The Bland Radio shaver inverter is one
of a multitude of such devices which appeared in the 1950's by various
manufacturers. It would appear that electric shaving had suddenly taken
off in the post war era.
Shaver inverters are simply a low power unit of no more than 15W designed to operate an electric shaver away from the power mains; either from a car battery of 6 or 12V, or a house lighting battery of the same voltage, or 32V. Some people adapted their car radios to do the same job by mounting a 3 pin socket on the side and including a switch to connect the radio's power transformer to the rectifier valve in the normal way, or to the socket.
Being designed for a shaver motor only, there is no attempt at RF suppression in these inverters. Besides, a radio of the time, using a good deal more than 15W would not be suitable anyway. In this article, one of the models from Bland Radio will be examined. Bland Radio was a prominent radio manufacturer in South Australia.
Bland Radio inverter installed in the glove box of what looks like 1950's Austin A40.
Note the rather long name, "Vibrator Transformer Electric Shaver Supply Unit". The Philishave specified uses a brush type AC/DC motor.
I obtained two identical units from eBay around 2008. Both were in their original boxes and looked little used. One was missing the vibrator, but otherwise the units were in as new condition. Interestingly, the seller turned out to be an acquaintance of mine, and when he learned of this, the single unit being sold became two, and at no extra charge. Unfortunately, nothing was known as to their history. Both were 12V models, with an output of 240V AC at 15W.
Because of the 100c/s output, the shaver has to be a motor driven type (i.e. AC/DC).
In a lot of ways, there is quite a resemblance to the Ironcore inverter described here. However, the Bland Radio provides AC output and not DC, and as far as vibrator power supplies go, it is a little unusual in the circuitry.
Internal view of one of the inverters. The transformer is about the size of a medium size speaker transformer, but unlike typical vibrator transformers, it is not centre tapped. The on/off switching is by the earth pin on the shaver plug.
Like the Ironcore inverter, this one has the same kind of Clipsal 3 pin socket that relies on the earth pin of the shaver plug bridging the split socket pin, to switch the unit on or off. Thus, the inverter runs only when the plug is inserted. At this point it's time to look at the interesting electrical design:
Circuit of the Bland Radio shaver inverter.
The most unusual thing about the design
is that a split reed synchronous vibrator has been used, effectively as
a DPDT switch. Thus the 12V is constantly reversed in polarity before being
fed to a single primary winding on the transformer. Normally, the transformer
has a centre tapped primary, and the vibrator switches as per a SPDT switch,
using the transformer winding to constantly reverse the magnetic field.
It is an interesting question as to why this was done. Perhaps it allowed
for a more compact transformer, or one cheaper to make, with half the primary
turns that would normally be used. But, surely that cost would be offset
by the more expensive vibrator. Perhaps it was just to do something novel
and different. Electrically, there's nothing at all wrong with the scheme.
However, for high power it could be dubious because of four contact sets
switching the DC, rather than the usual two. The effects of any contact
wear or misadjustment could be doubled, therefore.
It should be noted that this is an excellent way to convert a 6V car radio to 12V without having to change the transformer. By replacing the vibrator with a split reed type, wiring it as above, and feeding it into the existing transformer, but ignoring the centre tap, the same output voltage will be obtained.
An Oak/MSP vibrator type V2511 is used here. In earlier times it was also known as a 65UH. It has a seven pin UX base and is of the split reed type. For more information on this vibrator, go here.
Normally, a split reed vibrator is used in a conventional synchronous rectifying circuit, but with the secondary reed isolated from that of the primary, this enables a negative back bias supply to be obtained.
And so, they were commonly used with battery radios using directly heated valves, to avoid the need for a separate bias battery. Car radios with their indirectly heated valves can use cathode bias instead, so the split reed vibrator was not normally used here.
The vibrator is a 6V type, so to enable 12V operation the driving coil is powered via a 20 ohm 2W resistor. Why not use a 12V vibrator? Probably because in the split reed world, 6V types are much more common. After all, most vibrator sets using directly heated valves operated from 6V. Nevertheless, there is a 12V type, V5948 which was commonly used in AWA Carphones.
Because the split reed vibrator is meant to be used in a synchronous rectifying circuit, it should be pointed out that the timing for what are normally the primary and secondary contacts are slightly different, with the secondary contacts closing slightly later, and opening slightly earlier than the primary contacts. In this application, the difference is unimportant.
This view shows the other inverter from the other side (note the vibrator is not the same). The two wires leading into the primary winding can be seen here.
The secondary circuit is conventional,
with a 240V winding feeding the output socket and a .047uF 1600V paper
condenser for the buffer.
One inverter had obviously been used because it had been fitted with alligator clips wrapped with insulating tape, whereas the other one just had bare wire ends. I found a replacement vibrator for the one that was missing, electrically cleaned the contacts in it, and the other vibrator, and both inverters worked straight away.
Output waveform at 240V fed into a 15W light bulb. Because of the resistive AC load, the buffer capacitor cannot maintain the slope of the waveform when the contacts are in their off period. However, the loading prevents destructive voltages appearing. Nevertheless, the contacts are now making and breaking when full current flows and in theory vibrator life would be less when used in an AC inverter.
Of course, the paper buffer condensers
still need to be replaced. In fact, within an hour one did break down which
was no surprise. However, it did give time to measure the output, which
was 260V into a 15W lamp with around 13V input, with the waveform
shown above. The buffer capacitance appears to be correct, with no spiky
overshoot in the waveform.
Inverter powering a 240V 15W incandescent light bulb.
Again, like all these units, the absence of a fuse is rather disconcerting. Apart from overloading the inverter with a high powered appliance, failure of the buffer condenser will also cause damage. It is true however with something like this, and that people had more of a technical understanding of how things worked back in the 1950's, the user would probably be aware that something was wrong. However, it's just bad design not to include a fuse.
Here we see an 11W CFL being powered. Because it rectifies the incoming mains supply, the 100c/s is not a problem. Older CFL's with iron cored chokes are not suitable.
Like the Ironcore, a modern day limitation
is that most low power appliances have only a two pin plug. These will
not switch the inverter on, so the way around this is to use a 3 pin double
adaptor or power board, short of changing the plug. Of course back when
this inverter was designed, two pin plugs were not fitted to appliances,
so it was a non issue. Note that if an earthed metal appliance was plugged
in, that it could be live at 12V, depending on input polarity. While not
a shock hazard, it could cause a short circuit to a car body for example.
In reality of course, shavers were not earthed, so again is a non
issue when used as intended.
In summary, it's a novel design that works well for what it was intended for, still with plenty of applications in the modern day.