Introduction

Current limiting circuit often misinterpreted with current/circuit breaker. Unlike a fuse that break a circuit connection, a current limiter only limit the current at a predetermined level. Current limiting circuit can be as simple as a single resistor, but here I present an active current limiting circuit. With a resistor (a passive current limiter) the voltage drop is varied depending on the consumed current by the load. The higher the current is drawn by the load, the higher the voltage drop on that resistor. In many cases, this is not preferable.

In this active circuit, the current limiting circuit try not to drop the voltage if the current drawn by the load is below the allowable range. With this mechanism, in normal condition, the limiter circuit try not to dissipate the power, so almost all power is delivered to the load. If the load try to draw more than allowed, the current limiting circuit will now act as resistor, controlling it’s resistant value to limit the current to a predetermined level.

Figure 1. Current Limiting Circuit

Without the current limiter, the voltage source in Figure 1 should be directly connected to R load. R load here usually something that draw variable current (equivalent to a variable resistor), can be a battery to be charged or an amplifier circuit for examples.

How Current Limiter Works

Look at the Figure 1, output voltage at Q1 emitter act as a voltage follower, means that the voltage will follow its base voltage. Because the R sense value is chosen to be a low resistance, the voltage will be appear at load as a full voltage delivered from voltage source. Actually there is a little voltage drop caused by Q1 Vbe (base-emitter voltage) and the resistor R sense, but this voltage drop can be neglected. If the load now draw more current, at some level, the voltage drop across R sense will reach the level at a point where the transistor Q2 begin to conduct, and the current will flow from its collector to its emitter, decreasing the base voltage of Q2. Because now the Q1 base voltage decrease, the voltage output of the Q2 emitter will also decrease as it works as a voltage follower circuit. When this output voltage decrease, the current to the load will also decrease. After this point of allowed maximum current, the more the load try to draw more current (by lowering its internal resistance equivalence), the lower the output will be delivered to maintain a constant current.

How to Design, How to Choose the Component Values for This Current Limiting Circuit

1. Specify the maximum current to be limited Imax (for example 2 Amps)
2. Specify the voltage source needed by the load Vs (for example 12 volts)
3. Choose a transistor that can handle the Imax and Vs (for example X-type transistor with Vce max=40V, Ic max=4A, Hfe at Imax 2A =30).
4. Compute the Q1 base current Ib at maximum load current, approximate with Imax/QHfe (for example 2A/30=66.67 mA.
5. Compute the R bias value. If the voltage drop across R bias is assigned as Vb, the Rbias=Vb/Ib. Here we find something that isn’t clear yet. The voltage drop Vb is something we have to choose. Vb is the voltage drop across R bias at the maximum allowed current Imax. Vb will determine the total voltage drop caused by the current limiter circuit at the limiting point. At the limiting point (just before the limiting is triggered), the total voltage drop caused by the current limiter will approximate the Vbe+Vb +Vsense. The limiter gives almost only Vbe drop if the current drawn by the load is very small. Ideally, the Vbe is chosen as low as possible, but it means that the Q2 could possibly need to handle a very high current in case a short circuit happens (R load = 0). Lets try to choose 1 Volt for the example of Vb, then Rbias = Vb/Ib = 15 ohm.
6. Find the lowest possible of load resistance (when the current limiting circuit works to limit the current as the hardest effort). It is actually a complicated task, but we can simplify the problem by assuming a sort circuit might be happen, so our design is really safe and the calculation will be simple. For the safety of Q2, choose Q2 that can handle current of Vs/Rbias Ampere (12/15=0.8 A in our example).
7. Choose R sense, as (Q2 Vbe)/Imax , Q2 Vbe is the minimum voltage drop of base-emitter Q2, a voltage level that needed by Q2 collector-emitter to begin conducting. (For example 0.65V/2A = 0.325ohm).
8. The voltage drop caused by this current limiting circuit will be Q1 Vbe at very low load current consumption, and approximate Vbe+Vb+Q2Vbe just before the current reach the limiting point.

Thats what I can write about current limiter circuit, and I use many approximations and assumptions in presenting design guide. If you find something wrong with my design method then please let me know.

The Circuit Schematic

On circuit analyzed today, we will analyze AC Line Powered Pilot Light circuit designed by David Johnson. Using the circuit, you don’t need a transformer to power a standard LED. As always, David Johnson makes his design simple and smart, but he doesn’t talk much about his design. Let’s discuss his design, the formula behind the design.

