DE10242365B4 - Circuit arrangement for a LED array - Google Patents

Abstract

Circuit arrangement for a LED array with - two or more parallel to each other interconnected LED chains (chain x), in each of which at least one LED component (2) is arranged, with two or more LED devices (2), these serially interconnected with each other, wherein - the anode sides of the LED chains (chain x) are each coupled to the positive terminal of a supply voltage (UV), and the cathode sides are each coupled to the negative pole of the supply voltage (UV), wherein to each LED chain ( Chain x) in each case a control arrangement (RA x) for controlling a proposed current distribution to the individual LED chains is connected in series, and wherein the respective control arrangement (RA x) includes a bipolar transistor (Tx) whose collector terminal (Cx) each with the cathode side of the associated LED chain is connected and connect the emitter terminal (Ex) via an electrical resistance (Rx2) to the negative terminal of the supply voltage (UV) bar, wherein the base terminals (Bx) of the transistors (Tx) for balancing the current distribution are electrically connected to each other, and wherein a drive circuit, the base terminals (Bx) of the transistors (Tx) applied to a predetermined current, the drive circuit to the Positive pole of the supply voltage (UV) connectable, in the reverse direction to the supply voltage (UV) operated zener diode, which is connected to all base terminals (Bx) together.

Description

The present invention relates to a circuit arrangement for an LED array (an LED arrangement), in particular for a light signal device, with two or more LED chains (chain x) connected in parallel to one another, in each of which at least one LED component ( 2 ), wherein with two or more LED components ( 2 ), these are connected in series with each other, wherein the anode sides of the LED chains (chain x) each with the positive pole of a supply voltage (U) are coupled, and the cathode sides in each case with the negative terminal of the supply voltage (U _V ) can be coupled.

In LED arrays, different forward voltages of the individual LED components and the steep U-I characteristic of LEDs, which already causes a large current fluctuation with a small deviation of the forward voltage, lead to a deviation of the current from the nominal value in the individual LED chains of an array.

In conventional circuit arrangements of this type, in order to counteract this, for example, an electrical resistor connected in series with the series-connected LED components is provided in each LED chain. This resistance leads to a flatter U-I characteristic, which allows some restriction of the forward current in the individual chains. To achieve a corresponding accuracy in the power distribution to the individual LED chains, however, a relatively large voltage to the resistors is required, resulting in an energetically uneconomical overall system.

As an alternative solution to the problem outlined above, a very fine voltage grouping of the LED components is conceivable. However, the forward voltage of an LED is temperature-dependent, whereby in turn different temperature dependencies can occur between two LEDs. The grouping according to forward voltage is also very expensive, because a corresponding logistics and warehousing is necessary.

A change in the forward voltage on a chain, for example by shorting an LED leads to a strong redistribution of the chain currents even with a current setting by means of series-connected resistors.

The DE 198 04 891 A1 shows a circuit arrangement with four LED chains, two of which are connected in parallel to form a group. A transistor with an emitter resistor is connected in series to each of the groups. The base terminals of the transistors are connected to a control circuit.

The US 5 598 068 A shows an LED array with LED chains to which a transistor and a resistor are connected in series. The base terminals of the transistors are each coupled via a resistor to a potential node.

The US 5 278 432 A shows a circuit arrangement with an LED array. To each LED chain is a Darlington transistor whose base is coupled via a resistor to an operational amplifier, and a resistor connected in series.

The DE 100 17 878 A1 shows an LED array whose chains are connected in series with a resistor and transistors.

The US 6 161 910 A shows a circuit arrangement with an LED array. Each LED chain has a transistor and a resistor connected in series. The base current of the transistor is provided by an operational amplifier.

In 197 49 333 A1, an LED array is shown in which power sources are connected in series with LED chains.

The object of the present invention is to maintain a current distribution provided for the LED array as much as possible even when the forward voltage in the individual chains changes, for example during a short circuit of an LED component.

This problem is solved in a circuit arrangement of the type mentioned in each case with the features of claims 1 and 2, wherein each LED chain (chain x) each have a control arrangement (RA x) for controlling a planned power distribution to the individual LED chains in Series is switched.

In each case, a combination of a transistor Tx with a resistor Rx2 (in the emitter branch) is connected in series to the x LED chains in order to maintain a planned current distribution on the individual LED chains x. Where x denotes a running index of 1 to the maximum number of LED chains.

