Part 3: The Diode Dropper.

This was a very popular scheme with British and European TV sets during the 60's and 70's. The idea seems very simple at first glance. Ignoring D2 for the moment, it can be seen that because of D1, the heater string is presented with a half wave rectified sine wave. What this means is that D1 provides the circuit with half the power that would otherwise be applied. As a result, the heater dropper, R, dissipates much less power than in the conventional circuit. In this circuit, only the positive half of the mains cycle is used for heating, but it is immaterial what polarity is used in terms of the heater operation.

Let's have a closer look at how the circuit works with these waveforms:

The upper waveform shows the 240V 50c/s mains supply. Since the 240V is an rms value, the peak voltage is 240 x 1.4 = 340V. And because the frequency is 50c/s, one cycle takes 20ms.
Now, when the diode D1 is introduced in series with the supply, the voltage across Vh is shown in the lower waveform. Half of the sine wave has been removed, which means a resistive load (the valve heaters) will receive half power. The diode itself has a very small voltage drop; typically around 700mV. At a typical current of 300mA, the diode dissipates less than a quarter of a watt. It can be seen that we have halved the power to the load, without actually dissipating any significant heat in doing so.

A common misconception is that the rms voltage at Vh will simply be half of the mains supply. Nothing could be further from the truth, and circuits designed thus will result in valve damage. It's the power that's halved, NOT the voltage. Why is this so you might ask? Think of a resistive load connected to the 50c/s sine wave mains supply, like a light bulb. The entire power to light the bulb comes from the area under the curve of both the positive and negative half cycles, each of which take 10ms to go from 0 to peak. Thus one complete cycle takes 20ms.  It's the power that makes the filament glow, not just the voltage or current. Now, if we chop off one of these half sine waves, then obviously the area under the curve over the 20ms period is halved, and thus the lamp is fed half power. A diode is the ideal device with which to do this, and in fact "light bulb savers" were once sold on this principle. They were an adaptor containing a diode that could be plugged into a light socket. Hairdryers and brush motor power tools also use the scheme, with a diode connected in series when half power operation is desired.

Having established the load is fed half power by introduction of D1, we need to work out what Vh will be before calculating the value of the heater dropper resistor. As it happens, Vh only has to be calculated once; it is determined solely by the mains supply and not anything in the rest of the circuit.
1. As an example, lets say we have a 240R resistor across the 240V mains (R) . Current will be 1A (I) , and power 240W (P). Simple electrical theory there.
2. Now, insert a diode, and power will now be 120W, as we have halved the power by removing one half of the mains cycle.
3. Using the formula for power, P=I^2 * R, we can then determine what the rms current will be:
                                                  120=I^2 * 240
                                                  I^2 =120/240
                                                   I^2=.5
                                                      I=.707A

4. Now to calculate Vh; using V=I*R,
                                                  V=.707A * 240 ohms
                                                  V= 170V
You can use any load and any supply voltage, and you will find that Vh is always the supply voltage mutliplied by 0.707.

We can clearly see that for a 240V mains supply, Vh will be 170Vrms, NOT 120Vrms. Note that ordinary voltmeters will give an erroneous reading, due to the unusual waveshape, and presence of DC. Either use a CRO and calculate the area under the curve, or a true rms meter if you want to actually measure it. See the notes below on so called "true rms" meters.
For those that use 120V mains, Vh will be 120 * .707 = 85Vrms.  I've seen two examples on the internet of American restorers replacing the line cord resistor with only a diode, thinking the heater chain will be fed from 60V. Sadly, the valves in those sets are destined for early failure. Not convinced? You can easily demonstrate this by connecting a mains light bulb first through a diode, and then to a variac set at half mains voltage. The lamp fed via the diode is somewhat brighter isn't it?


Now that we know Vh is 0.707 of the mains voltage, the heater dropper (R) is calculated in the usual way.
The advantage of this circuit can be illustrated with a simple example. A television has a heater string requiring 150V at 300mA. For the conventional circuit with no diode, the resistor for 240V operation is 300R and dissipates 27W. Introduce the diode, and the supply becomes 170Vrms. Now the resistor is 67R and dissipates only 6W. If you're lucky and the heater voltages add up to 170V (240V supply), or 85V (120V supply), the scheme is at its most efficient, since no dropper resistor is required.
 
Mains Supply Voltage Vh
100 70.7
110 77.8
115 81.3
120 84.8
220 155.5
230 162.6
240 169.7
Vh calculated for typical mains supply voltages.

Measuring the Voltage.
Having ascertained that the resultant voltage cannot be read with an ordinary voltmeter, do not assume that a so called "True rms" digital multimeter will either. It seems that "True rms" multimeters cannot accurately read a non symmetrical waveform.
The following experiment will illustrate this. Four meters were connected across the mains. The AVO and UniVolt DT-830 are ordinary meters which respond to average voltage on their AC ranges, but display it as the equivalent rms sine wave value. The Meterman 85XT and Digitech QM1552 claim to be "True rms". As you will see, this is not entirely true.


Connected directly across the mains supply, the results are shown thus. The AVO shows about 230V.

Allowing for differences in meter calibration, the readings were as expected. Note that in the modern day, the mains waveform is not a pure sine wave. Loading by switchmode power supplies means the waveform is slightly flat topped. So it not surprising the two "True rms" meters show similar readings to each other, but different to the two average reading meters.
Next, a diode was connected in series with the mains to a 15W light bulb, to simulate the circuit of a diode dropper in a series heater circuit. The voltage across the light bulb was measured.


