A little advice needed

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Skeksis
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A little advice needed

Postby Skeksis » Mon Jul 11, 2011 5:06 pm

Hey guys,

This will be a horribly n00b-ish question so I apologise in advance... I'm still at the stage where electronics is horribly intimidating, not to mention infuriating :cry:

I have two ultrasonic distance sensors both of which draw too much power, I've been told this can be caused by blown op-amps. I was hoping to replace the chips but have no idea what I would be looking for as all identifying marks on them have been filed off, so my question is this: Can I just use resistors to limit the current drawn by the sensor boards or will I have to find out what these chips are to replace them?


Actually, while looking for an example image to show you all what I'm trying to fix I've found a photo of a board with surviving chip IDs which looks quite similar to the designs of the two I have. Would I be safe sourcing the chips in this image to replace the ones on my units?



Thanks in advance,

Skeksis
"Time is an illusion. Lunch-time doubly so." - Douglas Adams
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lilltroll
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Postby lilltroll » Tue Jul 12, 2011 1:31 am

The transducers are piezo-electric, that means that it looks like a capacitor seen from the electrical side (for frequencies that are not close to it's mechanical resonance fs).

Feeding a square wave (my assumption) into a capacitor is problematic, since that will result in a very large current at each flank. Somehow you need to limit the current e.g. band-limit the signal, there is many many ways to do that -> and serie resistors are maybe the most simple solution.
Check the impedance at fs for your piezo-device, probably choose a resistor that has maybe half of that value and also check the current specification on your Op-Amp.

The best way, but more expensive way, is to produce a clean sine-wave instead of a square wave.
Probably not the most confused programmer anymore on the XCORE forum.
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Skeksis
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Postby Skeksis » Tue Jul 12, 2011 2:36 am

Hi liltroll, thanks for replying.

It took reading through a couple of times but I think I can almost follow you. So using resistors to manage the power usage is the way to go? I think I'll have to do a load more research on the things to see if I can work it out.


Skeksis
"Time is an illusion. Lunch-time doubly so." - Douglas Adams
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lilltroll
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Postby lilltroll » Wed Jul 13, 2011 4:47 am

If you are using OpAmp´s, you may create a Low-Pass filter with help of some small capacitors in the feedback loop.

That way you can bandlimit the signal to 50-60 kHz ?
Your signal will then not have sharp transitions, and the maximum current flowing into the piezo will thus be less. (The capacitor will be charged over a longer period, since the voltage is changing more slowly)

Image

PS, I have built several systems with piezo-electric transducers and/or advanced distance estimators DS.
Probably not the most confused programmer anymore on the XCORE forum.

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