The resistor Rx2 influences the current distribution in the individual LED chains. The resistance Rx2 is inversely proportional to the associated collector current.

Depending on the embodiment, a corresponding drive circuit supplies the base terminals Bx of the transistors Tx with a predetermined current. Depending on a drive circuit, a bipolar transistor and an emitter resistor form a current source, which determines the current in the respective branch. This power source forms in each case a control arrangement RAx.

In addition, all the base terminals Bx of the transistors Tx are set to the same potential.

First, unclaimed embodiment:

The actuation by the drive circuit takes place once in that the drive circuit acting on the base terminals Bx of the transistors Tx with a predetermined current is provided in each case as a series connection of a diode Dx and an electrical resistor Rx1.

The diodes Dx and the resistors Rx1 allow the provision of a necessary for the function of the electrical control arrangement power supply of the respective transistor base. As the difference between the supply voltage, the forward voltage of an LED chain and the common base voltage of the transistors is applied to the drive circuit of a transistor its corresponding collector-base voltage. A change in the forward voltage of an LED chain (eg due to a short circuit of an LED) is intercepted by a corresponding change in the associated collector-base voltage so that the collector current and thus the corresponding chain current does not change or changes only very slightly , This is inversely proportional to the associated emitter resistor Rx2.

In addition, the diodes Dx prevent a flow of current from one LED branch to another.

Advantages:

If an LED in a LED chain fails due to a short circuit, the forward voltage of the LED chain decreases, but this is compensated by the associated control arrangement, as the collector-base voltage at the associated transistor increases. Since changes in the forward voltage of an LED chain via the electrical resistors Rx1 only the base current of the transistors Tx flows (about factor 100 ... 250 smaller than the collector current), the Rx1 can be dimensioned so that a very small change of Current through Rx1 (in the range <1 mA) already causes a large change in the voltage across the Rx1, whereby the different forward voltages in the individual LED chains can be compensated. The sum of the forward voltage of the LED chain and the collector base voltage of the associated transistor corresponds to the difference between the supply voltage and the base potential of the transistors, since all base terminals continue to have the same potential. The respective collector currents of the LED chains thus remain approximately constant.

If an LED in an LED chain fails due to burnout ("open contact"), the current through the defective chain no longer flows. The voltage between collector and base of the associated transistor Tx breaks down. At the base of the transistor of the defective chain, the potential must still be present as at the base inputs of the remaining transistors. This is achieved by the common electrical connection of the transistor bases, via which corresponding equalizing currents can flow. The intact LED chains initially take over the equalization currents. These additional partial currents then flow via the respective Dx and Rx1 and via the common electrical connection of the transistor bases into the base of the now diode-operated transistor Tx of the failed chain and through its emitter resistor Rx2. By interconnecting all base inputs at the same potential, the remaining intact chains continue to receive the current according to the distribution determined by the dimensioning of the emitter resistors Rx2. The emitter resistance values Rx2 are inversely proportional to the emitter current, which is the sum of the base and collector currents.

All further operating or fault states can be handled in accordance with the forward voltages of the LED chains between the extreme cases "short circuit" and "burnout" of an LED or LED chain.

In the case of the circuit arrangement according to the invention, when the forward voltage is changed (except for "open contact") in a chain, the collector or chain current intensity varies advantageously only by a few mA. Also, a change in the forward voltage in a chain due to short circuit of an LED, advantageously does not lead to the collapse of the power distribution. A costly grouping of the LED components after forward voltage is completely eliminated.

It is furthermore advantageous if the values of the drive resistors Rx1 in series with the diodes Dx are in the range from 100 ohms to 1000 ohms, since high compensation voltages can thus already be generated by relatively small currents. The Rx1 compensate for voltage differences to the LED chain voltages.

At the one transistor, a diode and two resistors having control arrangements compared to the second embodiment, only a very low voltage is necessary, what especially with longer LED chains leads to an energetically advantageous overall system.

