AC mains circuit : safety related question
AC mains circuit : safety related question
(OP)
Hi Folks,
In the attached application note figure 12 - Page 7, is there a need to select a safety approved (UL) resistor Rx/2 for any type of safety certification purposes? The IC HCPL-3700 is already providing safety rated optical isolation between 110/220V mains input and 5V logic output. Any thoughts?
Thanks
In the attached application note figure 12 - Page 7, is there a need to select a safety approved (UL) resistor Rx/2 for any type of safety certification purposes? The IC HCPL-3700 is already providing safety rated optical isolation between 110/220V mains input and 5V logic output. Any thoughts?
Thanks





RE: AC mains circuit : safety related question
In general though, UL is about fire and personnel safety, not just protection of the low voltage side from the mains side. I will bet that you are going to need fuses (of type specified in the UL standard) on the mains side of those resistors and possibly resistors with a short circuit current rating.
Best to you,
Goober Dave
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RE: AC mains circuit : safety related question
Yes, this is a power monitoring circuit interfaced to a micro to monitor the AC input. You are correct that UL is about fire and safety and therefore thought of isolation between primary and secondary circuits might be important. A user might come in contact with secondary circuit in a computing (VME,CPCI etc) chassis type environment. The UL rated short circuit current resistors are not available in all values (e.g. 19.1K,1%,1/2W and 36K,1%,1W). This is the reason this question came up. It is great you pointed out, the UL rated fuses before the resistors is already selected in the actual circuit. UL-60950 is the primary target for this application.
RE: AC mains circuit : safety related question
RE: AC mains circuit : safety related question
Unfortunately, 60950 is not one of the UL standards with which I am familiar. The interrupting rating may not be applicable.
There should be some table in the standard that discusses mains-voltage equipment and category code that must be satisfied for each piece. You have a copy of the standard, correct?
In general, safety-rated resistors may either eliminate the need for upstream protection or reduce the requirements of upstream components. You may not need them if the upstream protection is adequate. Hang on and see who else answers.
You might also consider posting a link to this thread in the UL forum (http://www.eng-tips.com/threadminder.cfm?pid=244).
Good luck with it!
Best to you,
Goober Dave
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RE: AC mains circuit : safety related question
The TT application note goes along with part of my last post:
"In general, safety-rated resistors may either eliminate the need for upstream protection or reduce the requirements of upstream components. You may not need them if the upstream protection is adequate."
Again, I'm not sure if 60950 would have any different requirements. Give it a look-see.
Best to you,
Goober Dave
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RE: AC mains circuit : safety related question
I will try going through 60950 again to see I find something directly related. I have posted this thread link on UL forum also.
RE: AC mains circuit : safety related question
RE: AC mains circuit : safety related question