Dummy load for Piezo transducer - Page 1
Forum section: electronics / projects

author: manytoolsmike
author_rank: Contributor
timestamp: January 27, 2022, 12:31:38 pm
content: Hello everyone,I need a dummy load that approximates a Piezo electric transducer load. the typical transducers I use are 20 kHz to 40 kHz resonant frequency and measure 4.6 nF with my multimeter (Brymen 869). I drive them at 100 to 300 Vac and up to 60 mA. The power factor (phase angle) is usually between 5 and 30 degrees so the real power is not more than a few watts. With a 500 Ohm resistive load at 20 kHz I measure 180 Vac and 36 mA.I want to make a load that behaves more like the Piezo transducer with a similar power factor. Is there is a combination of capacitors, resistors and inductors that will do this? If I can adjust the resistance and phase angle that is even better.Thanks for your help.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 27, 2022, 02:50:27 pm
content: It's harder than you think because the inductor values can get out of hand. Most are resonant and the power will vary wildly, depending on where you are on the resonance curve. There are some papers on modeling passive networks, though nothing comes right to hand. If you search on crystal circuit modeling, the principles are the same. I think Terman had some info in his old EE books, if you have access to those. This Maxim note should get you started-https://www.maximintegrated.com/en/design/technical-documents/app-notes/2/2127.html

author: manytoolsmike
author_rank: Contributor
timestamp: January 28, 2022, 07:19:10 am
content: Thanks Conrad,This is what I thought. I will try messing around with some circuit topologies I found online. The circuit shown in Fig 3 of the document you shared is what I had in mind. But Im not sure I can handle the math. Haha.The reason for making this is to have a standard reference to characterize drivers (signal generators). Production transducers have too much variation and then there are the environmental factors on top of that. The frequency response of a high power PZT transducer is very sensitive to ...... well everything! So I want a reference load that I can use to compare our driver's performance. A 500 Ohm Resistive load is in the ball park for Voltage and Amperage, but the ideal power factor means the power output is an order of magnitude higher, which heats up my drivers and output transformer.I want the load to be resonant, but I'm hoping for a flatter curve and more stability than I get from a production transducer.Thanks for your help.Mike

author: nfmax
author_rank: Super Contributor
timestamp: January 28, 2022, 09:30:02 am
content: The most popular(?) simple electrical model is the Butterworth-Van Dyke (BVD) model, which comprises a 'bulk' capacitance in parallel with one or more series RLC branches, each modelling a single resonance of the transducer. Note that piezoelectric transducers have much higher electrical losses than quartz oscillator crystals (obviously, or they couldn't emit any acoustic power!) resulting in more feasible values for the circuit elements.Try Googling around, or start from this article:http://www.mdpi.com/2079-9292/8/2/169. There are extensive references, and mention of the other common but more complicated models. However, I expect a BVD model with a single RLC branch would be adequate for the purposes of driver development.

author: manytoolsmike
author_rank: Contributor
timestamp: January 28, 2022, 10:14:33 am
content: Dear nfmax,This looks perfect. As with most of the papers I have seen the example frequencies are much higher than what I am using but I will follow the example shown here.Thanks a lot,Mike

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 28, 2022, 02:11:48 pm
content: @nfmax brings up a point not widely appreciated- if it didn't have loss, it couldn't do any work!