Second embodiment of the invention:

On the other hand, the actuation by the drive circuit is effected by providing the drive circuit, which supplies the base terminals Bx of the transistors Tx with a predetermined current, with a Zener diode reverse-biased to the supply voltage, the transistor side via a resistor FuB and a resistor Rz to all Basic connections Bx connected together. The zener diode and the resistor Rz make it possible to provide a power supply of the respective transistor base necessary for the function of the electrical control arrangement. In accordance with the difference between the forward voltage of the respective LED chain and the voltage dropping at the drive circuit (this is determined by the supply voltage and the common base voltage), a collector-base voltage drops at the respective transistor. A change in the forward voltage of an LED chain (for example due to a short circuit of an LED) is intercepted by a corresponding change in the associated collector-base voltage so that the collector current and thus the corresponding chain current does not change or changes only very slightly , This is inversely proportional to the associated emitter resistor Rx2.

According to this second embodiment according to the invention, the base current for the transistors only needs to be conducted past the array via a single current path.

Here, the supply of the base inputs of the transistors Tx is realized by a parallel branch next to the array in which the Zener diode is installed, which should have an approximately 1 V higher forward voltage than the LED chain x with the highest forward voltage. This will bring the transistors into a stable operating state.

Normally, only a very small current in the range of a few mA flows via this parallel branch, the basic supply line. This is sufficient to drive all transistors in the normal operating state such that they are conductive.

If an LED in a LED chain fails due to a short circuit, the forward voltage of the LED chain decreases, but this is compensated by the associated control arrangement, as the collector-base voltage at the associated transistor increases. The sum of the forward voltage of the LED chain and the collector base voltage of the associated transistor corresponds to the difference between the supply voltage and the base potential of the transistors, since all base terminals continue to have the same potential. The respective collector currents of the LED chains thus remain approximately constant.

If an LED in an LED chain fails due to burnout ("open contact"), the current through the defective chain no longer flows. The voltage between collector and base of the associated transistor Tx breaks down. At the base of the transistor of the defective chain, the potential must still be present as at the base inputs of the remaining transistors. This is achieved by the common electrical connection of the transistor bases, via which corresponding equalizing currents can flow. The parallel branch takes over the equalizing current. This additional compensating current flows through the Zener diode and Rz and via the common electrical connection of the transistor bases into the base of the now diode-operated transistor Tx of the failed chain and through its emitter resistor Rx2. By interconnecting all base inputs at the same potential, the remaining intact chains continue to receive the current according to the distribution determined by the dimensioning of the emitter resistors Rx2. The emitter resistance values Rx2 are inversely proportional to the emitter current, which is the sum of the base and collector currents.

All further operating or fault states can be handled in accordance with the forward voltages of the LED chains between the extreme cases "short circuit" and "burnout" of an LED or LED chain.

In the case of the circuit arrangement according to the invention, when the forward voltage is changed (except for "open contact") in a chain, the collector or chain current intensity varies advantageously only by a few mA. Also, a change in the forward voltage in a chain due to short circuit of an LED, advantageously does not lead to the collapse of the power distribution. A costly grouping of the LED components after forward voltage is completely eliminated.

A arranged in series with the zener diode fuse, z. As a melting resistance FuB, should advantageously prevent that when overloading the array, in particular, the transistors are destroyed.

It is also advantageous if the value of the drive resistor Rz in series with the Zener diode is in the range of 100 ohms to 1000 ohms, since this can already be generated by relatively small currents high compensation voltages. March compensates voltage differences to the LED chain voltages.

A further advantage results in both embodiments in that in each LED chain (chain x) a serially connected to the LED components fuse, for. B. a melt resistance (Fux) is arranged. In this way, single faulty LED chains are simply switched off when the current in the chain is too high, when the fuse trips or the melting resistance burns out.

Depending on the desired current distribution, the currents in the respective LED chains (chain x) are inversely proportional to the respective electrical resistances Rx2. Ie. The array can advantageously be made very flexible and provide an adapted current depending on the LED chain.

Further advantages, further developments and embodiments of the circuit arrangement for an LED array, in particular for a light signal device, result from the exemplary embodiments explained below with reference to the figures.

The circuit arrangement will be explained in more detail below with reference to two exemplary embodiments. Show it:

1 a detail of a circuit diagram of a first embodiment of the first, not claimed embodiment with npn transistors;

2 a section of a circuit diagram of a second embodiment of the first, not claimed embodiment with pnp transistors and

3 a circuit diagram of a large LED array using a circuit arrangement according to the second embodiment of the first, not claimed embodiment;

4 a detail of a circuit diagram of a first embodiment of the second embodiment according to the invention with npn transistors;

5 a section of a circuit diagram of a second embodiment of the second embodiment according to the invention with pnp transistors.

