DE29808365U1 - High voltage protection circuit with low input impedance - Google Patents

Description

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High voltage protection circuit with low input impedance

The invention relates to a protection circuit for an electrical measuring device, in which the measuring device can have a low input impedance when this is required for the measurement and with which the internal circuit of the electrical measuring device can be protected from damage by high voltage signals.

In general, an electrical meter cannot be switched to a low impedance input terminal when measuring a high voltage because this may damage the circuit of the electrical meter. Alternatively, the electrical meter may be equipped with a high voltage protection circuit for safety reasons in case a low impedance input terminal is selected for measuring a high voltage. Fig. 3 shows the circuit diagram of a conventional high voltage protection circuit. The high voltage protection circuit comprises a thermal resistor PTC and a clamping transistor Q. The activation current of the thermal resistor PTC is denoted by I _s (if the activation current is exceeded, the resistance value of the PTC is increased), the clamping current of the clamping transistor Q is denoted by I _Q and the maximum allowable voltage of the thermal resistor PTC is denoted by V _A as shown in Fig. 3a. Fig. 3b shows the equivalent circuit of the circuit of Fig. 3a. When an exceptionally high voltage exceeding V _A is applied to the circuit of Fig. 3a, the clamping transistor Q acts like a Zener diode. The clamping transistor Q is passed through a current which is greater than the activation current of the thermal resistor PTC. As a result, the thermal resistor PTC is in a high-resistance state to perform the protection function. At this time, 5 V _in =V _PTC +V _Q +V _R and V _in ^V _PTC =V _A , since V _PTC >>V _Q +V _R . Therefore, the protection circuit can only bear an input voltage up to V _A .

It is difficult to design the above-mentioned protection circuit in such a way

designed so that it can withstand an input voltage of up to 2V _A. If the protection circuit is designed so that it can withstand high input voltages of up to 2V _A , it will be large and expensive.
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Fig. 4a shows a circuit diagram of another conventional high-voltage protection circuit. The high-voltage protection circuit comprises a fuse FS, a thermal resistor PTC and a varistor VAR, where: I _s : the activation current of the thermal resistor PTC, I _Q : the clamping current of the clamping transistor Q, and

V _A : the maximum permissible voltage of the thermal resistor PTC.

The varistor VAR has the following properties:

1. the varistor can quickly absorb the rising voltage that exceeds V _A ,

2. the maximum clamping voltage of the varistor VAR depends on the maximum permissible voltage of the thermal resistance PTC,

3. I _VAR is the clamping current of the varistor VAR, and

4. I _LEAKAGE is the leakage current that occurs when the varistor does not perform the clamping function.

Fig. 4b explains the operation of the above circuit. The maximum input voltage cannot exceed the maximum allowable voltage of the thermal resistor PTC. The thermal resistor will be damaged if the clamping voltage exceeds the maximum allowable voltage V _A of the thermal resistor

PTC exceeds.

When the input voltage exceeds the clamping voltage V _Q of the clamping transistor Q, the latter acts as a zener diode and conducts the clamping current I ₀ . The thermal resistor PTC is in a high-resistance state when the clamping current I _Q exceeds the activation current I' _s ' of the thermal resistor PTC. Therefore, the thermal resistor PTC can perform a current limiting function. At this time, V _I/P =V _PTC +V _Q (V _0/p ) . If V _P1 . _C +V _Q >V _VAR , the varistor is in

low-resistance state and limits the voltage to V _VAR (like the Zener diode). The clamping current I _VAR of the varistor VAR is greater than the rated current of the fuse FS. The fuse FS will blow for protection.
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However, the circuit described above has the following problems:

1. The maximum input voltage is limited by the maximum permissible voltage of the thermal resistor PTC ( ^v var ^mu & must be less than V _A ).

2. The high leakage current of the VAR influences the measurement results in high-impedance measurements.

3. Replacing the fuse is difficult.

Fig. 5a shows an improved circuit compared to the circuit in Fig. 4a. The designations are as follows: Q: clamp transistor

R: heatable (inflammable) current limiting resistor PTC: thermal resistance

0 I _s : the activation current of the thermal resistance

PTCs,

V _VAR : ' the clamping voltage of the varistor VAR, I _VAR : the clamping current of the varistor VAR, ^ leakage ^: the leakage current that occurs when the varistor VAR does not perform the clamping function.

Fig. 5b explains the operation of the circuit of Fig. 5a. The current limiting resistor R is heatable and replaces the fuse FS in the circuit of Fig. 4a. The current limiting resistor R has a small resistance and can withstand the voltage V _A and the clamping current I _VAR . Since the I _PTC is very small, I _r =I _VA r- Furthermore, V _R =V _in -V _VAR , from which it follows that the voltage dropped across the resistor R is very high when a high input voltage is applied. Therefore, the resistor R should have a high voltage rating and a high power rating.

