over current or thermal protection for the electronic circuit

Hello, I have developed an high voltage capacitor discharge circuit using a voltage controlled switch. I would want to know the ways of protecting the circuit.

I thought of using PTC thermistor (PTC thermistor in series with charging resistor) which cuts off the supply to the circuit when there is a over temperature or over current is sensed, but since the voltage across the capacitor is high voltage (2500Vdc), Thermistor cant be used because it is only available for a maximum voltage of 150V dc in real life.

So it will be nice of you, if you could suggest me ways of protecting the secondary circuit against over current or over thermal protection in an high voltage (2500Vdc).

by yugendraraokn
January 23, 2017
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1 Answer

Answer by mikerogerswsm

This sounds like a job for an optocoupler. If you put a zener in series with your charging resistor you can steal a few volts to operate a sensing circuit and then send a signal via the optocoupler to shut down or crowbar the power supply. Hope this is helpful, mike.

+1 vote
by mikerogerswsm
January 24, 2017

Hello sir, Thanks for your reply. But adding zener diode in series with the charging resistor many not be possible as the voltage across the zener will be around 2500Vdc. In reality the zener diode with this must high voltage rating is not available.

My suggestion, What if i use a current leakage circuit breaker, which if detects a small current flowing in the ground of the load, then it must actuate the relay of the ELCB and there by tripping the DC Mains power supply to the circuit via a SPST.

by yugendraraokn
January 24, 2017

If you put a zener with say a 10V rating in series with the charging resistor it will pass the current and drop about 10V, from which you can power a floating sensing circuit. The sensing circuit needs to be in an enclosure which will stand the voltage.

by mikerogerswsm
January 25, 2017

Hello sir, Thanks for your response. But i am thinking of implementing a thermistor (PTC type) in joint with the charging resistor, which can act as a thermal protection to the charging resistor.

For an example: If the temperature of the charging resistor exceeds 45 degree then the thermistor must be able to cut off supply to the charging resistor, and once the temperature drops below 45 degree, the thermistor must be able to automatically connect the charging resistor in the circuit.

Plz do let me know if this option is possible, as when i had simulated the above circuit, the voltage across the charging resistor was around 2500Vdc.

Hoping for your response.

by yugendraraokn
January 27, 2017

The initial voltage across the 9K charging resistor is indeed 2,600 volts dc. It might be wise to split it into a number of resistors in series, supported by terminals mounted on a strip of suitable insulator such as glass. The number of resistors would depend on their ability to pass the required 300 mA instantaneously and to pass the total charge during the time the capacitor is charging plus the 300 mA passed during the time the switch is on. The 2M resistor might also need to be split into several in series. The capacitor is possible, dunno about the switch, it might need quite a bit of hardware to achieve that voltage sensing and switching function. Is the circuit used as a single shot or does it repeat? Does the voltage controlled switch stay on until reset or does it turn off after a specified time? Where do you buy 1pohm and what is the effect of switch voltage drop and wiring resistance? Er, take great care with such high dc voltages, have someone else present with strict instructions to operate the kill switch and phone for expert help BEFORE they try to resuscitate you and become incapacitated themselves. Been there, done it.

by mikerogerswsm
January 27, 2017

Hello sir, The voltage controlled switch is a spark gap which will conduct once the voltage across it reaches 2500Vdc. If you see the voltage across capacitor waveform you can observe that the waveform looks like a sawtooth waveform with its peak being Vpeak = 2500Vdc.

After discharge the voltage across the capacitor drops and raises again to 2500Vdc and it repeats itself in the form of charging and discharging at 2500Vdc.

The 1pohm resistor in reality is the equivalent of a spark rod, if you clearly observed the circuit diagram you can see that the capacitor discharge path is through the spark gap and finally to the spark rod.

I am also attaching the link for the images for the spark rod and spark gap, through google drive.

Link 1- https://drive.google.com/open?id=0B_wbak15T7m_Y09XNFdUZjUxZ0U Link 2- https://drive.google.com/open?id=0B_wbak15T7m_bnZsWFBZOVQ1S0U

by yugendraraokn
January 30, 2017

Wow, interesting! Nice to see the actual hardware in operation.

by mikerogerswsm
January 30, 2017

Hello sir, The circuit is working properly but i need to protect the charging resistor form the DC power supply, i.e, as soon as the charging resistor gets heated up, it must be automatically able to protect itself form overheating by disconnecting the charging resistor form the DC power supply.

Hence in my college project they are suggesting me to embedded a thermistor inside the hallow wire wound resistor for the protection purpose,

But i have never seen a thermistor being embedded into a resistor for the purpose of resistor protection.

Even in my college they are not sure of as to what might be BROWN COLOR component present in the charging resistor.

I am also attaching the images of the Resistor+thermistor component through google drive.

Plz go through and provide suggestions sir!.

Link-1: https://drive.google.com/file/d/0B_wbak15T7m_Zy15SzB2THhJYWM/view?usp=sharing Link-2: https://drive.google.com/file/d/0B_wbak15T7m_MVppdVlLMi1hQVk/view?usp=sharing Link-3: https://drive.google.com/file/d/0B_wbak15T7m_RnFia2c5YTVPN1U/view?usp=sharing Link-4: https://drive.google.com/file/d/0B_wbak15T7m_RzdZNXUxV3prVlk/view?usp=sharing Link-5: https://drive.google.com/file/d/0B_wbak15T7m_MGdqV2JoeDY2Nlk/view?usp=sharing

by yugendraraokn
January 31, 2017

Hello, So far as I can ascertain those RX20 resistors are made by Liwang Electric Technology Co http://liwang.win.mofcom.gov.cn/en/plate01/index.asp You might like to contact them, explain that you are doing a student project, and ask what the brown core material is. My own guess is that it seals the interior to prevent condensation and mould growth when stored in tropical conditions. Removing it should not cause you any problems. Hope this helps, mike

by mikerogerswsm
January 31, 2017
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