Main Components Functions

The capacitor act as a current limiter, equal to resistor in many LED design. Yes you can replace the cap by a resistor, but the current limiter will dissipate power and that’s not good. Using capacitor, you can limit the current without wasting the power.

The bridge diode function is for rectification, so the current will flow through the LED on both positive and negative cycles of the main voltage source. Without this bridge diode, the current won’t flow in both direction, and even won’t flow at all because the current limiter is a capacitor that block a DC current. You can use only a resistor and a LED in series to make a pilot light powered directly to line voltage, but you will see a rapid blinking light because the the LED will light only on the half cycle of the power supply.

How to Choose The Component Values

To choose the appropriate values for each components, you have understand how it works: the formula (sorry for those who don’t agree with the proposition that understanding the formula means understanding how the circuit works and vice versa).

The whole circuit can be modeled as a series of resistance/reactance. The formula of the capacitor’s reactance is

For 0.22uF, at 50Hz line frequency, the reactance will be -14469i ohm. That’s a complex number, and must treated with complex calculation. The current will be the source voltage divided by the total reactance of the capacitor, diode, and the LED. By assuming that the resistances of the LED and the bridge diode are much lower than the capacitor’s reactance, the current will approximate 110/14469A = 7.6 mA. Because the voltage source is normally stated in RMS (root mean square) value, the peak current will approximate sqr(2) 7.6 mA = 10.8 mA. Standard LED normally rated for 15-20 mA maximum current (mostly depends on its color), running the LED for 10.8 peak current is wise to keep the LED running for its specified life time.

Design Guide for Modification: Multiple Series LEDs

The circuit can be modified to make much brighter light by high power LED circuit as shown below:

Design Step:

1. Look at the LED’s data sheet, find voltage versus current graph. Choose a point where it will be used for the circuit, for example 2.6 Volt-18 mA (Vd-Id)
2. Find the total LED voltage by multiplying the Led’s operating voltage (Vd) with the total number of series LED (all LED are from the same type).
3. Find the voltage drop of the bridge diode (look at the current versus voltage at the point Id), if you can’t find its data then you can just simply measure it with a multimeter (the value would be slightly different because it use the meter’s operating forward current, but it’s OK).
4. Find the total voltage drop by the bridge diode and the LEDs by adding up 2-3.
5. Find the voltage that must be dropped by the capacitor Vc, Vc= Vs-(Vdtotal).
6. Find the capacitor’s reactance Xc, Xc=Vc/Id
7. Find the capacitor’s value C=1/(2.Pi.F.Xc)

In the step 5, try to use RMS value for Vs (110/220), then check if the 1.41*Id doesn’t exceed the maximum pulsed current, if it exceeded then use peak value of Vs at step 5. Any comments will be appreciated.

What do you think when you read the title of this post? Normally, in any measurement, noise addition to the signal acquired from a measuring transducer is the source of imprecision, then how can I say that we can also use a noise to improve the measurement precision? Yes, we can ad a noise to the acquired measurement signal to improve its precision.

Have you ever made repetitive measurement to obtain a measured value? You take the measurements for many times and then compute the averages of them. Usually, such multiple measurement to get more confident result is applied when a measurement is not precisely reproducible, so you’ll get confuse deciding whether you have to choose the value from the first or the second measurement. Just make multiple measurement and average it. If you get a same result from multiple measurement because you have a good enough instrument then it waste your time to repeat your measurement.

In analog-to-digital converter (ADC) , the precision of the reading depends on the bit resolution of the ADC design. Oversampling technique has been a common method to obtain higher resolution by averaging multiple reading. In the absence of noise, multiple reading will result in same values, making it useless to average them. The solution is by adding some noise to the signal that need to be measured by the ADC. Example of practical implementation of this technique, triangular dithering, is presented by Dave Van Ess in his article: Squeeze 10-Bit Performance From An 8-Bit ADC published in Electronic Design Magazine.

Triangle wave oscillator normally designed with a constant current source/sink to charge and discharge a capacitor. Reading an article on how to get 10 bit reading precision on 8 bit ADC on electronic design magazine today, I got something interesting on how to build a triangle wave oscillator: using two XOR-ed square wave oscillator. The picture below describe the concept:

Using this method, you can get a triangle wave oscillator with few logic gates plus some capacitors and resistors for square wave oscillators and simple RC-filters.

Source: Electronic Design Magazine