220v SSR relay voltage leak  - Page 1
Forum section: electronics / beginners

author: simonboydfoley
author_rank: Contributor
timestamp: March 02, 2018, 10:48:48 pm
content: I'm doing research into using a SSR to switch mains voltage to a 2Kw heater with thermal runaway protection based on Arduino mega code and a DBT11 temp sensor to keep temps in a small enclosure.(74x74x74cm) constant 60C Max current draw = 9 (10 amp safety margin). I understand that SSR do not provide infinite resistance and this will cause 20v leakage. Is there any reference documentation anybody can point me to on how to manage this issue to prevent current triggering electronics downstream of the SSR. It seems to me a ceramic bi pole capacitor in parallel with the main load should bleed current at a frequency greater than 30Hz to fix the Issue. any good docs people can point me at ?

author: kony
author_rank: Regular Contributor
timestamp: March 02, 2018, 11:05:30 pm
content: If it is safety related, I'd suggest contactor (combined with SSR if the device is regulating the heating element temperature as well).It's not really stray voltage per se, the SSR has some small off-state leakage current (you will find it in datasheet), so the voltage will be load dependent and shunting it with capacitor won't do much.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: March 02, 2018, 11:09:59 pm
content: I've always used Crydom SSR's. Check Google or Crydom for tech papers.http://www.crydom.com/en/Service/Crydom_TechTraining.pdfhttps://www.element14.com/community/servlet/JiveServlet/downloadBody/41989-102-3-246924/Solid%20Statements%20-%20Protecting%20AC%20Output%20SSRs%20against%20voltage%20transient%20phenomena.pdfOr from the output, drive an isolation transformer 1:1, as the leakage current will be too low for the transformer to "energize" itself.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: March 04, 2018, 09:15:55 am
content: I think SSR switching can cover a multitude of devices. I have used Triacs many times in the past and never been aware of leakage apart from the supporting snubber network. These snubber can be pain for contactors as well regarding LED lighting. Not sure from your post but what was the concern regarding the leakage. As others have said you should probably back up with a thermal fuse or mechanical thermal switch.Chris

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2018, 10:18:29 am
content: If this is safety critical, a thermal fuse should be connected in series with the heater, to prevent fire, if the solid state relay or temperature controller fails.

author: simonboydfoley
author_rank: Contributor
timestamp: March 06, 2018, 12:52:18 am
content: Thanks all,apologies for the slow response .... I need to see if I can set up email triggers. I will research the answers, have not touched electronics for 10 years so I'm a bit rusty. I like the idea about the thermal resistor but essentially I am using an raspberry Pi or an Arduino Mega to measure the temperature of the enclosure ( for a 3d printer) using a DHT11 Sensor in the enclosure. The heater itself will be a a commercial small space heater that I will switch on and off via the SSR being controlled via GPIO inputs to a FOTEK SSR-40-DA. This SSR will switch on and off the AC current to the heater. As you have to leave 3d printers running remotely I need to be extra cautious of the safety aspects. There is no need for a temperature resistor because the SSR circuitry will be downstream of the space heater which will be in the enclosure. Thermal runaway protection will be in software where the DHT11 sensor in the enclosure which will cut the power if the temperature does not rise quickly enough (heater malfunction / short) or if the temperatures rise too quickly. I will also use another SSR 40 DA to connect to the electronics of the printer (220v AC to a 360w 12v DC Power supply) which will be outside of the enclosure. Its the printer electronics that I really need to protect from the bleed voltage of the SSR otherwise it will keep switching itself on all the time and it also provides 12v to a heated bed on the printer in the enclosure. I will add a 10A fuse to the ssr electronics as well s surge protection device on the socket itself ... but to kill the 20v bleed voltage from even starting up the 12v melzi boards electronics ... i'm looking for common practice on how to kill the ~ 20v bleed voltage. I need to understand a SSR better as a capacitor in parallel with mains load resistance wont help as the AC frequency wont change to alter the impedance of the capacitor when it goes from 20V AC bleed to 220V AC mains (like an 10:1 switchable oscilloscope probe leverages). Im brain storming here and need to do more research. thx all for your help.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2018, 11:57:18 am
content: If you split your post into multiple paragraphs, it would be easier to read.I think you've overlooked the importance of a thermal fuse. Relays, including the solid state variety, have a nasty habit of failing on, which can cause a fire if it's over-temperature and can't turn off. When safety is concerned, you need to consider what can go wrong, more than what it should be doing, when it's working properly.

author: max_torque
author_rank: Super Contributor
timestamp: March 06, 2018, 12:08:12 pm
content: Quote from: simonboydfoley on March 06, 2018, 12:52:18 amWords that should strike fear into the heart of any engineer reading them!By all means, include suitable software functionality as a first line of defense. However you need to include a LAST LINE defense, using a thermal fuse to disconnect the heaters should a scenario occur that results in them staying on. Just sat here i can think of 5 or 6 likely to occur scenarios that can easily occur, from SSRs shorting on, to simple coding mistakes negating the safety cut off function etc etc.Most commercially sold resistive heaters actually already include that thermal fuse, so check, they may already be fitted!

author: wraper
author_rank: Unknown
timestamp: March 06, 2018, 12:25:02 pm
content: If you do care a tiny bit about safety, then don't use fake SSR from ebay. Not only they are not safe but also are way weaker than ratings written on them.

author: Beamin
author_rank: Super Contributor
timestamp: March 06, 2018, 01:32:34 pm
content: SSR aren't really relays at all but rather just transistor based switches in a convenient enclosure? Are they specifically engineered to act just like relays less the problems with inductor voltage spikes?

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: March 07, 2018, 02:07:56 am
content: Could be SCR's but they are mostly TRIACS not transistors. They also are convenient in that they have a heatsink.Quote from: Beamin on March 06, 2018, 01:32:34 pm

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2018, 02:49:12 pm
content: Quote from: Quarlo Klobrigney on March 07, 2018, 02:07:56 amIt depends on whether the SSR is designed to switch DC, AC or either. AC SSRs do use a TRIAC or two SCRs. A DC SSR will have a MOSFET output and one designed for either DC or AC will have two MOSFETs, connected back-to-back.