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Comments on Battery backup voltage regulator (an invention story)

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Battery backup voltage regulator (an invention story)

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Goals and objectives

Motivation. Inspired that there is a place on the web where simple but clever circuit ideas are encouraged, I decided to share another 1-transistor circuit trick. Besides, to make the story even more interesting than before, I decided to show, step-by-step, how the circuit was (could be) "invented"... as an example of an inventing procedure. Thus, visitors to the new platform will not only get acquainted with a simple but clever device but will also get an idea of how they can invent such devices themselves... and that would be great. So I have two goals - one is specific (1-transistor device) and the other is general (technology of invention).

Background. My story is based on an inherent BJT property that sometimes causes strange problems - when is fully on, the base-emitter junction actually connects, like a forward biased diode, the base to the emitter; thus the input voltage source is directly connected to the emitter. For example, this situation can happen in an emitter follower, if the emitter ceases, for some reason, to follow the base. Usually, this is an undesired situation but here I will show one of its useful applications in power supplies equipped with a backup battery.

History. This idea came to me in the early 90's when I had to develop a simple power supply for home burglar alarm systems (at that time, the electricity was often interrupted here and alarm systems had to be backed up; now it is mandatory). Although this was a long time ago, the idea is still relevant and worth considering. Here is a possible scenario of the "inventing" procedure...

“Inventing” the circuit

1. Unregulated power supply. If we supply the load directly by an unregulated power supply (consisting of a transformer, diode rectifier and filter) - Fig. 1, the voltage will vary for various reasons - because the mains voltage varies... or because the load varies... or both. For simplicity, I have designated the mains supply voltage BIN with the battery symbol.

Unregulated power supply

Fig. 1. Supplying a load by an unregulated power supply BIN (designated with a battery symbol).

2. Regulated power supply. So we have to somehow keep the voltage across the load constant. The best way to do it is like we do everything in life - to keep it equal to another but reference (desired) voltage VREF. For this purpose we compare them and change the voltage in the necessary direction until the difference between them becomes zero. This procedure is called (not very appropriate) "negative feedback".

Emitter follower

Fig. 2. Regulating the load voltage VL by means of the negative feedback (an emitter follower with a constant input voltage)

To implement it, we need a comparator and a regulating element. Both can be implemented by only one bipolar transistor - Fig. 2. The base-emitter junction is its (floating) input and the collector-emitter junction is the regulating element. The transistor "observes" the difference between the reference voltage VREF and the load voltage VL and changes its current until makes them (almost) equal. Figuratively speaking, the transistor acts as a "variable resistor" that changes its static resistance to change the current. This arrangement is known as "emitter follower".

The question now is, "Where do we get the reference voltage from?"

3. Battery backup voltage regulator. It is common to take it from the input unregulated voltage through a voltage divider of two resistors (bad) or a resistor and a zener diode (good). But here it turns out that we do not have a suitable Zener diode at hand... and then we come up with a simple "temporary" solution - to include a battery producing the voltage VREF - Fig. 3. Why not?

Battery reference source

Fig. 3. Using a battery as a reference voltage source ("Zener diode").

The battery provides the small base current needed to set the beta times higher collector current and, accordingly, the load current needed. As though, Bb tries to supply the load by injecting the small base current through the base-emitter junction to the load and Bin (T) "helps" it by adding beta times bigger collector current.

4. Battery backup voltage regulator. Here, too, chance intervenes and helps us make the "invention". Accidentally, the input voltage interrupts... but to our surprise, the load (alarm system) continues to operate at normal voltage. Why? Where does this voltage come from?

Battery supply

Fig. 4. An accidental invention of a battery backup supply - the battery supplies the load directly through the base-emitter junction.

Yeah, that is what it was - the battery voltage VB, which is transferred directly through the base-emitter junction as through a forward-biased diode. But wait... we can use this "failure" for something useful - to back up the supply voltage! This way we will not need additional diodes to switch the voltages.

This is a "2 in 1" solution since the single transistor does two things - regulates the input voltage and switches the voltages. Note that in the second mode (VIN failure) actually the transistor is not a transistor; it is used as a diode. The whole load current flows through the base-emitter junction acting as a diode. The battery Bb is also "2 in 1" since it serves both as a reference voltage source and backup battery.

5. Making the backup battery chargeable. The arrangement above works in this form as well. The base current is negligible and the battery will last maybe for a few months. But it is good to be rechargeable and to ensure its continuous charging. The simplest solution is through a resistor R - Fig. 5.

Charged battery backup voltage regulator

Fig. 5. Rechargeable battery backup voltage regulator

If you are afraid that the battery will overcharge, you can connect a Zener diode in parallel.

6. Charged battery supplies the load.

Charged battery supply

Fig. 6. The charged battery supplies the load directly through the base-emitter junction.

