DE2334409C2 - Circuit for an electric heater - Google Patents

Description

The invention relates to a circuit for a electric heating, with which you load via controllable bidirectional semiconductor elements in full-wave operation connected to the heating wave voltage! is and the ignition pulses for the bidirectional semiconductor elements by discharging one in each half-wave through one Back-rectifier and a capacitor charged with a voltage limited by a Zener diode can be triggered using optoelectronic couplers.

In the case of electrical heating, in particular of passenger coaches, there has been a transition to that which has been made up to now to replace the usual hot water system with an electrical direct heating system. It has, however found that the use of mechanical contactors to switch the heating power because of the Shift key was not suitable. Also the use of the phase control struc- tures common in the drive system to regulate the heating power is only conditional because of the associated harmonics useful. From DE-OS 21 36 319 a circuit for an electrical heater is now the opening described type known in which the Vollwelicnbetrieb ongoing heating is switched by controllable semiconductor elements, whose ignition pulse is in each case in Zero crossing of the supply voltage generated from a capacitor discharge via a pulse transformer will. The capacitor is charged from a supply transformer, which on the one hand clic power to ignite the semiconductor elements emits and on the other hand feeds vain zero voltage detector, which discharges the discharge in the zero crossing Capacitor caused by a transistor. There is a difficulty with such a circuit system in the fact that the usually very high operating voltage a power can be drawn, which both the Give the ignition power of the lead elements as well

must supply the auxiliary elements. The previously known solution uses a supply transformer, but spatially because of the performance to be provided by him and because of the required dielectric strength is too expensive.

The present invention is based on the object of specifying a circuit in which under Avoidance of complex transformers and heat-emitting series resistors: us one Auxiliary voltage are generated in the zero crossing ignition pulses. According to the invention, this object is achieved a circuit for an electrical heater of the type described above is achieved in that

a) the beginning of an ignition pulse due to the occurrence of a negative ignition voltage on a thyristor is determined, which switches through and the discharge of the charged capacitor to a light emitting diode causes while

b) the ignition pulse for the semiconductor elements applying voltage to the load from a auxiliary voltage source in a monostable multivibrator that is triggered by the opto-electrical coupler is generated and in a known manner via a pulse transformer to this semiconductor

< elements is placed.

The particular advantage of the circuit according to the invention can be seen in the fact that because of the very short-term power consumption of the light-emitting diode both on a transformer and on one heat-emitting series resistor can be dispensed with u; id with a low to earth Voltage a capacitor is charged, which can be made so small that it is just for the Brief actuation of the light emitting diode delivers the required power. In contrast, the capacitor in the previously known embodiment, apply the actual ignition power for the semiconductor elements. By using a thyristor with an ignition voltage that is negative to the anode, the relatively complex slot tension detector avoided and the Zündinipulse triggered as soon as the charging voltage for the capacitor as a result of Zero crossing falls below a certain value.

Another advantage can be seen in the fact that a galvanic light-emitting diode phototransistor system is used Separation is possible. This enables the usual circuits in railway systems with auxiliary control voltage Find use, and an existing mechanical contactor can ohrv: additional effort through a electronic to be replaced. In addition, this system has the advantage of being small with the same Power on the capacitor a large number of switching semiconductor elements can be operated. Through the Using a monostable multivibrator, the pulse width can be largely adjusted.

The figure shows, in a partially schematic representation, an exemplary embodiment of a circuit according to the teaching of the invention. The operating voltage present at 1 and 2 is fed through a voltage divider 3 and a rectifier bend 4 on the ground side of the operating voltage. The level of the resulting direct voltage is fixed with a Zener diode 5 and charges the capacitor 7 via the resistor 6. As soon as the charging voltage falls below a certain value as a result of a zero crossing of the operating voltage. it ignites the Thyrislortetrode 8. which enllädi the capacitor 7 through the resistor 9 to the light emitting diode IO, which together with the

3 4

jiuirunsisior Il an opto-electrical coupler the two with the load 16 in series

ld. The photorransistor II causes 17 to pass through each bidirectional half-core element

ulldurehgang from an auxiliary voltage source 12 instead of the bidirectional semiconductor elements 17,

; feed monostable multivibrator 13 triggered, the so-called triacs, can also be connected in opposite-parallel

: rst; irkl through a transistor 14, an ignition pulse thyristors are used,
I give the impulse transformer 15, which in turn

1 sheet of drawings

Claims (1)

Claim: Circuit for an electrical heater in which the load is via controllable bidirectional semiconductor elements in full wave operation to the alternating voltage is placed and the ignition pulses for the bidirectional semiconductor elements by the discharge one in each half-wave via a bridge rectifier and one through a Zener diode Limited voltage charged capacitor can be triggered using optoelectrical Couplers, characterized in that a) the start of an ignition pulse due to the occurrence of a negative ignition voltage a thyristor (8) is determined, which turns on and discharges the charged Capacitor (7) causes a light emitting diode (10) while b) the ignition pulse for the load to voltage legetxfen semiconductor elements (17) from a Hiifsspannungsquelie (12) in a monosilicate Multivibrator (13), which is activated by the opto-electric! Coupler (10, 11) is triggered, generated and placed in a known manner via a pulse transformer (15) to these semiconductor elements (17) will.