DC-DC converter

Thanks, Joe ... If you say so, I am inclined to believe it .

MichaelG.

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That said; I have yet to check how the discharge curves compare with the trusty SR44

Well, not sure yet that I believe it...

At least one of the advertised cells achieves this through trickery - a DC-DC built in to the cell, the latter appears evidently to be 3.6V chemistry. The DC-DC converts to 1.5V, and also seems to be the 'charger' when charging the cell from 5V ( a USB port, it would appear).

The advertised 1.5V rechargeable DURACELL's - I have failed to find an image of the cell, or a blister pack image of 'cells' actually showing the depicted 1.5V on the cell itself! Also, non of the MANY vendors of this specific cell have any in stock! Phantomware?

Anyway, assuming life is good, and the so called 1.5V rechargeables all use an integral DC-DC, maybe with all the fanfare regarding the caliper's noise sensitivity, the cell won't work anyway - perhaps the DC-DC noise will be an issue?

Regardless, I was just stirring a little - As Michael asked - show me a 1.5v rechargeable cell where the native chemistry results in a direct nominal 1.5v .

Joe

Well ... The Duracells are certainly off my list of candidates

Here’s the back of the package:

SoD: Ok as far as it goes. But I'd expect the problem to be due to noise direct from the DC-DC converter, not external pickup.

Andrew

That 'advertising' for you! Blatantly stating 1.5V but not letting one see the truth...!

Joe

Hidden in plain sight, Joe

The photo is grabbed from the link that you provided.

MichaelG.

Presumably the integrated circuit in a digital calliper uses the same type of CMOS as watches. Here, in the interests of best battery life, the FET device parameters are chosen to minimise the totem-pole currents (as well as otherwise operating reliably). The absence of external connection, and the fairly complete Faraday cage of the case means that watches are generally immune to ordinary levels of interference.

The same technology is not so sensible for digital callipers, particularly when an external connection is able to import RFI. It occurs that a possible investigation might be to increase the voltage of external supply slightly from 1.5v.

Initially, the supply current should be graphed against increasing voltage from 1.5v upwards to, say, 2.0v , or until the current indicates chip dissipation of a few milli-watts. For example, at 2.0v, 500 micro amps would result in 1 milli-watt, and hopefully such a small dissipation and over-voltage would do no harm. An increased supply voltage should provide higher noise immunity. Decoupling at the calliper would clearly still be essential.

Anyone looking for a project?

Method in my madness though! I tested my caliper with a linear PSU and 40cm leads and it was all over the place. Previously, I'd powered the same caliper from an Arduino with short 5cm leads and it was fine. (Arduino's on-board linear regulator, my potential divider.)

Thinking about it though the experiment isn't convincing. I ought to repeat it with a battery rather than a mains PSU. If I repeat with battery clean volts and the caliper still misbehaves, it must be noise picked up by the wires?

Induced noise is quite likely I feel because the caliper is high impedance? (I make 6uA at 1.55v about 260kΩ but you know what my maths is like!)

I still think my conclusions about the caliper being very sensitive to low volts are valid though. I found:

My caliper fails absolutely below 1.48V and was reliable only above 1.51V. The SR44 cell produces 1.55V, and this voltage appears to be critical : a drop of 0.05V is enough to upset this caliper, at least with my noisy supply.

No harm in checking again especially as I qualified the conclusion with 'at least with my noisy supply'. Playing electronics is more interesting than tidying, which is what I'm supposed to be doing! I might have a glass of wine and do the experiment in a drunken haze.

Dave

Edited By SillyOldDuffer on 07/04/2020 15:45:07

How about using screen leads, grounding the outer screens ONLY at the Caliper end? Dunno if a common mode choke will work along with some decoupling caps. these will need be near the input.

Sounds like there's very little decoupling in the calipers and a high impedance on the supply line to save battery drain..... try and get the external battery to work and then the external power supply.

Dave

What do you need 1.5V AAAs for?

Alkaline AAAs drop to 1.2v about half way through their discharge life, NiMH AAA by about 15-20% (to 1.3V with low currents).

If you only need low power for relatively short periods you could use NiMH ones and recharge frequently?

Neil

Edited By Neil Wyatt on 09/04/2020 13:05:45

Found something interesting about my Lidl Caliper powered from a clean battery yesterday - it can interfere with itself.

As the caliper only consumes 6 microamps, a 250kΩ load, I'd assumed a 9V battery and 10kΩ potentiometer would be stiff enough as a potential divider to power the caliper.

Not so! The caliper outputs about 140mV of noise at the battery terminals. This sufficient to cause a poorly regulated supply to dip below critical voltage, causing misreadings.