Conclusion

In this story, I have shown how another simple 1-transistor circuit is "invented". This is an example of how chance can help us come up with a new idea and its implementation.

This is also an illustration of the inventive "principle of combination" - two functions are combined in one transistor (regulation and switching) and two things in one battery (reference voltage source and backup battery).

battery BJT voltage-regulator circuit-design
posted about 4 years ago
1y ago by Lorenzo Donati‭
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Circuit fantasist‭
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1 comment thread

General comments (24 comments)
General comments
Olin Lathrop‭ wrote about 4 years ago
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It is hard to see the circuits from your schematics with all those lines in them. There is just too much clutter all around.

Circuit fantasist‭ wrote about 4 years ago · edited about 4 years ago
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@Olin Lathrop‭, I use simple conceptual circuits with a small number of generic elements. This allows me to use the rest of the space to visualize the invisible electrical quantities voltage and current. My goal is to reveal the idea and explain the circuit in order to deeply understand it. Some draw voltmeters and ammeters for this purpose; I prefer more abstract voltage bars and current loops colored red and green. But these are only means; the end result is only important.

coquelicot‭ wrote about 4 years ago
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A tinkerer solution, probably not standard EE. But that's what I am, after all, a tinkerer. So I give you a +1 for the simplicity of this circuit. I would have done it in a somewhat "more standard" way with the battery connected to the Vcc+ via a diode, and the base of the transistor driven by an R-zener as you've evoked previously. A bit less simple but perhaps more logical.

Circuit fantasist‭ wrote about 4 years ago
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@coquelicot‭, You probably understand that here I wanted to show something more original than usual. BTW the input source may be weak since the alarm system does not normally consume much current. But when the bell turns on and consumption increases sharply, the backup battery turns on in parallel and helps the weak input voltage. I also remember I had come up with an original circuit for AC power supply through the bell above the apartment front door so there was no need for additional wires.

tlfong01‭ wrote about 4 years ago · edited about 4 years ago
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@Circuit fantasist, I can barely follow your story. There are some confusions: (1) What exactly is the thing labeled "Bb"? If in your final schematic, is it true that there is no longer any Bin, and is now moved to Bb? (2) Your circuit seems to work, but there is one big problem: The load current now flows through R, which of course wastes power, and (3) Can you give an example of how to calculate the value for R? (4) You can use my example: (a) Battery = 12V, Load = 10 Ohm, I ~= 1.2A.

Circuit fantasist‭ wrote about 4 years ago · edited about 4 years ago
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@tlfong01, I will answer you but later because after a while I will have to start an online exercise with my favorite students on how to measure the diode IV curve. They are IT beginners and I will have to discuss exactly the questions you ask. If you don't mind, we will come in here and try to answer your questions:) You can get an idea of how I conduct these excersises from https://www.researchgate.net/post/How_do_we_investigate_semiconductor_devices_in_the_educational_lab.

tlfong01‭ wrote about 4 years ago
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@Circuit fantasist, no hurry and no problem at all. Actually I forgot most of the regulated power supply I learned ages ago. But I still remember at that time I used a NPN BJT called 2N3055 to step down voltage and of course wasted a lot of energy, in this "linear" regulator power supply. Your battery regulated PSU is not I have not thought off, so need to think about it for a while. Cheers.

Circuit fantasist‭ wrote about 4 years ago · edited about 4 years ago
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@tlfong01‭, Thanks for the response; it means a lot to me. I have conducted an interesting on-line exercise using various web resources including Codidact. We considered this story as an example of how a BJT can act as a diode. Bb means "backup battery". Bin is the input unregulated power supply. In the "final schematic", when there is no Bin, Bb begins performing its role. Usually, the return current can not enter Bin since there are backward biased diodes inside it...

Circuit fantasist‭ wrote about 4 years ago · edited about 4 years ago
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... Also, R is high enough and the current and wasted power should be small. I am not a fan of calculations but I think R could be defined as follows: R = (Vin - Vb)/(Ich + IL/(beta+1)).

tlfong01‭ wrote about 4 years ago
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Thank your for your clarification. Your answer seems to have added one more confusion: "Usually, the return current can not enter Bin since there are backward biased diodes inside it...". What do you mean by "backward diodes inside the "battery"?

Circuit fantasist‭ wrote about 4 years ago
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@tlfong01‭, Thanks for the response. It is strange why others keep silence... Browse the web for diode rectifiers (e.g., https://en.wikipedia.org/wiki/Diode_bridge) and you will see that, in this case, the diodes inside the rectifier are backward biased... and they stop the return current.

tlfong01‭ wrote almost 4 years ago · edited almost 4 years ago
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(1) It is strange why others keep silence - I don't feel strange because I understand you and the EE SE Q&A situation better. (2) When I asked about the backward biased diode inside the battery, you diverted to bridge rectifier. It appears that you mind is hopping randomly, ... (3) When I worried that the series R in your invention might waste power. you replied that R is high resistance, therefore loses small power - strange, are you sure you know what you are talking about?

