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I have a project I am working on where I want to power a "self-drive" piezo buzzer for 20-30 seconds. However, in order to provide the power to this piezo, I would like to ideally use capacitors in conjunction with a PMOS transistor to trigger the buzzer. (The alternative is to use a battery with a 555 timer to control the amount of time the piezo is on). However, I am trying to convince myself one way or another whether capacitors can deliver enough power to the piezo for that amount of time. I have not chosen a piezo yet, but I did find a 5V self-drive piezo here . However, a data sheet was not provided with the piezo to indicate average power consumption.

The bottom line: is this possible to do (without super capacitors)? Or am I going to have to go with a battery configuration?

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Guessing without a datasheet is not the best. You can expect that to use around 25mA give or take 10 or 20mA. Let's say 30mA and 30 seconds and you will power it at 6V and allow it to fall to 4V (average voltage 5V and assumed constant 30mA, which won't be true, but is close enough for a WAG without a datasheet).

So \$C = \frac {2(0.03 \cdot 5 \cdot 30)}{V_f^2-V_I^2}\$ = 0.45F

Since the energy consumed is the integral of i(t) * v(t) over time, and the energy in a capacitor is \$ \frac{C V^2}{2}\$

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  • \$\begingroup\$ I actually had just worked the math and I got something similar: i = 10mA, v = 5v; P = i*v = .05 W; E = P * (time_on) = P * 20s = 1 J; E = 1/2 * C * V^2; C = 0.1 F. Vf = Vi(e^-t/(tau)); R = -20s /ln (Vf/Vi)/ C = 493 ohm. Is this the correct way to approach deriving Tau ? \$\endgroup\$ – cg14 Jun 14 '16 at 18:30
  • \$\begingroup\$ Additionally, assuming this is correct just a relatively large capacitor is needed in order to make this configuration work? \$\endgroup\$ – cg14 Jun 14 '16 at 18:32
  • \$\begingroup\$ I'm not going to slog through your math, but yes a large capacitor in the 0.1-1F range that does not have too much internal resistance to supply the required current. \$\endgroup\$ – Spehro Pefhany Jun 14 '16 at 18:33
  • \$\begingroup\$ Understood (I more or less included it in case some one else wanted to see what you gave in more detail.) Instead of using something that falls into the super capacitor range, I am assuming I could potentially use a capacitor array instead? Super capacitors tend to run a little expensive. \$\endgroup\$ – cg14 Jun 14 '16 at 18:36
  • \$\begingroup\$ 45 10,000uF caps might be a bit pricey too. \$\endgroup\$ – Spehro Pefhany Jun 14 '16 at 19:51

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