Problem with LTC3855 sparkfun breakout board

Thread Starter

etamar

Joined Feb 28, 2021
7
Hello all,

Using SparkFun breakout board (link) for the LTC3855 (datasheet), I assembled the following circuit (taken from LTC)

1614517590716.png

The differences are -
1. Transducer - instead of the MIDEV21BL I tried using the following piezo elemnts:
a. 1614517729977.png
b. 1614517789799.png(datasheet)

2. There is no MICROPROCCESSOR connected to the output.
3. I used the ADALM1000 to probe different pins (link), Its internal resistance is 1Mohm
4. the GND is the ADALM1000 GND



PROBLEM IS: by bending the sensor i got a steady rise in VIN voltage, up to 1V, BUT NOT ABOVE IT. (both elements should output way above 1V when bent/pressure applied over them)

I tried replacing VIN capacitor to 100uF, and reached the same maximum voltage (the bending took a bit longer)

I tried taking out the probe, bending for about a minute, and the probing the voltage - BUT IT STAYED 1V. (I thought maybe the leakage was to high)

Do you have any thoughts how to solve it? VIN should be able to charged up to 20V.

THANK YOU!
 

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Thread Starter

etamar

Joined Feb 28, 2021
7
Hi eric, thanks for your help

From my understanding, their difference is mainly affecting the charging time (ie, how much power a single bent can supply).

Both of the tranducer reach voltage much higher than 10 V, thus should at least charge VIN to above 1V.

What do you think?

thanks,
 

Thread Starter

etamar

Joined Feb 28, 2021
7
EDIT - the original post is (probably) not true...
The "proof" (i think) is that both 10 and 100 uF where charged to the same value of 1V. meaning the problem is not the piezo as a power source. \
At the bending instant the voltage is much higher than 1V, so i expected to at least see a slow rise in Voltage, regardless the piezo source


ORIGINAL POST:
After another look at the datasheets, I think the problem is that the piezo require continuous movement at resonance frequency to reach the high output voltage.

Ill try to by another piezo element suited for "bending"...

If anyone has any insights - Ill be happy to hear them :)

Thanks
 
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Thread Starter

etamar

Joined Feb 28, 2021
7
Hello there

I tried connecting today the adalm 5V output directly to VIN... again VIN was stack at 1V,
The (second) weird thing is VCC began to charge up REALLY slowly (for the current it drew from adalm1000 it should have charge the 47uF 10 times while it was turned on)

What do you think? :)
 

Yaakov

Joined Jan 27, 2019
2,381
After another look at the datasheets, I think the problem is that the piezo require continuous movement at resonance frequency to reach the high output voltage.

Ill try to by another piezo element suited for "bending"...

If anyone has any insights - Ill be happy to hear them :)

Thanks
Are you expecting the piezo to continuously provide output when it is *not* moving, that is, if it is held flexed but static?
 

Thread Starter

etamar

Joined Feb 28, 2021
7
H
Are you expecting the piezo to continuously provide output when it is *not* moving, that is, if it is held flexed but static?
Hi

No. I continually bended the piezo and the voltage built up slowly.

Problem is with the 1v maximum limit, which appears also when I connect pz1 and pz2 directly to 5v source and gnd (with some resistor to prevent the chip to heat up)
 

ericgibbs

Joined Jan 29, 2010
12,217
hi eta,
Please post your circuit showing the resistors etc ... and their values.
What type of voltmeter are you using to measure the Vout .?
E
 

Thread Starter

etamar

Joined Feb 28, 2021
7
Hi

I attached two photos from different perspective.

Main idea - to charge Vin from 5V source, and to check how much it changes.

Charge was made using 5V source and 2Mohm resistors -
the current where VCC stop rising is 4V (the 5V source - 1V that VCC get stuck on) means that from the source there is current of 2uA
From LTC datasheet:
1614965971643.png

There be a steady rise in Vcc, at least when VCC = 1V

What do you think?

Thank You
Etamar
 

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ericgibbs

Joined Jan 29, 2010
12,217
hi eta,
Found a LTSpice LTC3588 model, using a MIDE V22BL transducer, the results look OK.
Note the Piezo is at @ 41Hz and its amplitude, V(p1,p2)

This should be a good reference point for your project.
E
AAA 1224 18.39.gif
 
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Thread Starter

etamar

Joined Feb 28, 2021
7
Thanks, I'll try to test the circuit with spice as well
Either way, the circuit attached above with direct 5V src behaves funny.
I'll update here later this week the simulation result
 
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