Volt drop for current measurements

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Steve Smith
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Joined: Sun Aug 25, 2013 7:22 am

Volt drop for current measurements

Post by Steve Smith »

With the introduction of the Milliohm & Motor tester https://www.picoauto.com/products/milli ... tester-kit it has opened my eyes to a fact that I have not considered for many years, “everything has a resistance

Whether it be a length of cable or in the example below, a “blade fuse” while their respective resistance will be low, they still have a resistance value we can use to our advantage (assuming we can measure low resistance)
Image 1
Image 1
Using the milliohm meter above, we conclude the resistance of our serviceable 15 A blade fuse is 4.5 mΩ.(Refer to Image 3)

So how could this be useful and why do I need to use a milliohm meter to check a fuse?

The immediate answer is you do not need a milliohm meter to check a fuse, but if you wish to manipulate its resistance to determine current flow then a high accuracy milliohm meter is the order of the day

The resistance values of blade fuses are well documented and here is one such example https://m.littelfuse.com/~/media/automo ... asheet.pdf

Below I have highlighted the resistance values of 3 blade fuses, 5, 10 & 15 A using the link above
Image 2
Image 2
Focusing on the specified resistance value for a 15 A fuse we have 4.8 mΩ (Cold) in the table above; below we have measured 4.5 mΩ @ approx. 20°C (300 µΩ difference) which I think we can accept
Image 3
Image 3
So why measure blade fuses if resistance specifications are available?

Not all fuses are made equal, and the following forum post highlights such a scenario viewtopic.php?p=101571#p101571

I guess the answer must be, there is no harm in measuring to qualify and confirm the specifications as there are varying fuse types and manufacturers across the globe

With this in mind, I have also measured 5 & 10 A blade fuses from the same vehicle (VW e-Golf) and compared them to the specifications above
Image 4
Image 4

Above we have our 5 A fuse measured at 17.3 mΩ (Spec = 17.85 mΩ Cold) and our 10 A fuse at 7.0 mΩ (Spec = 7.7 mΩ Cold)

The variables we need to consider with such measurements are the fuse material (tin or silver), temperature (i.e., what is “Cold” in terms of a temperature value) and of course the difference between various manufacturing processes

Based on the above, to summarize we have:
5 A fuse measured 17.3 mΩ : Specification 17.85 mΩ : Difference 550 µΩ
10 A fuse measured 7.0 mΩ : Specification 7.7 mΩ : Difference 700 µΩ
15 A fuse measured 4.5 mΩ : Specification 4.8 mΩ : Difference 300 µΩ
(All fuses above are OE taken from our VW e-Golf)

So how does this information help with diagnosis?

This is where Ohms Law once again comes into play looking at current measurements via our blade fuses

To measure current via a blade fuse will typically require intrusion, i.e., the fuse needs to be removed, inserted into our “fuse extension” and then refitted into the fuse box. https://www.picoauto.com/products//brea ... -leads-kit

This process has the potential to rectify a possible connection error (simply by removing and inserting the fuse extension) or, resetting the offending component protected by the removed fuse (i.e., an ECU)

What if we could measure current flow via the fuse without removal!

Given we know the resistance of the fuse, if we can measure the voltage drop across the fuse (where access allows) we can determine the current flow via the fuse. (Voltage / Resistance = Amps)

In the image below we are measuring across a 5 A blade fuse fitted to a fuse extension within the engine bay fuse box of our VW e-Golf.

Note. The fuse extension is used purely to compare the values obtained by the current clamp with our current calculation (The fuse in question is SB18 for Battery monitor control unit [J367])
Image 5
Image 5

Below we have the results from the above measurement, Channel A has the BNC+60 A current clamp capturing 28.00 mA current flow, Channel B reveals the volt drop across our 5 A fuse (364 µV) and the Math Channel returns the current flow (21.04 mA) based on Ohm law using the voltage on Channel B divided by 0.0173 Ω (17.3 mΩ) The formula used is B/0.0173
Image 6
Image 6
Based on the results above we are close with a deviation of 6.96 mA between the current clamp and our volt drop technique. So, which one is correct?