At the in 1 partially shown embodiment, a plurality of LED chains 1 , in each of which a plurality of LED components 2 connected in series with each other, connected in parallel. The LEDs can also be different from each other, z. B. with respect to their semiconductor materials. In every LED chain 1 is optionally a serial to the LED components 2 switched melting resistance Fu1, Fu2, Fu3 ... Fux. In series with each LED chain x is still a npn transistor T1, T2, T3, ... Tx whose collector terminal C1, C2, C3 ... Cx each with the cathode side of the associated LED chain or possibly is connected to the interposed melt resistance Fu1, Fu2, Fu3 ... Fux and its emitter terminal E1, E2, E3 ... Ex each connected via an electrical resistance R12, R22, R32 ... Rx2 to the negative terminal of a supply voltage U _V. is. The transistors Tx are designed in the illustrated arrangement as commercial npn transistors. The anode sides of the LED chains x are each connected to the positive pole of the supply voltage. Between the cathode side or possibly the melting resistance of each LED chain x and the respective base terminal B1, B2, B3 ... Bx of the associated transistor T1, T2, T3 ... Tx is in each case a series circuit of a diode D1, D2, D3 ... Dx and an electrical resistance R11, R21, R31 ... connected. The base terminals B1, B2, B3 ... Bx of the transistors T1, T2, T3 ... Tx are connected together. Where x denotes a running index of 1 to the maximum number of LED chains.

The functional principle of this circuit arrangement is described for x = 3:
At the resistors Rx2 (with x = 1, 2 or 3) a voltage URx2 = Rx2 * Ix (with x = 1, 2 or 3) drops when current is applied with the current intensity Ix (with x = 1, 2 or 3) , At the base-emitter path Bx-Ex (with x = 1, 2 or 3) of the transistors Tx (with x = 1, 2 or 3), a voltage of about 0.65 V occurs, so that in total in each case the base -Embitterstrecke Bx-Ex and the associated electrical resistance Rx2 a voltage of Rx2 · Ix + 0.65 V generate. As a result of the fact that the base inputs Bx of the transistors Tx are electrically connected to one another (short-circuited), the current through the various branches (chains) is distributed via the transistors Tx as determined by the respective voltage (Rx2 * Ix + 0.65 V). in the different chains is determined. If the total array current I _{ges is to be} equally divided between the different chains, then all Rx2 must have the same resistance value; If the different chains are to be energized differently, this can also be realized very simply by means of different Rx2. The values of the Rx2 are inversely proportional to the desired current values.

Because all the base terminals Bx of the transistors Tx are set to the same potential and because of the constant voltage between base and emitter of 0.65 V, the same voltages must also be applied to the respective emitter resistors Rx2. These same voltages are caused by the respective emitter currents constructed, which flow through the emitter resistors Rx2 and resulting in the sum of the respective base and collector currents.

A change in the forward voltage of an LED chain (for example, due to a short circuit of an LED) is intercepted by a corresponding change in the associated collector-base voltage, with the result that the collector current does not change or changes only slightly.

Only with "open contact" of an LED chain, the voltage at the corresponding emitter resistor must be maintained by a corresponding change in the associated base current. The required base current change is made possible via the common electrical connection of the transistor bases.

The supply of the base inputs of the transistors Tx is realized via the diodes Dx (with x = 1, 2 or 3) and the resistors Rx1 (with x = 1, 2 or 3).

• 1) Sicherstellung der Betriebsbedingung der Transistoren Tx (Spannung an der Kollektor-Basis-Strecke Cx-Bx).
• 2) Unterdrückung von Querströmen zwischen einzelnen Ketten, d. h. dass über die gemeinsame elektrische Verbindung der Transistorbasen kein Strom von einer LED-Kette x in eine andere LED-Kette y fließen kann (verursacht durch Potentialunterschiede in den einzelnen Ketten z. B. wegen unterschiedlicher Durchlassspannungen oder kurz geschlossener LED).

The diodes Dx have a double function here:

• 1) Ensuring the operating condition of the transistors Tx (voltage at the collector-base path Cx-Bx).
• 2) suppression of cross-currents between individual chains, ie that no current can flow from one LED chain x into another LED chain y via the common electrical connection of the transistor bases (caused by potential differences in the individual chains, for example because of different forward voltages or shorted LED).

