PTC Thermistor Relay Circuit for Inrush Current Limiting

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Solution Overview

Problem

Conventional circuit arrangements for controlling electrical consumers, such as electric motors, lack an effective safety mechanism to prevent overheating in case of a malfunction of the control device, which can lead to excessive inrush current and potential ignition.

Innovation Solution

A circuit arrangement featuring a PTC thermistor as an inrush current limiter and a switching device, such as a relay, that is bypassed by the control device during normal operation but switched off in case of a malfunction, redirecting all power through the PTC thermistor to limit power consumption and prevent overheating.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional circuit arrangement with an enable pin on the power module is used to control safety, then the control device can switch off the power module, but the structure becomes more complex and the safety function is not sufficiently reliable

Engineering Contradiction:

Improvesafety functionVSAvoidcircuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The safety function is extracted from the power module's enable pin control and relocated to the inrush current limiter circuit. The relay contact in parallel with the PTC thermistor serves as a dedicated safety mechanism that independently limits current when the control device fails, simplifying the overall control structure while enhancing reliability.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The PTC thermistor is pre-positioned in parallel with the relay contact to provide automatic current limiting before damage occurs. In the event of control device failure, the PTC thermistor's positive temperature coefficient特性 automatically limits the inrush current, providing a cushioning protective effect that prevents overheating and ignition without requiring complex active control.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

2Duration of action of stationary object

If the switching device remains closed during malfunction to maintain power supply, then the electrical consumer can continue operating, but the inrush current becomes excessive and causes overheating or ignition

Engineering Contradiction:

Improvecontinuous operationVSAvoidoverheating and ignition risk

Core Design Contradiction:

Duration of action of stationary objectVSObject-affected harmful factors

Solution Approach 1:

The PTC thermistor's inherent property of increased resistance at elevated temperatures is utilized to convert the harmful effect of continuous power supply during malfunction into a beneficial protective mechanism. When the relay contact opens due to control device failure, all current flows through the PTC thermistor, which automatically limits the current through its positive temperature coefficient, preventing overheating while allowing continuous operation at reduced power.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Object-affected harmful factors

If the switching device is opened to limit inrush current, then overheating is prevented, but the electrical consumer cannot be operated

Engineering Contradiction:

Improveoverheating preventionVSAvoidelectrical consumer operation

Core Design Contradiction:

Object-affected harmful factorsVSProductivity

Solution Approach 1:

The circuit employs dynamic switching through the relay contact that can transition between closed and open states. During normal operation, the relay contact remains closed to bypass the PTC thermistor and allow full power to the electrical consumer. Upon detecting control device malfunction, the relay contact dynamically switches to the open state, activating the PTC thermistor for current limiting while maintaining the circuit in a controlled operational state rather than complete shutdown.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This solution ensures that the electrical load cannot overheat even if the control device malfunctions, as the increased resistance of the PTC thermistor limits energy delivery to the inverter and connected consumer, effectively preventing overheating and ensuring safety.

Implementation Method 1

The inrush current limiter has at least one PTC thermistor. The electrical resistance of the PTC thermistor can increase linearly or non-linearly as the temperature rises

Methodology Applied

Scientific EffectPositive temperature coefficient (PTC): Thermistor

Implementation Method 2

its ohmic resistance increases and thus limits the power consumption of the circuit arrangement

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Data Source

PatentEP2667471B1Circuit configuration for controlling an electricity consumer

Publication Date: 2019.02.27 DIEHL AKO STIFTUNG & CO KG

Figure 1

AI summary

The arrangement has a rectifier (16) attached to an alternating current (AC) input (10), and an inverter (18) with switching elements (19) attached to the rectifier. An AC output (20) of the inverter is connected with an electrical load (22), and a starting current delimiter i.e. PTC resistor (28), is arranged between the AC input and the rectifier. A relay (30) is switched parallel to the delimiter and switched ON according to switching-on or starting-up process by a controlling unit (24) to bridge the delimiter. The relay is switched OFF during malfunction of the controlling unit. The inverter is selected from a single-or multiple rectifiers.