Line Conditioning, Line Isolation, or...?

xanatos

Hi, I have a project that has been installed in a factory where the AC power is very noisy/spikey. I have one of those little surge supressor cubes installed (Delta, 05DKAG5, http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=05DKAG5&x=18&y=19 but they have a HUGE pump motor that they fire off on occasion, and it cooked the display in my project even with the surge supressor. So now we want to go with something more robust to condition the power to the box.

A bit of info about the box itself - the surge supressor currently only protects power to the logic/control section. There is a 40A solid state relay that controls a small (1/2HP) motor from the box. That leg of the power is NOT protected (and should be fine that way).

Here's the thing - if I need to use an external power conditioner of some sort, it will need to handle the full load of both the logic (~1/4 amp) AND the motor current (including startup surge - which can be in the 40 to 60 amp range).

Is there something better I should be using in place of the Surge Supressor that I can use IN the box to continue to protect the logic only, and if not, should I be using a line isolation transformer, or a power conditioner, that can handle the startup surge?

The client knows their power is horrid, and it really is. The lights in this place are always flickering, dimming, flaring due to their dozens of pumps, solenoids, and who-knows-what that are always firing on the lines.

Thanks for the advice,

Dave

PS., Just a side note - while the display got cooked, the BS2px was absolutely fine! Robust little things!

Tubular

You can look at a ferroresonant power conditioner. They are chunky little things but do a good job.

xanatos

Yikes! I can't even imagine the size of one of those I'd need to handle a 60 amp startup current! :-) I just looked at one that was the size of a (large!) shoebox and it only put out 4.2A max... Odd that the spike I got that killed the display did nothing to the supply, or the microcontroller and associated circuitry. Just the display (must have been because the display wasn't a Parallax product! :-)

Tubular

I don't think you'd need one rated at 60 amps (in the same way the circuit powering that 1/2 hp motor is probably only rated at 15 or 20 amps). We have a "large shoebox" one here that is rated at 750 VA, it weighs a heap, but its good for plugging inline and making these kind of issues go away.

Tell us a bit more about the layout of your system - including some distances etc. It would obviously be preferable to have the motor bypass the filtering but I gather there are internal space limitations in your box? Also is (was) the display running off a 5v or some other rail?

tonyp12

>AND the motor current
why is that?, the SSR will provide isolation to the BS2 and why not isolated the 5v going to the the BS2 with a 5v to 5v dc isolator:
http://www.mouser.com/Power/DC-DC-Converters/_/N-5gc7Zscv7?P=1yzssi7Z1yzvepkZ1z0x0e6Z1z0sptpZ1z0x09lZ1z0x0e5Z1z0x0e1Z1z0x04hZ1yzt19vZ1z0x08d&Ns=Pricing%7c0

xanatos

Yes, the 5v is powered from the AC that is inside the box, which is switched on/off by the SS relay which is controlled by my logic board. The circuitry inside - which I designed and built - is rated for 40A. I believe in over-engineering! :-) I usually rate my relays to handle the expected surge current, despite the fact that the relays are rated to take massive spikes over their rated current on a once-in-a-while basis. But I think about things like locked rotors, etc., and the possibility of a failure in the breaker on the box....... this probably borders on engineering paranoia, but it usually saves me when everything else around my box blows up and my box is the only thing that still works.... usually. :-)

What I did inside the box is to mount a standard AC outlet. The AC to this outlet alone has the surge supressor on it. The logic board is powered by a 5vdc 2A regulated switching "wall wart" with all sorts of built in overcurrent protections (short it out and it just shuts off until the short is removed, etc. VERY good units.) I do have SOME room inside the box, but not much more than about 3 x 3 x 6 inches. If there was something that could plug into that internal AC outlet, that I could in turn plug the power cube into, that would fit - it would be good. I can also add a power conditioning circuit inline with the 5V output.

The other paths from box "externals" into my logic are (should anyway) all be completely isolated. The switch sensor input is running on an optoisolator, and the output signal goes to a solid state relay (which has an optoisolator input section).

