BeStar Electronics BMT1212H09  magnetic transducer polarity? - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: March 02, 2015, 12:02:05 pm
content: Hello,In one small research project for this week, where magnetic transducer will be used instead of piezoelectric, to try create accoustic resonance at given frequency in 0.5m-1.0m long closed cylinderI've got in my hands thispdf: BeStar Electronics BMT1212H09 MAGNETIC TRANSDUCERDidn't used those magnetic transducers, so google'd thisElectromechanical transducers: General Overview and Driving Circuits.This they wrote makes sense and driving circuit similar to piezoelectric circuits are mentioned as expected:QuoteWhen looked to its data sheet ensured that looks like those magnetic transducers has driving coil:Quotewhere :QuoteSo, calculated coil inductance, while I might be interested in driving it at lower frequencies, butthis bloody (+) mark means on thisBeStar Electronics BMT1212H09- additionally this pin is longer than second like in THT LEDs where positive (+) is marked to easy determine its polarity?Those pins ar econnected to internal coil (inductor) and measured its resistance from both sides within specs 140 Ohm and there should be no difference how bloody inductor is energized, especially when I'd like drive this magnetic transducer using AC with low power H-bridge +/-12V to force it to oscilate at choosen frequency, so any ideas why this magnetic transducer has this (+) mark on longer pin?Of course, I do not care too much what they drawn and will use AC to push this thing to the limits, but that is interesting why this (+) mark is there-nothing about polarity in its datasheetUpdate: The only explanation of this (+) might be maybe its internal mechanical design, so while magnet is there, than we need apply given polarity to create sound wave in good direction, while opposite movement is limited by inductor coils?Anyway, we should be able force it back faster with reverse pulse, so maybe it makes sense to drive this thing in H-bridge in a awy that we first enargize this thing with polarity marked by manufacturer and at another half of the cycle forcing go back with reversed polarity...It costs below $1 so no worryand then teardown if it fails, but I do not think so

author: eneuro
author_rank: Super Contributor
timestamp: March 02, 2015, 06:54:48 pm
content: Yep, connected this thing directly to low power +/-5V 2phase H-bridge controled by AVR ATTiny85 and no problem what so ever so farThis magnetic transducer outputs quite nice sound- tested in the range 300Hz-3000Hz, but probably AC coupling by adding cap in series will be good idea, so its time to play with this circuit for a while in circuit symulator, to see how it will perform with reactance chosen based on those hints from link:http://en.wikipedia.org/wiki/Capacitive_couplingSo for adequate low frequency response, the capacitors used must have high capacitance ratings. They should be high enough that thereactanceof each is at most a tenth of the input impedance of each stage, at the lowest frequency of interestHopefully, physics works and this confusing (+) longer pin mark on thisBeStar Electronics BMT1212H09 MAGNETIC TRANSDUCERlooks like doesn't matter in this application-as wrote above I've programmed ATTiny85 to send positive pulse first to this (+) marked pin, than reversing voltage to finish one period cycle, so I always start with suggested by manufacturer positive voltage applied to this (+) marked pin and no smoke smell so farNow, it is time to implement more interesting part of this research project and let this this power up accoustic resonance box and create some interesting music.Hopefully, awfully annoying garden monsters are not audiophile and this magnetic transducer sound should be fine in 3 phase configuration