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
- Specify the maximum current to be limited Imax (for example 2 Amps)
- Specify the voltage source needed by the load Vs (for example 12 volts)
- 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).
- Compute the Q1 base current Ib at maximum load current, approximate with Imax/QHfe (for example 2A/30=66.67 mA.
- 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.
- 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).
- 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).
- 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.
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September 24th, 2008 at 10:28 pm
I teach community college so rewrote part of your piece to fit their level. Thanks for the post. It was very helpful. You can use what I did or not. Thanks again. Also labeled B C and E on the diagram for them.
Current limiting circuits can be confused with current/circuit breaker. Unlike a fuse that breaks a circuit connection, a current limiter limits the current (to the designed value). Current limiting circuits can be as simple as a single resistor but here I present an active current limiting circuit. With a resistor (a passive current limiter) its voltage drop increases with increasing load current. The higher the load current the higher the voltage drop on the resistor, resulting in a decreasing load voltage. This is usually undesirable.
Active current limiters have two major advantages over fuses and current limiting resistors. First, they are much faster acting than fuses so do a better job of protection (not to mention that they don’t have to be replaced). Second, they do not cause a reduction in load voltage.
Figure 1. Current Limiting Circuit
In this active current limiting circuit, the load voltage remains constant when load current is within the allowable range. It is also desirable that the limiting circuit not dissipate additional power, which means almost all the power is delivered to the load. But when the load draws more than allowed, the current limiting circuit begins to act as a resistor. The value of resistance increases to limit current to the designed level.
Without the current limiter, the voltage source in Figure 1 is directly connected to R load. R load here represents a resistance that draws a variable current. Batteries under a charge or an amplifier circuit are examples.
How the circuit Works
See Figure 1. The key to this circuit is that emitter followers (Q1 and Q2) act like a switch, turned on and off by voltage at their base. When base voltage is high, transistor current is high and it acts like a closed switch. When base voltage is low, there is no current flow – switch off.
Normally the base voltage of Q1 (controlled by R bias and V ce across Q2) is sufficiently high for Q1 to operate in saturation as a closed switch (I ce is at maximum). This is because R bias and the V ce across Q2 act like a voltage divider, where the higher the resistance – the higher the voltage across that resistor. When Q2 is turned off, it has a high voltage drop (acts like an open switch). The open switch condition of Q2 – V ce, when compared to the R bias voltage, is so large that Q1 is turned on by the large V b of its base.
How load current is controlled is the function of the R sense resistor. Its value is small, so under normal conditions load current does not create an appreciable voltage drop. This means that full current and voltage are presented to the load. But when current approaches the maximum design limits, load current through R sense is sufficient to develop a voltage at the base of Q2. This voltage is large enough to begin to turn Q2 on. Because of the current gain of Q2, this transition from an off to an on switch is rapid.
When the Ice current through Q2 increases, its current must come from the source and that effectively shunts current from R sense while increasing the voltage drop across R bias. Two things begin to happen. The voltage drop across Q2 (V ce) falls rapidly and the voltage drop across R bias increases. This reduces the base voltage of Q1, turning it off and closing the Q1 switch. When the Q1 switch is closed, current flow to the load stops. Transistors are capable of operating millions of times a second, so the transition from off to on actually controls the load current at a level set by the design value of R sense.
The remainder is unchanged.