None of the meters show 170Vrms or anything close to it, when the diode is introduced. In fact, the three digital meters read much the same, despite one not being true rms.

Interestingly, neither of the True rms meters showed 170V (or 165V with the 234V mains previously measured). In fact, all the digital meters, including the average reading DT-830 show around 130V. The AVO shows about 112V. We can now see the trap of using a "True rms" meter to measure the voltages in a diode dropper circuit. The valve heaters would be overloaded if such a meter was used to determine the dropper resistor value.

So, what is the problem? It seems "True rms" is not true rms at all, at least with consumer or technician grade meters. It appears that "True rms" only applies to symmetrical waveforms. Connected to the output of a square wave inverter (i.e. symmetrical waveform), the two "True rms" meters both read much the same voltage, which in turn was different to the average reading DMM.


Connected to a square wave supply, the AVO reads about 242V. This and the DT-830 show a higher voltage than both the "True rms" meters.

From this we can at least see that the "True rms" meters are not merely ordinary meters labelled thus. There is something genuinely different in them. Nevertheless, they still can't read asymmetrical waveforms, and are not suitable for testing diode dropper circuits. In fact, a real true rms meter should be able to read DC, since rms represents heating power. Neither of the meters tested can do that.
In summary, if a meter does not show 0.707 times the supply voltage when a diode is introduced, it is not really true rms. You'll have to just accept that the supply to the heater string is 0.707 times the supply voltage when a diode is introduced, and make calculations for the resistor from that.

Problems:
Diodes do fail, and when they do it's almost always in short circuit mode. This means the valve heaters would be over run, and probably without the user realising it. This is where D2 comes in. Should D1 fail short circuit, D2 will conduct when Vh goes negative and blow the fuse (F). Surprisingly, I had not seen D2 in any commercially made circuit, until I stumbled upon this article in Television, for August 1971:


The diode dropper had been in use for a few years, so it is strange that it took this long for someone in the TV manufacturing world to come up with using the additional diode for protection.
Instead, most British TV sets used other schemes to alert the user of D1 failure, but they didn't actually protect the heaters. Some methods are described in the above article. Another method was a reverse biassed diode between the heater string and the audio output valve cathode, which was not bypassed by the usual electrolytic capacitor. If D1 failed, 50 cycle AC would be fed into the audio stage making it impossible to listen to. This method was used in the Thorn R2M chassis. Either way, the set would be made unwatchable, and the owner would hopefully switch it off before the valves and CRT were damaged. It can be imagined however, that where the set was left running unattended, damage would be done. Some set owners continued to run faulty sets anyway, as long as some semblance of image appeared on the screen, or just to hear the sound. Yet, just the simple addition of D2 would have completely solved the problem.

It is sensible to include a small capacitor, say .01uF 1kV, across the diodes to minimise high voltage spikes damaging them, and possibly a mains VDR across the supply straight after the fuse. Most restorers and manufacturers don't bother with D2 to their possible detriment. For typical heater strings of up to 600mA, D1 and D2 can be 1000V diodes such as 1N5408, and the fuse 1A.

One disadvantage with this scheme is that it loads the mains supply asymmetrically, introducing a DC component, which can saturate transformers and cause electrolysis in the mains distribution system. In practice, with a domestic radio or television set fed from the public mains the DC component will be insignificant, since it is tiny fraction of the overall power of the local stepdown transformer. Where it may be problematic is with 120V equipment run from 240V via a small stepdown transformer. A rough rule of thumb would be to rate the transformer at twice the stated power consumption.
Inverters, particularly vibrator types, might not like the assymetical loading either, but at least this time the power factor isn't reduced, and the supply frequency is irrelevant.

In Australia, the supply authorities disliked TV sets with half wave rectifiers because of the DC component, and is one reason why there were very few here, apart from the general Australian phobia with live chassis construction. The method of earthing used here (MEN - Multiple Earth Neutral) means that depending how good the consumer's neutral connection is, and what the voltage drop is across the neutral suppy wire, more or less neutral current actually flows through the mains water pipes. Obviously, if the pipes are at a DC potential to earth, corrosion will result.
One wonders what it was like in the UK during peak viewing time with all those live chassis TV's taking not only a 300mA bite every half cycle for the B+, but when diode droppers were introduced, this was doubled. It surprises me so many sets were made like this, when in actual fact it is possible to largely cancel out the asymmetrical loading. It's true that the earthing system used in the UK is not so problematic with the scheme, but one must wonder about so many sets running off the local stepdown transformer.

If we reverse D1 and D2, the heaters draw power only on the negative cycle, and the B+ only on the positive cycle. And, as European series heater TV valves draw 300mA, then the loading is much more even. The last series heater live chassis TV set sold in Australia, the Thorn R2M did just this. I discuss the TV set power supply problem with the Ekco TX287 article.

Of course, the diode dropper works on AC only. Fed from DC mains, and depending on the polarity, the heaters would either not work at all, or be over run. D2 would provide no protection in this case. However, in the present day, the chance of plugging into DC mains is so remote this possibility can be discounted - except when powered from a switchmode inverter with a certain fault condition.

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