasitic drains on Modern cars with CAN-BUS and other networked components, made worse by upwards of 30minute shut down cycles, would be impossible by pulling fuses (you'd wake up all sorts of unrelated ECU's in the process), so a non-intrusive test is needed.

Worse, the problem may only happen after a series of triggers, so once reset, the drain disappears for days, weeks at a time, losing the "error state" would result in the drain being reset and not a repair, the car would be back sometime in the future with the same problem and a dissatisfied client.

If you're not confident in the quality of the fuses or they are the sealed type, then a choice has to be made about swapping with quality fuses with "probe holes", sadly this might reset the issue, but at least you'll be able to tell the client the facts, and be in a position to properly diagnose at the next visit. Far superior to be in control and honest !

I think in future (maybe even today) just isolating the draining circuit may be misleading as it's entirely possible the culprit could be a sensor/other ECU sending a "wake up" message, but draining very little current themselves in doing so, especially relative to the drain of the innocent and not at fault "awoken" ECU, and that the awoken ECU could be replaced and achieve no improvement! So soon you'll need to "listen" to the native CAN-BUS/Other networks (not the diagnostic socket which will probably be the wrong side of a gateway node!) to see why the ECU was awoken to find the true source of the awaking and hence the drain! Phew!

There is the ability to use 4 channels of the older scopes (so gain 1 extra channel and avoid all the duplicated Maths channels) by "floating" the scope's "Ground" to the fuse board "Supply Side Voltage" however this is fraught with risks and in a busy workshop might lead to someone with less than 1000% appreciation of the risks blowing up the scope, or even PC attached. The theory being that the laptop would need to be battery powered, with no link to the Vehicle or to the Mains. That the scope would use only 1 ground lead, into the "Supply Side Positive" and then the 4 channels can use Custom Probes to provide a direct reading of the volt drop, converted into Amps using the fuse resistance.

Personally I would advise against this, and based on the risk, just buy a new naked 4425! It has this ability DESIGNED in and protection to avoid any mistakes!

As I said before, when we have our new 4425 I'll finish testing and then publish both the Maths Channel and Custom Probe definitions for all to share. :D

Happy waveforms!
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Re: Custom Probe - combining 2 channels

Postby Robski » Sat Jan 09, 2016 4:11 pm

............... I'll carry on with my DMM :roll:

One thing for a fact is I won't be buying the new scope, if I'd have known about TiePie before buying my 4423 I would have bought one as it already had differential input channels & sure connect.............. if I bought the new scope how long before another comes out ? & then I'll have 2 collecting dust on the shelf at the value of £1k +................
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Re: Custom Probe - combining 2 channels

Postby FioranoCars » Sat Jan 09, 2016 4:30 pm

Quite right a DMM can do mode of this, and a little look up table will give reference amps/milli-amps for your mV readings ... but I don't have 4 hours to watch the DMM until something happens!

The scope as a datalogger is a life saver, especially for overnight or over weekend parasitic tests (no, I have drawn the line, I'm not camping out in the workshop to gaze at a DMM, no client is worth that! :D )

It's these long time test, when a clamp either can't power on for that long, or just wanders due to coil in-stability that created a separate need to fuse based volt drop testing...

I've not seen the TiePie before, but it's not vastly different in price to the Pico stuff, and you can always ebay your old Pico kit, it makes good money ... or better off it here on the forum ... hey Pico, maybe time to start a dedicated forum to "kit for sale" ? I did buy a Hantek 8 channel and although it's used for very specific tasks, it's no replacement for the Pico, but they both have a valid place in the weapons cupboard !

We've bid on lots of Pico on ebay, but always made too rich money :(

I guess competition is good, anything that incentives Pico to utilise firmware upgrades or other patches to unlock the potential of the existing hardware would be good news ... while clearly the reverse is true, no upgrade path or big expense to have the latest features is always going to annoy those who have invested bug bucks already, we hate to see our investment devalued!

Keep Scoping!
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Re: Custom Probe - combining 2 channels

Postby Robski » Sat Jan 09, 2016 4:51 pm

you won't catch me sat for hours neither waiting for DMM to change, I would employ amps clamp first for an idea of time/consumption/location but I'm not here for you to tell me how to find a draw & vice versa, if I really wanted to I'd go spend on a Flir not a 4425.
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Re: Custom Probe - combining 2 channels

Postby Steve Smith » Mon Jan 11, 2016 12:38 pm

First and foremost THANK YOU Richard for taking the time out to post all the information above that has helped me and countless others.

I would like to create a New Feature Request here at Pico based on all the above and so would like to clarify,
You would like a manageable solution for multiple Custom Probes:

A customisable "Library" section of the Probe menu would assist with the filing of multiple custom probes built for fuse volt drop testing?

This is how I envisage the menu structure and file tree could look:
1. Create a new custom probe as required using the custom probe wizard

2. Perhaps at the final stage "Custom Probe Identification" (where we name the probe) specify a file location other than the default Library section of the probe menu?

Once complete, the probe menu would contain additional folders other than "Standard" "Automotive" "Library" and "Loaded"

When selecting a probe the user would have access to a folder called “Custom” containing Fuses, Pressure sensors etc.? (See image below)

FILE TREE SUGGESTION.jpg


Moving onto the original Software Differential Probe suggestion, you would like to see a look up table incorporated into the maths channel wizard?

This could be implemented after entering the equation?

Any clarification here would be most appreciated as I would like to get these feature requests filed.

Thank you again, take care…….Steve
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