So now that I've described all this - how about if I just attach a picture of the innards..... :-)

Best,

Dave

xanatos

tonyp12 wrote: »

>AND the motor current
why is that?, the SSR will provide isolation to the BS2 and why not isolate the 5v going to the the BS2 with a 5v to 5v dc isolator:
http://www.mouser.com/Power/DC-DC-Converters/_/N-5gc7Zscv7?P=1yzssi7Z1yzvepkZ1z0x0e6Z1z0sptpZ1z0x09lZ1z0x0e5Z1z0x0e1Z1z0x04hZ1yzt19vZ1z0x08d&Ns=Pricing%7c0

The need for conditioning of the motor current as well was only in the event of needing to use an external conditioner of some sort. I am looking at the DC/DC converters you mentioned now, and honestly, I never knew that a 5v to 5v DC/DC converter existed. I would have thought such a thing was called a "wire" prior to this! :-) This is rated 1kv isolation and is quite tiny... this may well be a useful lead. Thanks!

davejames

xanatos wrote: »

So now that I've described all this - how about if I just attach a picture of the innards..... :-)

Dave - NICE layout!

Phil Pilgrim (PhiPi)

With the door closed, your electronics are in close proximity to the noisy power line wiring. I wonder if you might be getting some inductive pickup.

-Phil

xanatos

Thanks, davejames! :-)

Phil - Could be - but there are about 4" of separation between the wires on the door and the highest point on the backplane wiring, or the door switch (it's a deep box, even though it doesn't look that deep in the photo. It's about 7" deep, with the backplane being raised 1.25" off the cabinet back and the door itself actually rides about 1" above the lip of the cabinet)... But since it can't hurt, I'll add some shielding to that as well when I button it all back up.

I ordered some of those 5 to 5 dc/dc converters just as a "try & see" experiment. Still think it's off that only the display cooked and nothing else....

Thanks for the ideas - and any others who wish to chime in are all welcome! :-)

Dave

Duane C. Johnson

Hi Dave:

While your layout looks very nice I think I see some problems.

It looks like some of the wiring on the "hot" side is bundled with that on the "control" side. Lots of noise can be coupled across.

My main suggestion would be to redress the wiring to separate things. Possibly all hot stuff at one end of the enclosure and control at the other end.

You might want to put the control hardware in a separate aluminum box mounted on the cover. This box covers the processor and display. This should be bolted to the cover to act as a shield.

Duane J

kwinn

@xanatos

Switching regulators are not a good choice for a power supply from a noise filtering perspective. They operate at high frequencies, use small capacitors, transformers, and ferrite cores that do not absorb or filter short high energy pulses well. Better to use a linear supply where the greater mass of the transformer copper windings and iron core can absorb part of the noise spike energy and the larger capacitors filter out some of what is left.

Best to isolate both the power input and signal I/O of the control circuitry from the power as much as possible. You already have a good start by using an optically isolated SSR to drive the motor on the output end.
Add a power supply that has a good surge supressor (inductors, capacitors, and TVS) on the input of the transformer and good filter capacitors on the input and output of the regulators for your control circuit. Keep the control circuit wiring and boards as far from the high voltage/current wiring as possible, and connect the power supply transformer and ground directly to a breaker panel line, neutral, and ground.

xanatos

Hi Duane, kwinn,

Thanks for your feedback. Duane, while it looks like hot side stuff is bundled, it's not hot. The lines labeled P, P, and HS are all 5v (actually open-collector) pulsed lines, so no problems there. The lines running to the top of the solid state relay are 5Vdc and Gnd, so no problems there. The only other lines in the bundle are the actual 5v supply output lines. All the HV stuff (120vac) is confined to the bottom of the backplane.

The solid state relay is isolated from the control circuit by virtue of the optoisolator input section of the relay. The HS line is, while still a low-voltage signal (roughly 2.5v at the logic board), isolated with an optoisolator. The only unisolated lines are the tow "P" lines, which are an open-collector sensor. One of the P lines is the 5V out from my logic, the other line is the signal in. So... there should not be any HV and LV wires bundled together, and I have done what I can to isolate everything else.

Really, at this point, I am looking at replacing the 5V power cube with something more "massive" - meaning transformer (kwinn) - and other surge supressing items. Also specifically protecting the display itself, given that it seems the most sensitive item to spikes (it was the only item cooked in the surge).

Thanks for all of your thoughtful suggestions... It definitely got me looking at several angles.

Dave