Power Supplies And Their Capacitors.

Power Supplies And Their Capacitors ~

By Bill Jones

The circuits used for smoothing rectified alternating current (AC) voltages have taken many forms over the years.  The early radios used batteries and it was not until the mid-twenties that AC voltage became available.  This AC was a new technology and welcome for radio because it was no longer necessary to buy expensive batteries.  The AC did present a problem because radio tubes require direct current (DC) for their operation. The radio tube filament did not necessarily require DC but the other tube elements did. The AC had to be changed or rectified to produce DC for the tubes.  The rectifier tube was well understood (diode) because it was the first type of tube to be invented. Since electrolytic capacitors were yet to be invented the filtering or smoothing of the rectified AC was done by using the paper capacitor along with large inductors.  The paper capacitor was physically large and heavy, plus the capacitance values were only in the range of two to four microfarads.  This capacitance value was limited by cost and the acceptable size of the radio. It was possible to obtain reasonable performance by using filtering techniques as shown in the Figure 1-C. below.  In Figure 1C the inductors L1 and L 2 were physically large, and heavy but necessary because of the paper capacitors. The inductor contributes greatly in the smoothing of the rectified signal and it was easy to create an inductance value that would lead to a relatively smooth DC.  The capacitor will reduce the ripples because of its low impedance and the inductor acts as a high impedance to the ripples.  

This article concerning the filter came about because of a 1928 Philco model 511 that I had in for repair. I do enjoy going over the circuits and trying to imagine what the designer had in mind as the radio was in the design stage.  The Philco 511 brought to mind some of the many circuit variations that the AC power supply has had over the years.  These variations were primarily caused by the improvements in the capacitor.  Of course the speakers were also a major consideration because better speakers produced a lower audio frequency response, and power supply hum is a fairly low frequency. With better speakers available it was necessary to improve the power supply ripple.   While paper capacitors could always be used it would require the addition of inductors to the power supply to reduce the ripple. The radio speaker went through many variations along with the capacitor.  The moving coil speaker with a field coil was invented very early before its time, and was not ready for use until the necessary tubes and power supplies were available.  The first production loud speakers used permanent magnets with the exponential horn, and these were used for some years.  

After AC power supplies became available radio tubes improved rapidly.  It was recognized that the moving coil loud speaker was a necessary addition to radio in order to obtain desirable audio response.  It was immediately noted that the field coil could be used for an inductor for power supply filtering. The loud speaker field coil was used in a number of ways in power supply circuits.  The speaker fields were even placed in the center tap of the power transformer thus providing a large negative power supply voltage that could be used in the first automatic gain control circuits.  This negative voltage was used on the cathode of the AGC tube and the plate of the AGC tube was set near zero volts and the plate would go negative when an RF signal was placed on the AGC tube grid.  These early circuits were complicated and difficult to service.

The loud speaker field was normally used as shown in the (B) circuit of Figure one.  After the permanent magnet moving coil speaker became available it was not unusual to find a filter inductor L 1 on the chassis (as in circuit B) with the filter capacitors being only around eight to twelve microfarads.  The electrolytic capacitor greatly improved the AC power supply by reducing its size.  The first electrolytic used a liquid electrolyte within the capacitor and the life of these capacitors was limited because the liquid would leak from the capacitor.  With the invention of the solid electrolytic, it quickly replaced the liquid electrolytic.  In its original form the solid type of electrolytic was still only around eight to twelve microfarads.  Volume-wise the solid electrolytic was in the order of five times smaller than the wet type.   I began to service radios in the mid-forty’s and always had to replace the wet electrolytic with a solid electrolytic.  The wet type was not available because it was recognized that the solid type was far superior.

Today we have much smaller capacitors available, and they can be in the range of one hundred microfarads with a high working voltage specification, and reasonably priced.  I am not thinking about the lower voltage computer type capacitors that are in the thousands of microfarads because, while impressive, they can be physically large and expensive.  It should be noted that I try never to change the appearance of an antique radio for improved performance.  When I encounter an old set that has had wet electrolytics I may open the can and place a solid capacitor inside.  The early wet capacitors were often in brass cans and they are very attractive when polished.

I have wondered what limits should be placed on the size of the electrolytic capacitors when used on the older sets.  Using the original size that was in the radio is always safe but I do enjoy a hum free radio.  I no longer keep the eight to ten microfarad capacitors in my stock.  I began to wonder if using larger capacitors could cause a problem with the rectifier tube. The rectifier tube has a large variety of specifications, but here I am interested mainly in the specified   peak current, and average current as well as short term  maximum peak currents,  and the so called  ”hot switch current”.  My concern is how a larger capacitor will affect the life of the rectifier tube.

