Power supply mains transformer protection - Page 1
Forum section: electronics / beginners

author: RazSlack
author_rank: Contributor
timestamp: June 03, 2014, 09:43:30 pm
content: Hello there folks!I am building a linear power supply at the moment, and I'm trying to make the mains input decently protected. Now the specific part I want to make good protection for, is the mains transformer itself. Here's a picture of my current design:http://kepfeltoltes.hu/140603/Screenshot_-_06032014_-_09_29_30_PM_www.kepfeltoltes.hu_.pngThe transformer I am using is a toroidal transformer, that I'll wind up some extra secondary coils for fixed output voltages and power source for a meter as well. My main concern is PROTRES as shown in the picture. Question is, is it doing anything good there, or should I place something else there instead, or should I just omit that piece entirely?

author: kripton2035
author_rank: Super Contributor
timestamp: June 03, 2014, 09:46:40 pm
content: 3 varistors between live, neutral and earth would be a better protection IMHO.

author: RazSlack
author_rank: Contributor
timestamp: June 03, 2014, 09:56:43 pm
content: Should varistors go before, or after the fuses?

author: madires
author_rank: Super Contributor
timestamp: June 03, 2014, 10:06:35 pm
content: Quote from: RazSlack on June 03, 2014, 09:56:43 pmAfter the fuses and a single varistor between line and neutral is sufficient.

author: retrolefty
author_rank: Super Contributor
timestamp: June 03, 2014, 11:31:39 pm
content: I question the use of separate fuses on both the neutral and hot leads. I'm pretty sure that violates NEC rules/recommendations.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 03, 2014, 11:59:11 pm
content: You never want to be in a situation where a neutral fuse as blown but the hot as not, if there is any chance of coming in contact with the neutral part of a circuit.

author: Monkeh
author_rank: Super Contributor
timestamp: June 04, 2014, 12:28:05 am
content: Quote from: theatrus on June 03, 2014, 11:59:11 pmThe chance of coming in contact with the neutral should be exactly the same as the line. They are both live conductors.It is a non-issue inside an appliance.

author: hamster_nz
author_rank: Super Contributor
timestamp: June 04, 2014, 02:15:26 am
content: Quote from: Monkeh on June 04, 2014, 12:28:05 amAgreed - treat neutral is if it is live - who knows? you might have a break in the mains lead, or a fried rat somewhere in the in-wall wiring.However, in the spirit of "fail safe" not "fail to a death trap" shouldn't you only put a fuse the live, so any overcurrent event will isolate the device from the phase? Or is the neutral where you are not tied back to Ground potential at the switchboard (like it is here in NZ)?Mike

author: ejeffrey
author_rank: Super Contributor
timestamp: June 04, 2014, 02:41:27 am
content: Quote from: retrolefty on June 03, 2014, 11:31:39 pmThat is true in the US, but in much of Europe plugs are unpolarized or have inconsistent wiring and the standard is to switch and fuse both wires. This is required for double insulated devices using the unpolarized ungrounded europlug.It isn't really a problem as most anything with a metal chassis is grounded now. The US style might be theoretically safer, but remember that in the case of a live-ground fault the hot fuse is going to be the one to blow anyway, while a live-neutral fault should still be insulated from anywhere a user can touch.

author: Monkeh
author_rank: Super Contributor
timestamp: June 04, 2014, 02:53:52 am
content: Quote from: hamster_nz on June 04, 2014, 02:15:26 amNeutral, by definition, is a grounded conductor. Whether youhavea neutral or not, however.. not all systems do. And say you've got a 120-0-120V system like the US and you're a 240V device, neither conductor is neutral, both could short to ground if present. Both must be fused in this circumstance.Also, never assume you know which is line and which is neutral; not all plugs are polarised, even if you have a polarised socket, many standard connectors (IEC C7/8, anyone?) are not polarised.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 04, 2014, 04:00:29 am
content: Hmm, I see the distinction of fusing both ends in equipment in this case.For fuse applications in plugs (like UK plugs), are there a pair of fuses?