If the input voltage exceeds the terminal voltage V _Q of the

clamping transistor Q, then it acts like a Zener diode and conducts the clamping current I _Q . The thermal resistor PTC is in high resistance state when the clamping current I _Q exceeds the activation current I _s of the thermal resistor PTC. Therefore, the thermal resistor PTC can perform a current limiting function. At this time, V _in =V _PTC +V _Q . When V _PTC +V _Q >V _VAR , the varistor VAR is in low resistance state and limits the voltage V _PTC +V _Q to V _A (like the Zener diode). The VAR conducts a current I _VAR , causing the voltage across the resistor R to rise. The voltage is V _R =V _in -V _A , since v _VAR =V _PTC =V _A. V _R =V _A when V _in =2V _A . Therefore, the circuit can withstand 2V _A input voltage.

However, the circuit described above has the following problems:

1. The current limiting resistor R should have a small resistance value and be able to withstand high power and voltage. Therefore, the current limiting resistor R is large in size. The current limiting resistor R will burn out if its rated power is not large enough.

2. The high leakage current of the VAR influences the measurement results in high-impedance measurements.

The invention solves the problem of eliminating the problems described above and of creating a measuring device that can withstand a higher voltage. This is achieved according to the invention with a protection circuit for an electrical measuring device, which has a first thermal resistor, a second thermal resistor, a varistor, a first clamping transistor, a second clamping transistor, a current limiting resistor and a buffer resistor. The current limiting resistor is connected with one connection to the first thermal resistor. The first thermal resistor is connected in parallel to the buffer resistor. One connection of the first thermal resistor is connected to the second thermal resistor. The varistor is connected to the node between the first and second thermal resistors.

connected to suppress the leakage current when the varistor is not working. The emitter of the first clamp transistor is connected to the varistor, and the collector and base of the first clamp transistor are connected to the ground. The activation current of the second thermal resistor is smaller than that of the first thermal resistor (the internal resistance of the second thermal resistor is larger than that of the first thermal resistor). The other terminal of the second thermal resistor is connected to the emitter of the second clamp transistor. Through this protection circuit, the internal circuit of the electrical measuring instrument can be protected from damage caused by high input voltage.

The invention is explained in more detail in the following detailed description with reference to the accompanying drawing, in which:

Fig. 1 is a block diagram illustrating the application of the low input impedance protection circuit to an electrical measuring instrument;

Fig. 2ä shows the circuit diagram of the protection circuit according to the invention;

Fig. 2b shows an equivalent circuit to the circuit of Fig. 2a;

Fig. 3a shows the circuit diagram of a conventional protection circuit for electrical measuring instruments;

Fig. 3b shows an equivalent circuit to the circuit of Fig. 3a;
Fig. 4a shows a circuit diagram of a conventional protection circuit in which a fuse is used for the electrical measuring instrument.

Fig. 4b shows an equivalent circuit to the circuit of Fig. 4a.
5 Fig. 5a shows the circuit diagram of an improved circuit compared to the protection circuit according to Fig. 4a; and

Fig. 5b shows an equivalent circuit to the circuit according to

I ft ft · · ft · · · · · · ·

&igr;*·· ·· · · ft I

Fig. 5a shows.

The following terms are used below:

PTCl PTC2 VAR: Qi =

_R1 :

_R2 :

VARS ·

PTCl

PTC2

Ql

Q2

Rl

_C02

S2

first thermal resistance

second thermal resistance

Varistor

first clamp transistor

second clamping transistor

Current limiting resistor with small

Resistance value

Resistor with high resistance value

Clamping voltage of the varistor VAR

Voltage of the first thermal resistance PTCl

Voltage of the second thermal resistor PTC2

Emitter voltage of the first clamping transistor Ql

Emitter voltage of the second clamping transistor Q2

Voltage across resistor R

Clamping current of the first clamping transistor Ql

Clamping current of the second clamping transistor Q2

Activation current of PTCl

Activation current of PTC2

Fig. 2a shows the circuit diagram of the protection circuit according to the invention and Fig. 2b shows an equivalent circuit. The protection circuit can prevent damage to the electrical measuring device caused by its incorrect operation. Such incorrect operation is, for example, when the user mistakenly tries to measure a high voltage (e.g. IkV) at the resistance, current or other low-resistance inputs. The protection circuit has a first thermal resistor PTCl, a second thermal resistor PTC2, a varistor VAR, a first clamping transistor Q ₁ , a second clamping transistor Q ₂ , a current limiting resistor R ₁ and a buffer resistor R ₂ . The first thermal resistor PTCl and the second thermal resistor PTC2 have the following properties: 1. The internal resistance of the PTCl is smaller than that of the

PTC2.

2. The activation current of the PTCl is greater than the activation current of the PTC2.

3. PTCl and PTC2 both have a maximum allowable voltage V _A .

4. PTCl and PTC2 have low impedance when not activated and high impedance when activated.

One terminal of the current limiting resistor R ₁ is connected to the PTCl. The PTCl has a maximum allowable voltage V _A and a positive temperature coefficient. The PTCl is connected with one terminal to the buffer resistor R ₂ and with its other terminal to one terminal of the VAR and one terminal of the PTC2. The other terminal of the VAR is connected to the emitter of the first clamp transistor Q ₁ to suppress the leakage current that occurs when the VAR is not in operation. The base of the first clamp transistor Q ₁ is connected to ground.

