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The Care and Feeding of LM317 and LR8 Integrated Circuit Regulators, Particularly in Valve Circuits

 Bruce Heran   USA Flag   To email Bruce, type out the email address.

The Care and Feeding of LM317 and LR8 Integrated Circuit Regulators, Particularly in Valve Circuits

This is the second mini text on subjects that continually show up in the forums. The first was on Grounding and Shielding. This one concentrates on how the LM317 and LR8 voltage regulators are used in some of the projects posted on the site. It does not cover all the uses of either type though. The regulators have a number of things in common and some distinct differences. Both are three terminal devices, input, output and adjustment. Both can be used as pass through voltage regulators and both can be used as constant current sources.

LM317 Voltage Regulators

The LM317 voltage regulator is available in several packages from a low power TO-92 size to a rare but very large TO-3 size. The most common size packages are the TO-220 types. There are a few different types of the TO-220 as well. The two of most interest in the posted projects are the standard LM317 and the LM317HV. The primary difference is the standard one is rated for a maximum voltage of 37 volts and the HV one is rated at 57 volts. Both types work the same and are very useful as regulators such as in providing DC heater power to small tube projects. Depending slightly on brand they can handle up to just under 1.5 Amps with a maximum dissipation of 15 Watts (with a suitable heat sink). I use the LM317 in projects that need a stable low noise heater supply. You need to keep in mind that in order to regulate properly the input voltage should be about 4 Volts above the desired output voltage. I also find LM317 regulators to be very useful in making constant current sources (CCS). Those of you familiar with any of my push-pull power amplifiers have undoubtedly noticed them in the cathode circuits of the output tubes. As a CCS you program the regulators differently from when used as a voltage regulator. As a voltage regulator you need to feed a fraction of the output voltage to a resistor divider with the middle point of the divider going to the adjust terminal. The voltage regulated (Vreg) is equal to 1.25 times the value of 1 plus the ratio of the lower resistor of the chain to the one going to the output terminal. Vreg=1.25(1 + R2/R1). When used as a CCS the LM317 becomes a two terminal device. The input is connected to the most positive end of the circuit and the adjust terminal to the most negative. The output terminal is connected to the most negative end through a programming resistor. The approximate value of current programmed is equal to 1.25 divided by the resistor in Ohms. The value will be in Amperes. For example a 10 Ohm resistor will set the current close to 0.125 Amperes (125 mA). When used either as a voltage regulator or CCS there is a minimum voltage necessary between the input terminal and the output one. This varies somewhat but a good estimate is 3 to 4 volts. If you go below this threshold the performance can be extremely non-linear. There is a bit of controversy about the noise performance of LM317 in audio circuitry. As with any device there is some noise associated with it. I have found that when used as a cathode CCS the actual noise is no worse than a resistor would be and really is so low that it is not a factor. These are robust devices, but it is possible to damage them. The most common way is over voltage. I have accidentally applied as much as 100 Volts DC and had some survive. That is not usually the case. When they do fail, it is nearly always as a direct short. In CCS applications if you measure across a failed LM317 (while in circuit) all you will generally see is the value of the programming resistor.

LM317 / LM338 / LM350 Voltage Regulator Calculator Circuit

Figure 1: LM317 / LM338 / LM350 Voltage Regulator Calculator Circuit

LR8 Voltage Regulators

LR8 regulators are available in three main styles designated by a letter and number suffix (K4, N3, and N8). They all work the same but carry different dissipation ratings and have different cases. They are LR8N3 - TO-92 - 740mW, LR8N8 - TO-243AA (SOT-89) - 1.6 Watts and LR8K4 - TO-252 - 2.5 Watts. I prefer the N3 for low power uses but its dissipation is only 740 milliWatts. The LR8K4 is the choice for higher power uses with a heat tab that is soldered to the PCB. The N8 version is about halfway in between the other two. All LR8 regulators are high voltage devices. They are rated at 450 Volts. They are hooked up the same as the LM317 regulator but the resistor values and programming formula are different. I recommend reading the information on the Supertex.com web site before coming up with anything above a simple circuit using these devices. The basic formula for use as a voltage regulator is Vreg= 1.20(1+R2/R1) + IADJ X R2. R1 and R2 are in the same locations as in the LM317 regulators and IADJ is the current flow through R2. I find these regulators greatly enhance the hum and noise reduction in power supplies feeding preamplifier tubes as long as the difference between the input and output exceeds 25 Volts. While these regulators have fairly amazing specifications for voltage, they are rather easily damaged. If you accidentally short one out or even attach a new capacitor of over about 1uF to the output terminal when it is powered up, there is a very high probability that it will be destroyed. Typically it will short through and act like a small value resistor between the input and output terminals. If you measure approximately the same input and output voltages on one when powered up it is almost certainly cooked. One thing that I have found helpful is to use a reversed polarity rectifier like a UF400X series diode from the input to output terminals. This prevents excessive reverse voltages on the LR8 (higher output side vs input side). The LR8 regulator also exhibit another behavior that can cause consternation. If the internal protective features of the IC determine that its dissipation will be exceeded, it will shut down and there will be little or no output. It will not reset until all the voltage is removed. It will keep doing this until the load side issue is resolved. This behavior can also occur if the filter capacitor on the output side is too large. It will sense the inrush current and think is it "a constant current" and shut down immediately. No harm will come to the IC, but this safety feature can really mess with your mind if you are not aware of it.

Integrated Circuit Regulators in Valve Circuits

The above information on solid state devices might seem a bit strange in the context of valve amplifiers, but they perform functions extremely well, that while possible, are difficult to duplicate solely by valves. Valve audio purists will cry foul, but using three terminal voltage regulators and constant current sources can easily turn a modest project in to an exceptional one. In my opinion, when all is considered, audio is about the listening experience, and ought not be all that concerned as the manner it is accomplished.

Good Listening
Bruce

About the Author

Bruce Heran is the VP for Design and Support of Oddwatt Audio. He has been involved in the field of electronics in one form or another for nearly 50 years. Bruce is a strong supporter of DIY audio projects and generally specializes in vacuum tube designs for Hi-Fi. One of his main goals is to educate and encourage new comers in the building of affordable high performance audio equipment. For more DIY audio building tips and techniques from Bruce see these articles:

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