Most of the radios I service are of a 1940 to1950’s style and use a filter capacitor connected directly to the diode rectifier cathode, i.e. the input capacitor as in circuit (A). For each AC cycle, this type of connection will cause large peak currents in the rectifier diode. These currents are large because the diode only conducts for a short period of time. During this conduction time, the large current peaks will flow through the diode to charge the input capacitor. The current peaks, when they are averaged, will be equal to the DC current drawn by the radio. The filter will then act as a means to average the rectifier tube output.  Very few radios will provide a perfect DC output voltage and there will be a small amount of ripple on the averaged DC, and the size of this ripple determines how good the filter circuit is.  It is not unusual to find a one or two volt ripple on the DC. Of course this ripple is necessarily determined by how the radio gain is distributed between RF and audio.  If the radio has  high gain audio circuits it will require a much better filter than those sets that use a low audio gain.  

Two of the most common rectifier tubes will be discussed here, the type 80 family and the 35Z5 family.  The type 80 tube has been used for many years and appears in most of the older AC transformer radios. The 5Y3 and the 5Y4 have the same specifications as the type 80. While the peak current allowed for these tubes is relatively high the DC current – or average current –that is allowed is much less. For our purposes we will assume some typical values that will cover most of the radios that use the type 80 family. We will assume a transformer output of 300 AC volts and a transformer resistance of 150 ohms to the center tap.  The output of the power supply will be 290 volts and provide 125 milliamperes from the full wave rectifier.  This will give an average current of 62.5 milliamperes per rectifier diode.  If a 20 microfarad capacitor is used on the rectifier cathode, the peak current will be 5.8times the average current per diode. Calculations are made using the Radiotron Handbook.  The peak current is then 362 milliamperes and this is less than the allowed 440 milliampere allowed (specified).

Increasing the capacitor to a 100 microfarad results in a peak current for the diode of only 375milliamperes and this is again less than the maximum allowed peak current. At first glance this is surprising.  Reference to the tables in the Radiotron Handbook shows the type 80 diode to have a large plate resistance of about 400 ohms. This large resistance, in combination with the 150 ohms to the transformer center tap will allow only a small increase in the peak current as the filter capacitor is increased. Larger rectifier tubes such as the 5U4 will give different results, as will smaller tubes such as the 35Z5. Tubes similar to the 35Z5 are the 35Z4 and the 35W4. These tubes most often use the circuit shown in Figure 1. (A).

The 35Z5 is a low resistance tube compared to the type 80. Since the tube is used in half-wave applications it will require a larger capacitor than the full wave circuit. However, it is used in small radios and most do not reproduce 60 cycles very well. The 35Z5 tube specification allows a 40 microfarad capacitor on the cathode. Calculations for the 35Z5 tube show that with 100 microfarads on the cathode the allowable peak current is not exceeded.  It is at the allowable maximum value of 100 millamperes DC current with 117 volts AC, without a pilot light. Most of the sets using this tube require currents much less than 100 milliamperes and the peak current specification would not be exceeded if a 100 microfarad capacitor was used. However, since the tube has a very low anode resistance (about 50 ohms), AC line transients can cause excessive peak currents and short-term AC power losses may present a problem.  It is usual to connect the 35Z5 plate to the pilot light tap because this connection gives full brightness from the lamp when the plate current flows through the lamp. In such case, an AC power loss for a moment will cause the radio DC voltage to approach zero and if the AC comes back on before the rectifier cathode cools down the peak current specification would probably be exceeded. It would be safer to use a smaller filter capacitor on the rectifier diode. Other means of protection consist of a resistor of around 30 ohms in series with the rectifier in the plate or cathode circuit and some sets use a type 51 pilot light instead of a type 47. Some of the AC/DC sets will have three filter capacitors. This is ideal because the input capacitor can be made small for tube and lamp protection and the output capacitors large for hum reduction.  The type 80 is specified such that it will allow a momentary high peak current but the 35Z5 does not appear to have such a specification. The type 80 has a “hot switch” specification that allows a current of 2.5 amperes for 0.2 seconds.

It is seldom necessary to use a capacitor much larger than 40 microfarads on the cathode of the rectifier of either the type 80 or the 35Z5. This size capacitor will normally reduce the hum to a reasonable level. If it does not, there is likely something else wrong with the radio.  In most cases, it is possible to use larger capacitors safely on the type 80 tube because of its high impedance. While calculations indicate that the 35Z5will allow a capacitor much larger than 40 microfarads, it is not recommended where there is a pilot light. Where there is no pilot lamp on the radio it is not unusual to see 50 to 80 microfarads on the cathode of the 35Z5.It is apparent that the type 80 tube family is quite forgiving of high peak currents, and also it is not likely to approach the allowed current specifications because of its high plate resistance.  The 35Z5 family of tubes is not so forgiving because of their low plate resistance. It is not difficult to injure this type of tube with a large input filter capacitor mainly because of its low plate resistance.

Questions or Comments? Please e-mail me at whnj@att.net  Thanks, Bill.