Using a 1kΩ potentiometer to divide 9V down to 1.55V is far more stable. With a 10kΩ pot the caliper fails at 1.48V, with a 1kΩ pot it worked down to 1.40V

With the caliper operating correctly at normal full battery (1.55V), I injected noise into the supply. 9mV of external noise is enough to randomly toggle the minus sign. With 40mV of injected noise, the digits start flipping occasionally. At 90mV they flip continually.

In short, the caliper is:

• intolerant of low battery volts - the caliper stops working when its SR44 cell drops 0.15V from new.
• sensitive to electrical noise on the supply, including it's own!

Going to take it apart later. I expect to find no decoupling because the caliper normally relies on the battery to smooth out internal noise and eliminate external noise.

Making calipers work with an external supply is probably quite simple - relatively low impedance volts (like my 1kΩ potential divider), plus a decoupling capacitor at the caliper terminals to get rid of any noise. A shielded or twisted cable decoupled at the power supply as well would be prudent.

For a small fee I might measure how many volts are needed to kill the caliper. It took an accidental 1.9V without complaint. Fussier about too few volts than too many.

Dave

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I don’t, Neil

Joe was picking-up on the exchange that I had with ndiy

[ your examples nicely demonstrate my original point ]

MichaelG.

Edited By Michael Gilligan on 09/04/2020 14:12:01

So add a capacitor to your 10K version to reduce the source impedance

Neil

It is only to be expected that the calliper IC will draw current spikes that translate to voltage noise. In a static state, CMOS logic draws only nano-amps. The operating current is due to charging and discharging internal capacitances on each clock edge. There needs to be adequate decoupling to limit the voltage excursion this causes. A battery may just be good enough, but a suitable capacitor should be better.

In view of the very small margin below the battery voltage for the chip to work correctly, I again suggest investigating a slightly higher external supply, but still with adequate local decoupling at the battery terminals, or even better as near the power connections on the chip as possible.

Do we know the clock frequency used? An electrolytic may not be best – a suitable ceramic is generally better at higher frequencies. The leads should be short to avoid adding stray inductance in series with the capacitor. A correctly designed surface mount capacitor fitment would be best, but it may be a bit late for that!

F

I haven't read the whole thread, but have you re-soldered the battery connections !

I've got/had a couple where the display couldn't make up its mind and re-soldering the battery connections on the board cured the issue !

10uF from the pot wiper to ground didn't make any difference. It helps when placed across the battery contacts, the difference being eliminating the length of the leads.

Macolm makes a good point about finding the clock frequency - the spikes are caused by square waves and will have a substantial higher frequency component. If it's easy to do I'll identify the clock frequency and try various decoupling values.

The surprise to me is how critical about power this caliper is, but it's not a showstopper.

Dave

I'd be surprised if the clock frequency is more than a few kilohertz. The process isn't doing anything particularly fast or complicated so a high clock speed isn't needed. In CMOS circuits the dynamic power consumption is proportional to frequency and the supply voltage squared.

Think I'll stick to using mechanical micrometers - no batteries needed. I don't trust all this electronics nonsense.

Andrew

Edited By Andrew Johnston on 09/04/2020 19:05:15

I don’t have a feel for the core frequency. Certainly the data output is low kilohertz bit rate. However, the individual elements of the capacitive vernier can only be 10 or 20 pf. Secure motion tracking as well as accuracy might need a higher frequency, and of course, this could be the part that is upset by low supply voltage or interference.

Unfortunately, measurement is not particularly straightforward without specialised testgear.

F

Investigators may be interested to read Meyer’s patent for the capacitive system [now widely known as Chinese]

US4437055A

This, and his other patents are readily available [*] on espacenet: **LINK**

https://worldwide.espacenet.com/patent/

MichaelG.

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[*] Just search for: "MEYER HANS ULRICH" [CH]

So I opened it up and performed a post-mortem!

The photo shows the interesting top side of the circuit board. (On the other side is the slide movement detector, just a lot of parallel tracks.)

The toothy tracks on the edges are for the control buttons. There's an unused set marked ?, don't know what the missing button is for.

The silver tube clock crystal runs at 153.6kHz which is low radio frequency. (BBC R4 Long Wave is on 198kHz) No idea why 153.6kHz, which seems an odd number? Maybe Michael's link to the patent explains!

Anyway there are only 3 passive SMD components in the whole shebang, shown ringed in red. In the bottom right hand corner are pads for another, missing, SMD component, ringed in black. The pads connect across the supply rail and are certainly intended to take a decoupling capacitor.

This digital caliper was made by a bunch of cheapskates! I expected more for £4.99, including VAT.

Dave