Since changes in the forward voltage of an LED chain via the electrical resistors Rx1 only the base current of the transistors Tx flows (about factor 100 ... 250 smaller than the collector current), the Rx1 can be dimensioned so that a very small change in the Current through Rx1 (in the range <1 mA) already causes a large change in the voltage across the Rx1, whereby the different forward voltages in the individual LED chains can be compensated. To achieve the necessary compensation voltages to the Rx1 at relatively low currents, the Rx1 advantageously have values in the range of 100 to 1000 ohms.

The stability of the circuit to production-related fluctuations in the current amplification factors B (= collector current I _C / base current I _{B of} the transistors) is ensured by the control arrangement.

In a further variant (eg for safety-relevant applications), melt resistors (fuse, safety design) Fux (with x = 1, 2 or 3) are preferably additionally in the LED chains 1 integrated, which reliably prevent too large currents through an LED chain, by in the worst case, if z. B. experiences a chain x the double target current, burn and cause an "open contact". Then no current flows at all through this "defective" LED chain x. The remaining "intact" LED chains x - 1 receive the current again according to the given distribution.

In the case of an open contact of an LED chain, according to the first embodiment, a current which is sufficient to maintain the required voltage at the Rx2 of the disconnected LED chain must additionally be conducted via the Rx1 of the still "intact" LED chains.

With a number of at least three LED strings, in the case of "open contact" according to the first embodiment, the Rx1 must absorb smaller equalizing currents than according to the second embodiment the single resistor Rz.

The use of a partial flow for control by each chain increases the reliability of the system. When using resistors with 1% tolerance for Rx2 results in a base current fluctuation of only 2% and thus a large uniformity of the total current distribution.

The circuit arrangement according to 1 is expandable by any number of LED strings as shown.

The melt resistors are optional and are only for additional protection (eg for safety-relevant applications). Commercially available melt resistors can be used which permanently interrupt the flow of current from a defined power (burn through).

To achieve a uniform base current split for all chains, it is necessary that the Rx2 all have the same value. But there are also conceivable applications where you want to selectively control different streams in the individual chains. This is easy to implement with different Rx2 and the desired currents can also be set exactly in this case. The values of the Rx2 are inversely proportional to the desired current values.

At the diodes Dx must drop such a large voltage that the transistors get into a stable operating condition. For example, LEDs could even be used here, which would then also act as an indicator for different forward voltages in the individual chains.

In principle, the resistors Rx1 need not necessarily all have the same value, but in order to optimally design the reliability and the symmetry of the arrangement, the same resistance values should also be present here.

The transistors Tx are designed in the illustrated arrangement as commercially available same npn transistors.

The circuit can, as in 2 shown to be constructed in an analogous manner with pnp transistors. Here are the circuit parts with the transistors Tx, the electrical resistors Rx1 and Rx2 and the diodes Dx between the anode sides of the LED chains 1 and the positive pole of the supply voltage U _V.

The in 3 The circuit diagram shown shows the use of a power distribution circuit in a large LED array, for example, the signaling technology. In this case, the LED chain currents of the LED array are monitored by an unspecified described here and as a "black box" additional monitoring circuit.

4 shows an example of the second embodiment according to the invention with the Zener diode and the resistor FuB as impressing and driving circuit for the base current. This circuit can be extended by any number of LED strings as shown.

The melt resistances are optional and serve only for additional protection. Only in series with the Zener diode, a resistance Rz should be connected in the amount of 100 ohms to 1000 ohms to generate high compensation voltages at relatively low currents in a change in the forward voltage of an LED chain. In this case, causes a small change in current through the series resistor Rz a sufficiently high voltage change to the series resistor Rz.

For a uniform base current distribution in all chains, the Rx2 must all have the same value here as well. For special applications but also different Rx2 may be necessary, for. B. in combinations of different LED colors. The values of the Rx2 are inversely proportional to the desired current values.

The forward voltage of the zener diode must be large enough to bring the transistors into a stable operating state, i. H. the forward voltage must be approx. 1 V higher than the voltage in the chain with the highest forward voltage.

The transistors Tx are designed in the illustrated arrangement as commercial npn transistors.

