Exploring the Voltage Sense on Small wall wart SMPSs

RE: Exploring the Voltage Sense on Small wall wart SMPSs

shabaz — Sun, 28 Aug 2016 14:15:12 GMT

Hi John!

Great article! The TL431 is really cheap as you say, so is extremely popular. By the way there is also a less-popular part called TLVH431 which is a lower-Vref (1.24V) equivalent (it doesn't come in TO-92 package though).

RE: Exploring the Voltage Sense on Small wall wart SMPSs

jw0752 — Sun, 28 Aug 2016 02:22:54 GMT

Inspired by [mention:4f57fc9d538949ad9eb336ddb9469bb8:e9ed411860ed4f2ba0265705b8793d05] I decided to see if I could make some generalizations related to heat dissipation in this simple circuit involving a current limiting resistor and a Zener Diode.

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For the purposes of description of total energy dissipation of R1 and ZD1 I will call this circuit "The System". The energy dissipation of R1 will be called PWR1 and the energy dissipation of ZD1 will be referred to as PWZD1. For simplicity's sake I am going to assume that all components are linear over the voltage ranges involved and that their values are not affected by heating. This false assumption of course puts my results in the category of rough guesses at best.

I have run a series of calculations on this circuit using a fixed voltage for Vin and a fixed resistance for R1. From the results of these calculations I have extrapolated to a general model and I have found that the following generalizations can be made:

No. 1 For a given voltage Vin and a constant current limiting resistor R1 the Zener ZD1 experiences the maximum energy dissipation when its value is one half of Vin.

No. 2 Total energy dissipated in The System increases as the value of ZD1 decreases from Vin to zero volts. When the value of ZD1 = Vin the energy dissipation is zero watts and when the value of ZD1 = 0 the dissipation is (Vin)^2 / R1 watts.

No. 3 Maximum Current in the system is limited to Vin / R1 for all values of ZD1.

No. 4 Max Energy dissipation of R1 is (Vin^2) / R1 watts. or Maximum PWR1 = (Vin^2) / R1 watts

No. 5 Max Energy dissipation of ZD1 is (Vin^2) / (4*R1) watts. or Maximum PWZD1 = (Vin^2) / (4R1) watts

These of course are all simple results using Ohms law and related power calculation formulas. This exercise was needed to help me better understand some of my own questions after reading what [mention:4f57fc9d538949ad9eb336ddb9469bb8:e9ed411860ed4f2ba0265705b8793d05] wrote above in his caution to me above about heat and power considerations. Since I had them on paper I thought I would share them.

John

RE: Exploring the Voltage Sense on Small wall wart SMPSs

jw0752 — Thu, 25 Aug 2016 06:57:31 GMT

Building a 2.5 Volt to 35 Volt variable Zener Substitute for the Lab:

Hi Guys,

As a follow up on this project I saw the possibility to build a Variable Zener Substitute for my Lab. As I mentioned above the TL431 is a very versatile component. This simple little circuit will allow me to dial in any shunt voltage level between 2.5 volts and 35 volts with a ability to sink 100 mA. Here is the schematic for the device which is basically taken from the Data Sheet.

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When using the device it is important to size an external series resistor or use other current limiting means so that the TL 431 is not required to sink more than 100 mA. I noticed in the data sheet that there are several applications where the resistor divider to the TL 431's reference pin is not tied directly to the cathode and so in my circuit design I have installed a jumper that can be removed to separate the top of the divider from the cathode of the TL431. I also installed a cathode test point and an anode test point so it will be easy for me to attach a meter during my experiments. The resistor divider is a 20 turn 100 K potentiometer so I have quite fine control of the voltage. My best meter is 4 digits below 6 volts and I was able to control the TL 431 to single millivolts on this scale. Here are a couple pictures of my finished prototype.

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John

RE: Exploring the Voltage Sense on Small wall wart SMPSs

mcb1 — Tue, 23 Aug 2016 07:25:45 GMT

John

I'm sure you will find some form of driver chip that excites the transformer, and thus the stepdown function.

I suspect that the feedback will simply stop the driver chip, and therefore the system is regulated.

Very interesting

Mark