To answer this question, we need to understand the limitations of both measurement techniques and the low level of current & voltage they are measuring

Current clamp drift must always be considered and to ensure our clamp remains on 0 A (Zeroed) we need to periodically disconnect the clamp from the signal source and the scope. Upon reconnection of the BNC+ clamp, the new “Zero” is defined and the clamp can be reapplied to the signal source

Equally, zeroing the clamp in the vicinity and orientation of signal to be measured (especially for very low current) will help minimize the effects of the earth’s magnetic field should the clamp be rotated or repositioned after zeroing.

Finally, the physical capability of the clamp (TA473) without resolution enhancement is 10 mA and so I think we can agree the BNC+ 60 A clamp does a remarkable job of capturing 28 mA thanks to filtering and scaling within PicoScope 7

Now let us look at the limitations of the voltage drop method which predominately surround “noise” (EMI Electromagnetic Interference) which is inherent in the working environment

Note how we are measuring to micro volt (µV) level and therefore require sufficient resolution in order to return an accurate measurement. Environmental “noise” may well intrude into the probe and therefore we are required to perform the Zero offset function in order to provide the scope/software with a true 0 V reference point without any "noise" intrusion

The following forum post describes the Zero offset procedure viewtopic.php?p=104062#p104062

Once the Zero offset has been applied to the probe (x1 test lead) do not be surprised to see the scope over-range with the detection of EMI when the test leads are not connected across the fuse (i.e., test lead 4 mm banana plugs are open to free air) This is normal and the correct signal will be displayed once the probes are connected across the blade fuse
Image 7
Image 7
With both measurement techniques (current clamp and volt drop) scaling and filtering will be required in order to provide a clear display of captured values (Refer to image 6)
Below we apply the same measurement technique to our 10 A blade fuse (SB16 Charge unit 1 [AX4])
Image 8
Image 8
Finally, we capture the current flow via our 15 A fuse SB3 Engine/motor control unit (J623) below
Image 9
Image 9
Note in all captures above, I have chosen a time base of 100 ms/div to allow for live updating of the math channel. At 200 ms/div and above, we need to wait for the capture to reach the end of the buffer before the math channel is momentarily revealed

Another option when measuring at time bases of 200 ms/div and above, is to create a custom probe for the x1 test lead connected across the fuse under test. In this scenario there is no need for a math channel but you may find yourself with many custom probes to cover all manner of fuse resistance values

For example, to create a custom probe for our 5 A fuse with a resistance of 17.3 mΩ our linear equation entry into the custom probe wizard would be:
Y = mx + c
0.0173 V / 0.0173 Ω = 1 A ∴ 0.0173 V (or 17.3 mV) = 1 A
17.3 mV / A
To now find multiplication factor to determine what 1 V is equal too
1/0.0173 V = 57.803
∴ Y = 57.803x + 0 (+ 0 = zero offset)

See below for custom probe entry into PicoScope 7 Automotive software
Image 10
Image 10
Below we have applied all 3 combinations of current measurement via our 5 A blade fuse in following order:

Channel A BNC+ 60 A current clamp (measuring current via 5 A fuse)
Channel B Voltage drop across the 5 A fuse
Channel C Custom probe across the 5 A fuse
Math channel B/0.0173 (Returns current flow calculation using Ohms law)
Image 11
Image 11
So where and why would such measurements be necessary?

1. Parasitic drain measurements spring to mind where we need the vehicle in a “true” sleep state. By “true” I mean the vehicle has been allowed to fall asleep without the intrusion of pulling fuses during an attempt to isolate an offending parasitic drain circuit (pulling fuses during “sleep” can reset ECU’s or wake networks)

2. Any low current measurement where high resolution and accuracy are required without the fear of inherent current clamp “drift”

3. Where a user does not have the luxury of a BNC+ current clamp or fuse extension, the volt drop method of current measurement provides a viable alternative

4. Where fuse access is limited, the thought of adding a fuse extension and current clamp may be near impossible, whereas “reaching-in” with back pinning probes to measure across the fuse could be the solution

A word to the wise, the above volt drop measurement techniques are only applicable to floating ground scopes (i.e., 4225, 4225A, 4425 & 4425A) as we are not measuring reference to ground but instead, across the fuse. More information on floating ground architecture can be found here https://www.picoauto.com/library/traini ... ing-inputs

I hope this helps, take care…….Steve

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