Circuit fantasist‭ wrote almost 4 years ago · edited almost 4 years ago
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@tlfong01,‭ BIN actually represents a diode rectifier (although it can be battery as well). The diodes inside this rectifier would be backward biased by the backup battery voltage Vb applied through the resistor R. Really, R will waste power... but what do we do? We can only decrease the difference between VIN and Vb... to some extent. The current flowing R is small since it only charges the battery (the base current is negligible). Most power is wasted in the transistor.

tlfong01‭ wrote almost 4 years ago
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@Circuit fantasist, Most power is wasted in the transistor.. This is another confusion. I thought you said that Bin is replaced by Bb. So the current goes first to R, before to the transistor. In other words, power is wasted in both R and transistor. If your final version does NOT show Bin but only Bb, perhaps there will not be any such confusion.

Circuit fantasist‭ wrote almost 4 years ago · edited almost 4 years ago
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@tlfong01‭, Thanks for the attention to my humble "invention". "Bin is replaced by Bb" is said in the sense that, when the mains supply fails, already Bb but not Bin supplies the load. Bin stays at its place but it does not produce power (that is why it is drawn in light gray).

tlfong01‭ wrote almost 4 years ago
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@Circuit fantasist, Thank you for your clarification. So (1) Bin is not a battery, but an unregulated power supply consists of (a) AC transformer, stepping down AC mains from say 200VAC to 12VAC, (b) Bridge rectifier to convert 12V AC to 12V DC, (c) a big decoupling capacitor, perhaps? (2) Bb, on the other hand, is a real battery, and might be chargeable. Good, I got a better picture now, though not all the details. Thank you for your help again. Have a great weekend. Cheers.

Circuit fantasist‭ wrote almost 4 years ago
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@tlfong01‭, Exactly! (1) is true... It is an unregulated power supply consisting of (a) + (b) + (c) cascaded... I frequently use the battery symbol to represent a voltage source... And (2) is completely true. I will write you a message later in the chat...

tlfong01‭ wrote almost 4 years ago
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@Circuit fantasist, OK. So I understand ‭your "Battery backup voltage regulator". I also agree with you saying that your students understand you story. However, there is a big problem. I think most first time readers won't understand you story for at least two reasons: (1) You are using misleading names and symbols, eg, battery symbols to represent (AC to transformer to rectifier to capacitor) unregulated power supply. (2) Using real battery as backup, but not mentioning it beforehand.

tlfong01‭ wrote almost 4 years ago
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That is way I mentioned that you are diverting often, and you mind is hopping randomly around. If you are giving a lecture in class, I think there is no problem, because your students can raise their hand ask you to clarify, or ask classmate sitting next to clarify. But as a paper in the Q&A/forum/paper site here, it is almost no one will understand you. Of course your schematics cause extra problem. This is a serious communication problem. You might like to comment before I suggest something.

Circuit fantasist‭ wrote almost 4 years ago
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@coquelicot, can you explain to me what does "tinkerer solution" ("tinkerer") mean? I guess but I am not sure...

coquelicot‭ wrote almost 4 years ago · edited almost 4 years ago
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@circuit fantasist. That means a man building things not rigorously, in order for it to work in the given case, but not according the EE rules of the art. A good electric engineer would never have done that this way. /continued

coquelicot‭ wrote almost 4 years ago
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For example, regulating the voltage with the battery as a voltage reference is bad in general, and using the base of the transistor to pass the backup current is bad design as well. Also, charging the battery this way is bad. At the price of few basic added components, you can do everything right. Your circuit may perhaps be relevant if you want your item be very cheap, for very tolerant loads. Tinkerer solutions are pleasant sometimes, that's why I've upvoted it.

Circuit fantasist‭ wrote almost 4 years ago
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@coquelicot, Thanks for the comprehensive explanation. I imagined something similar but now it is quite clear to me. As you probably noticed, in my circuit stories I try to show non-standard circuit ideas. My goal is to arouse interest in creative thinking leading to invention of circuits. In this story, I show how we can use the situation in an emitter follower when, for some reason, VCC drops…

Circuit fantasist‭ wrote almost 4 years ago
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... So far, the transistor is able to resist the input voltage through its emitter voltage. Now the latter disappears and the input voltage is applied directly through the base-emitter junction to the emitter (load). In some cases, this is undesirable... and in others it can be useful. BTW we can observe the same phenomenon in the circuit of the capacitive multiplier when the input voltage drops.