"GregS" wrote in message
...
In article , "ian field"
wrote:

"Jeff Liebermann" wrote in message
. ..
On Fri, 19 Sep 2008 17:34:40 +1000, Franc Zabkar
wrote:

On Thu, 18 Sep 2008 13:59:55 -0700, Jeff Liebermann
put finger to keyboard and composed:

On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
wrote:

To clarify the circuit, the postive side of the speaker receives 17
volts
thru one diode and a resistor and the negative side is connected via
transistor to ground and the transistor is driven by 4001 IC.

What value resitor?
What's the part number on the speaker?

I just happen to have the manual for a similar Heathkit GC-1005
"Electronic Clock" handy. No specs on the speaker (401-163). The
circuit is similar in that the speaker has one lead going to the
collector of an MPS-A20 and the other to 18.5VAC (not DC) through a
diode and 1200 uf to ground for a DC voltage of 25.7VDC. No resistor
in series.

So, let's do the math. My guess(tm) is that 1 watt will be
sufficiently loud to wake the dead. Yours has about a 15VDC swing.
Assuming a 50% duty cycle and a pure sine wave (yeah sure):
Power = E^2 / R
1 watt = 15^2 / R
R = 225 ohms.
So, it's probably a high impedance speaker of some sorts. I have the
digital clock and an LRC meter and could probably measure the
impedance. If you can't get the info any other way, bug me and I'll
rip it apart.

The GC-1107 supplies the speaker via a rectified 13VAC source and 150
ohm 1/2W resistor. That's a DC supply of 18V.

I believe maximum power will be transferred to the speaker if it has a
resistance equivalent to that of the series resistor, ie 150 ohm. In
this case, when the transistor is turned on, the current will be
18/300 = 60mA. Assuming a square wave signal with a duty cycle of 50%,
the power dissipated in the speaker will then be 9V x 60mA x 0.5 =
270mW.

Therefore I'm guessing that the speaker has an impedance/resistance of
at least 150 ohms and a power rating of at least 0.5W. If the
speaker's impedance were any less, then the dissipation in the
resistor would increase.

If we accept that the speaker should dissipate less than 270mW in both
clock circuits, then in in the GC-1005 case we have ...

Power(max) = 0.27 = 25 x 25 x 0.5 / R(min)

So R(min) = 1157 ohms

- Franc Zabkar

I like your calcs better than mine. That suggests that Heathkit would
have used something like a high impedance earphone "speaker" in the
design. That's possible and probably would work quite well.

However, I tore apart a similar Heathkit clock, with the identical p/n
speaker, and measured 41.5 ohms DC resistance. Adding the inductance,
that I didn't bother measuring, will produce about 50 to 60 ohms
impedance. Obviously, this is not the optimum power transfer design,
but that's what Heathkit apparently used. The important thing is that
a common 4, 8, or 16 ohm speaker will NOT work.

In this situation I'd probably nick a replacement speaker from an old
pocket
radio along with the O/P transformer, in which case the resistor could
probably be omitted without overstressing the driver transistor or
supply..

Actually, radios of that vintage are probably worth a bit, but many people
will have such transformers in the junk box, Maplin among others still
stock
the Eagle Electronics LT700 transformer that would do the job.


I remember ordering some 100 ohm paper cone speakers from Mouser many
years ago
for my Johnson walki-talkies. I know they still have some 100 ohm
speakers.

greg


The highest I've seen were 150 Ohm as used in the Philips EE kits, they were
driven class A by a single transistor - an AC128 in the EE20, the EE1003
used a BC148 - those horrible "lockfit" transistors which were quite large
for only 220mW, one of the two BC148s in the kit had a pressed steel 'heat
fin' that didn't fit at all snugly to the transistor and stayed pretty much
cold as the transistor got hotter and hotter.