On Thu, 29 Oct 2020 03:43:19 -0700 (PDT), "
wrote:


Do you think I'll need to limit the voltage across the buzzer (2 (/3)
diodes in series in parallel with the buzzer?) and that the existing
bell draws at least 25mA?

That would depend very much on how long your delivery individual holds down the button.

If it's anything like the Hermes delivery at 8 last night, too long
(or two many times). ;-(

But a couple-of-ohms, 1-watt resistor, or so, in parallel to the buzzer should give all the protection you need.

So if using a resistor in that role I really need to match the
resistance to the resistance of the solenoid to give me the right
voltage drop over the buzzer (potential divider), hoping the worst
case is not too much voltage across the buzzer and when the batteries
are on the way out, enough for the buzzer bit to still work?

If I had a suitably low value (ww?) pot, I could put that across the
buzzer then tune it for the best sound with the Ni-Mh cells (then
replace it with a fixed resistor etc)?

One watt is crazy-overkill, but cheap enough.

Why wouldn't a 'voltage' regulated solution be better, given how
little voltage we have spare to play with and the fact that the
voltage will change with time?

It looks like you can get 1.5V zeners so as long as they can manage
the current though the existing bell and the back EMF etc, wouldn't
that be the more subtle way of managing it all?

idea

I just stuck my DMM across the contacts on the bell push (at the bell
itself) and with a 6V battery supply I measure ~750mA (dropping to
~730 after a few seconds). That puts the bell solenoid resistance at
around 8 ohms (plus what I would get via the cabling etc).

If the buzzer draws 25mA at 1.5V then the buzzers resistance is around
60 ohms (I know that isn't ever going to be a pure resistor, unlike
the saturated solenoid) so adding that in series with the bell is
going to give me around 70 ohms and a maximum current of ~85mA.

That's roughly 5V across the buzzer and 1V across the bell.

Putting a 2 ohm resistor in parallel with the buzzer will give me just
over 10 ohms in total, so back up to around 600+mA, and so 1.2V across
the buzzer and 4.8V across the bell (and I know it works with 4.8V's
worth of rechargeables. ;-)

However, aren't we wasting energy in a resistor system, rather than
clamping voltage using a (say) a zener or is it the same thing?

Cheers, T i m