5 shows one to 4 similar circuit, wherein instead of the npn transistors, pnp transistors are used.

LIST OF REFERENCE NUMBERS

1

LED chain

2

LED component

Fu

fusible resistor

D

diode

R

resistance

March

Resistor in series with zener diode

T

transistor

e

emitter

B

Base

K

collector

U

tension

I

electricity

RA

control arrangement

x

running Index

Claims (11)

Circuit arrangement for an LED array with - two or more LED chains (chain x) connected in parallel to one another, in each of which at least one LED component ( 2 ), wherein with two or more LED components ( 2 ), these are connected in series with each other, wherein - the anode sides of the LED chains (chain x) respectively to the positive terminal of a supply voltage (U _V ) are coupled, and the cathode sides in each case to the negative terminal of the supply voltage (U _V ) can be coupled, wherein for each LED chain (chain x) in each case a control arrangement (RA x) is connected in series for controlling a planned current distribution to the individual LED strings, and wherein the respective control arrangement (RA x) contains a bipolar transistor (Tx) whose Collector terminal (Cx) is respectively connected to the cathode side of the associated LED chain and the emitter terminal (Ex) via an electrical resistance (Rx2) to the negative pole of the supply voltage (U _V ) is connectable, wherein the base terminals (Bx) of the transistors (Bx) Tx) are electrically connected to each other for balancing the current distribution, and wherein a drive circuit, the base terminals (Bx) of the transistors (Tx) subjected to a predetermined current, wherein the drive circuit to the positive pole of the supply voltage (U _V ) connectable, in the reverse direction to the supply voltage (U _V ) operated zener diode, which is connected to all base terminals (Bx) together. Circuit arrangement for an LED array with - two or more LED chains (chain x) connected in parallel to one another, in each of which at least one LED component ( 2 ), wherein with two or more LED components ( 2 ), these are connected in series with each other, wherein - the anode sides of the LED chains (chain x) respectively to the positive terminal of a supply voltage (U _V ) are coupled, and the cathode sides in each case to the negative terminal of the supply voltage (U _V ) can be coupled, wherein for each LED chain (chain x) in each case a control arrangement (RA x) is connected in series for controlling a planned current distribution to the individual LED strings, and wherein the respective control arrangement (RA x) contains a bipolar transistor (Tx) whose Collector terminal (Cx) is respectively connected to the anode side of the associated LED chain and the emitter terminal (Ex) via an electrical resistance (Rx2) to the positive terminal of the supply voltage (U _V ) is connectable, wherein the base terminals (Bx) of the transistors ( Tx) are electrically connected to each other for balancing the current distribution, and wherein a drive circuit, the base terminals (Bx) of the transistors (T x) supplied with a predetermined current, wherein the drive circuit as a connectable to the negative terminal of the supply voltage (U _V ), in the reverse direction to the supply voltage (U _V ) operated Zener diode, which is connected to all base terminals (Bx) together. Circuit arrangement for an LED array according to one of claims 1 or 2, characterized in that a fuse is arranged in series with the Zener diode. Circuit arrangement for an LED array according to one of the preceding claims, characterized in that in series with the Zener diode, a resistor (Rz) is arranged. Circuit arrangement for an LED array according to claim 4, characterized in that the value of the resistor (Rz) in series with the Zener diode is in the range of 100 ohms to 1000 ohms. Circuit arrangement for an LED array according to one of the preceding claims, characterized in that in each LED chain (chain x) is arranged a serially connected to the LED components fuse. Circuit arrangement for an LED array according to one of the preceding claims, characterized in that the ratios of the currents in the respective LED chains (chain x) to each other by the values of the electrical resistances (Rx2) are set to each other. Circuit arrangement for an LED array according to one of claims 1 or 2, characterized in that the values of the electrical resistances (Rx2) are in the range of 10 ohms. Circuit arrangement for an LED array according to one of the preceding claims, characterized in that the LED array is a light-signaling device. Circuit arrangement for an LED array according to claim 3 or one of claims 4 to 7 or 9 with reference back to claim 3, characterized in that the fuse connected in series with the Zener diode is a melt resistance (FuB). Circuit arrangement for an LED array according to claim 6 or one of claims 7, 9 or 10 with reference to claim 6, characterized in that the fuse connected in series with the LED components is a fusing resistor (Fux).

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