author: Monkeh
author_rank: Super Contributor
timestamp: June 04, 2014, 04:06:38 am
content: Quote from: theatrus on June 04, 2014, 04:00:29 amNo, because our plugs are polarised and the polarity is explicitly tested.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 04, 2014, 08:23:25 am
content: We used to get a lot of US equipment intended for their 240v system which had a fuse in both legs.Pretty sure it wasn't entirely legal in OZ,but they "slipped through the cracks".At one time,the orientation of "Active" & "Neutral" on power sockets ("GPOs") was a "recommendation" rather than a hard & fast rule.I found that out the hard way!We were fitting out a building with Surveiilance cameras.They had "pan & tilt" heads,which worked on 240v (made in dear old Blighty!).The motors had multiple connections to control the motor direction,plus cams which were adjusted to prevent the thing from smashing a lens against the wall,or other obstruction.The local "Smart Person" made up a box with multi contact keyswitches to enable us to perform these adjustments,as the operators console had not yet arrived.In the spirit of the "KISS" rule,he only switched the active line!The manufacturers insulated the Active side of the wiring in the pan & tilt head,but left the Neutral side uncovered.The process was:Climb up the ladder,pop the panel open,wait while the "other guy" moves the thing around to as far as you want it to go,adjust the cams to make that the limit,do the other end of the travel,check,& so on.Between each operation, the controls would be set to the "off" position,but the switch on the GPO powering the box wasnotturned off.There I am,on top of the ladder,the controls "off".I try to reset the cam,my finger brushes over a microswitch,& next thing,I'm picking myself up from the floor.Yep, reversed GPO connection!

author: RazSlack
author_rank: Contributor
timestamp: June 04, 2014, 07:22:25 pm
content: Regarding the double fusing: You made me question my design, and for a good reason actually. I am no expert in electric shock protection, but I live in europe, and we either use grounded plugs, or ungrounded (so called double insulation) plugs. I did some reading on this, and according to the local double insulation standard, double insulated devices are not grounded, and they must NOT be grounded at all. Question is, why? Most of the box enclosure I will use is mostly plastic, with the back and front panels only being aluminum. Should I ground those, or not, if I don't use double insulation?Anyway, I totally forgot about adding MOV protection to the circuit, here's a possible design for it, if I ground the panels, but the fusing is still a big questionmark, considering the fact that I don't know which lead is going to be live when connected to mains.http://kepfeltoltes.hu/140604/Screenshot_-_06042014_-_07_00_21_PM_www.kepfeltoltes.hu_.png

author: abaxas
author_rank: Regular Contributor
timestamp: June 04, 2014, 08:18:29 pm
content: What about a NTC doodar for current inrush?A hamster farting is enough to trip most modern consumer units.

author: RazSlack
author_rank: Contributor
timestamp: June 04, 2014, 11:35:50 pm
content: Quote from: abaxas on June 04, 2014, 08:18:29 pmYeah, adding an NTC sounds good, but while I'm at it, I think I should also add a relay there as well. Oh boy, this is getting really nasty! Am I adding too much protection, or is this all actually done in devices? The transformer in question is a toroidal core 230 to 2x30v (nominal 160VA), with 3 extra secondary coils added manually.

author: retrolefty
author_rank: Super Contributor
timestamp: June 04, 2014, 11:46:30 pm
content: Quote from: RazSlack on June 04, 2014, 11:35:50 pmCould be. There comes the time when adding more stuff just increases the chance for more components to fail in more ways.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 04, 2014, 11:49:45 pm
content: Quote from: RazSlack on June 04, 2014, 11:35:50 pmI think you are letting the excitement of the crowd carry you too far.A single fuse should do a proper job. That is what most gear uses.Now, depending on the inrush current, you may need a slo-blow fuse.Now, if you are way out in the jungle in the 3rd world operating on a generator powered by monkey dung, maybe you need some extra MOVs, etc.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 05, 2014, 01:46:18 am
content: Quote from: RazSlack on June 04, 2014, 07:22:25 pm"Double Insulated" has a very real Regulatory definition in most countries.Your box doesn't meet that definition,so I would strongly advise that you earth any exposed metalwork.

author: RazSlack
author_rank: Contributor
timestamp: June 05, 2014, 10:32:56 am
content: You're right, I'm making this a bit too much of a hassle.Yeah, I already ditched out the double insulation idea, since I want mains earth access from the power supply. This means I should remove one of the fuses as well. I'll add in an NTC (I don't want the transformer to go HUMM on power up.) and keep the MOVs, because the switch I'll use is a really old vintage valve era clunky switch that is susceptible to arcing. After all this, I'll just call it a day and finish the project.Thanks for the tips.

author: Kjelt
author_rank: Super Contributor
timestamp: June 05, 2014, 12:37:05 pm
content: How about a sparkgap for lightning protection

author: retrolefty
author_rank: Super Contributor
timestamp: June 05, 2014, 01:21:21 pm
content: Quote from: Kjelt on June 05, 2014, 12:37:05 pmOnly if wired to a 8 foot ground rod driven through the floor and into earth.