The second thermal resistor PTC2 has a smaller activation current than the first thermal resistor PTCl and the same maximum permissible voltage V _A . In addition, the second thermal resistor also has a positive temperature coefficient. The other terminal of the PTC2 is connected to the emitter of the second clamping transistor Q ₂ . The base and the collector of the second clamping transistor are connected to ground.

0 If the input voltage exceeds the clamping voltage V _Q2 of the second clamping transistor Q ₂ , the second clamping transistor Q ₂ acts as a Zener diode and conducts the second clamping current I _Q2 . The second thermal resistor PTC2 is in a high-resistance state when the clamping current I _Q2 exceeds the activation current I ₃₂ of the second thermal resistor PTC2. The second thermal resistor PTC2 has a smaller activation current than the first thermal resistor PTCl, so the second thermal resistor PTC2 acts first.

At this time, the input voltage is applied across the second thermal resistor PTC2 and the second clamping transistor Q ₂ (V _in =V _PTC2 +V _Q2 ). If V _PTC2 +V _Q2 >V _VAR +V _Q1 , then the varistor VAR and the first clamping transistor Q ₁ act together as a Zener diode and limit the voltage across the second thermal resistor PTC2 and the second clamping transistor Q ₂ to V _VAR +V _Q1 . If the current I _VAR exceeds the activation current I ₅₁ of the PTCl, then the PTCl is in a high-impedance state to perform protective functions. The input voltage is applied accordingly across R ₁ , PTCl, PTC2 and Q ₂ .

At this time, V _in =V _R1 +V _PTC1 +V _PTC2 +V _Q2 . Since R ₁ is very small and Q ₂ has a small clamping voltage, the voltages V _R1 and V _Q2 can be neglected. Therefore:

V _in = V _PTC1 + V _PTC2 (1)

From this equation it can be seen that the protection voltage according to the invention is twice the voltage of the conventional protection circuit.

The buffer resistor R ₂ is connected in parallel with the PTCl to prevent the currents I _Q2 and I _VAR from becoming extremely small due to the increasing impedance of the PTCl. If I _Q2 <I _S 2/ then the PTC2 stops working and its resistance value decreases. At this time Vp ₁₁₀₁ =V _1n and the PTCl could be damaged by the high input voltage applied to it. Therefore, the parallel resistors R ₂ and PTCl have values such that the current I _Q2 can exceed the current I _S2 .

0 Therefore, the PTC2 can remain in a high-resistance state so that the input voltage is applied across PTCl, PTC2 and VAR.

Fig. 1 shows the block diagram of an electrical measuring device for resistance and capacitance measurement. The electrical measuring device has a protection circuit 1, a low-impedance current source or signal source 2, a microcomputer controller 3, a high-voltage current limiting circuit 4, an A/D converter with high input impedance 5 and a numerical

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Display 6 on.

If the input for resistance measurement is used normally, the controller 3 controls the current source 2 in such a way that a measuring current flows through the protection circuit 1 to the component to be measured. This generates a voltage across the component to be measured. The generated voltage is applied to the A/D converter 5 via the current limiting circuit 4 and converted into a digital signal. The microcomputer controller 3 sends the digital signal to the numerical display 6.

If a high voltage source is mistakenly connected to the resistance measurement input with the low input impedance, the high input voltage is applied to the protection circuit 1 and the current limiting circuit 4. At this moment, the high input voltage activates the protection function of the protection circuit 1, which limits the voltage on the internal circuit to a lower value.

The protection circuit can be divided into two stages for protection against high input voltage. The first stage has the PTCl and R ₂ connected in parallel with it and the VAR 5 and Q ₁ connected in series with it. The second stage has the PTC2 and Q ₂ .

Claims (3)

10 Claims 1. Protection circuit with: a current limiting resistor (R ₁ ); a first thermal resistor (PTCl) with a maximum permissible voltage V _A , which is connected with one terminal to the current limiting resistor (R ₁ ); a buffer resistor (R ₂ ); a second thermal resistor (PTC2) with maximum voltage V _A , which is connected by one terminal to the first thermal resistor (PTCl) and has a larger internal resistance value than that of the first thermal resistor (PTCl); a varistor (VAR) having one terminal connected to the junction between the first thermal resistor (PTCl) and the second thermal resistor (PTC2) and the other terminal connected to a first clamping transistor (Q ₁ ) to suppress the leakage current, the second thermal resistor (PTC2) having its other terminal connected to the second clamping transistor (Q ₂ ); 2. Protection circuit according to claim 1, wherein the emitter of the first clamping transistor (Q ₁ ) is connected to the varistor (VAR), and the collector and the base of the first clamping transistor (Q ₁ ) are connected to ground. 3. Protection circuit according to claim 1, which has two stages, the first stage comprising the first thermal resistor (PTCl) and the buffer resistor (R ₂ ) 0 connected in parallel thereto and the varistor (VAR) and the first clamping transistor (Q ₁ ) connected in series thereto, and the second stage comprising the second thermal resistor (PTC2) and the second clamping transistor (Q ₂ ).