800V DC Thermal cutt-off switch - Page 1
Forum section: electronics / projects

author: sanwal209
author_rank: Regular Contributor
timestamp: January 24, 2018, 12:16:48 pm
content: Hi Guys,I am looking for a thermal cutt-off switch for 800V DC and 6A but couldn't find any. If anybody used such type of switch please do let me know.Thanks

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2018, 01:24:01 pm
content: Quote from: sanwal209 on January 24, 2018, 12:16:48 pmA smaller thermal switch, such as a bimetal strip, controlling a contactor or power transistor?

author: sanwal209
author_rank: Regular Contributor
timestamp: January 24, 2018, 01:25:36 pm
content: That is plan B if we cannot find any thermal switch with that power rating.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2018, 01:32:43 pm
content: Does it have to be resettable or one shot? Is it safety critical?The problem is breaking the arc, when the switch opens.

author: sanwal209
author_rank: Regular Contributor
timestamp: January 24, 2018, 01:42:26 pm
content: Doesn't matter one shot or resettable.

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2018, 09:08:20 pm
content: Breaking 800VDC at significant current is a big ask.Any possibility that you could switch earlier in the chain? If it is mains powered equipment maybe the primary feed or AC side? Some more detail of what you are trying to protect would be helpful.

author: sanwal209
author_rank: Regular Contributor
timestamp: January 25, 2018, 10:26:46 am
content: This module is used in Electric vehicle and we have 800V DC battery. Module generates high voltage PWM for heater. We want to protect that heater with over temperature.For example if power mosfet gets short then PTC or over temperature switch cutoff supply to heater.

author: CM800
author_rank: Frequent Contributor
timestamp: January 25, 2018, 11:15:23 am
content: I would instead consider a DC contactor rated for 800V @8A or more, plus a 24V thermal cut out linked through to the contact.I highly doubt you will find an 800V 8A thermal switch off the shelf, short of mass production, probable custom designs.

author: madmac
author_rank: Contributor
timestamp: January 25, 2018, 12:53:03 pm
content: Low cost solution might be high voltage DC fuse and a thyristor crowbar to blow it on overtemp. The fuse would also protect against other faults.

author: Gyro
author_rank: Super Contributor
timestamp: January 25, 2018, 06:45:15 pm
content: Lateral thinking.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 25, 2018, 07:05:15 pm
content: Even a fuse will be expensive and physically big for that breaking capacity. A simple solution is a second mosfet operated by a thermal switch. This ensures there is not a single point of failure (PWM mosfet). Be sure to ensure it turns of fast, and is protected from inductive spikes or it will fail.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2018, 07:16:18 pm
content: Hmmm.... seconding the transistor, I think.Transistors can be made very reliable, with active voltage and current protections. They switch fast enough to protect against short circuits, and with a little inductance and filtering added, can current limit a troublesome load, with very high efficiency to boot. And you can put two in series for redundancy. (Si or SiC transistors are plentiful in that range, or an IGBT can be used if you don't mind a little more conduction loss.)Medium voltage DC is nothing to trifle with.I'm guessing there's quite a lot of engineering going into this design already; why skimp on a potential shitshow (forever-arcing thermal switch or contactor; heater short; heater break and arcing)?Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 25, 2018, 07:50:34 pm
content: Use a transistor and a low voltage over temperature sensor of your choice.Even if a thermal swith would exist for that voltage/current it would be so expensive ( clearance distance, arc rupture, UL flammability, etc) that you wouldnt even consider it eventually

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2018, 08:27:45 pm
content: Trying to interrupt 800VDC with a thermal switch is ridiculous. As others have said, use a low voltage thermal switch and use it to interrupt the control. If you want an extra layer of safety you could have a crowbar circuit that will blow the fuse if the temperature rises beyond the cutoff of the primary protection.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 25, 2018, 08:54:53 pm
content: mouse trap connected in series as a spring-loaded contact (big clearance) as disconnector with explosive trigger (blasting cap) ?