This is another in my collection of Terado inverters. Again, it's an ebay purchase bought for minimal cost and looks like it's had very little use.
Input is 12V DC and output is 110V 60c/s at 75W continuous or 100W intermittent. Like the other Terados, this one is very compact.
The circuit.
As far as the primary side of the circuit
goes, it is completely standard. A series driven 60 cycle vibrator is used;
Terado type 1007. As this is a 6 volt type, a 22R resistor is in series
with the driving coil, for 12V use. It appears that Terado might have wanted
to minimise their vibrator types by simply using a 6 volt version in all
models. Some RFI filtering is provided by the .01uF ceramic condensers
across the vibrator contacts.
Input filtering is by a choke and 1uF
paper condenser. A 20A fuse is provided on the input. 10A would be more
appropriate, as allowing for inverter inefficiency, 100W could be drawn
without blowing the fuse.
The secondary circuit is unusual. When
tracing out the circuit I noticed one of the buffer capacitors connected
to an otherwise unused transformer connection. The other buffer capacitor
was connected across the 110V output socket as one would expect. Tracing
out the transformer connections, and measuring the voltages, showed that
the secondary terminal to which the second buffer was connected provides
350V. The obvious reason for doing this is to avoid having a physically
large buffer capacitor - which would not fit into the case. As the reflected
capacitance decreases by the square of the turns ratio, by providing a
higher voltage tapping the buffer condenser value can be significantly
decreased. The catch is that its voltage rating has to be increased, but
it still ends up being physically smaller.
Having this 350V overwind gave me an idea.
Looking at the gauge of the winding wire, it appeared to be the same as
that used for the 110V section. So, with a rearrangement of connections,
it should be possible to get 350 -117V = 233V.
A simple reconnection of the output socket obtains 230V from this
inverter.
Indeed, a 240V light bulb connected between
the 110 and 350V connections lit up to normal brilliance. It proved the
point, but for the sake of originality I didn't want to permanently modify
the circuit. Given my collection of U.S. appliances, I wasn't too keen
on having a two pin socket producing 230V. One could inadvertently plug
something in assuming it was only 110V. As it is, I have more than enough
240V inverters.
It's worth noting for those who like playing
around with inverters, that further adjustment of output voltage can be
had by connecting the transformer secondary in series with one or both
halves of the primary. Connecting it in phase will increase the output
by about 12 or 24V, and out of phase, the voltage will be dropped by the
same amount.
Above the chassis is mostly taken up by the transformer.
There was little restoration required. The buffer condensers were replaced and the input cable also needed replacement. A two pin polarised plug was fitted to the end for use on my domestic 12V supply. The vibrator, being series drive, worked straight away. Because of the crimped can construction, I have not attempted to open it.
The yellow wire running into the blue condenser is the 350V terminal.
"Tuffy Terado" had good advice but I wonder how many users actually
took note. I will never forget the angle grinder I once saw being used
on a vibrator inverter - because "it works".
It is doubtful that anyone took note of steps 1 and 3. Noticeably
absent is a recommendation to always use the inverter under load.
This is typical of the Terado advertising in the 1950's and 60's.
As can be seen, they had a multitude of inverter models.
Performance of the inverter is like any
other properly designed unit. The no load current at 12V is 3A which is
rather high, but again demonstrates that the inverter should be used loaded.
Then, efficiency improves markedly. As an experiment, I found that changing
the 350V buffer condenser to .47uF dropped the no load current at 12V to
1A. However, once the secondary was loaded by more than about half, the
output waveform was showing undesirable characteristics. With the correct
1uF, the waveform reverted to normal.
Without opening the vibrator to inspect
contact size, it does seem unusual that the vibrator appears to only have
a single set of contacts. For an inverter of this size, I would expect
at least a dual interrupter type. Of course, it is possible that it is
dual interrupter but with the contacts parallelled inside the vibrator.
