Isolation transformer - surge protector.

[KE6DF]

I've been looking for a solution to the problem of high line voltages on old equipment designed for 115vac.

I recently picked up an old Stancor isolation transformer that has primary taps for 105v, 115v, and 125v. The output is 115v.

The 125v tap works great with our 123-124 V line voltages.

The transformer is rated at 1KVA.

Anyway, I've been using it as is, but I think I'll mount it in a cabinet with a line volt meter and a circuit breaker and a switch to select different primary taps.

I was also thinking perhaps it would be useful to put some kind of surge protector circuit in the box with the tranny.

What is the circuit used for surge protectors? Is it just a cap across the lines to short out transients? Some kind of LC filter?

Or is this not even necessary/useful when using an isolation transformer?

[KE6DF]

I guess I'm making progress answering my own question.

Been reading about metal oxide varistors (MOVs).

[Opcom]

Those will do. Also, "Transils", a solid state Zener-like clamp is faster and a good compliment to MOVs, or just by themselves as they are generally used. Info on the is at ST.com, and other semiconductor manufacturers pages. That can be had at digi-key.

In another thread, it was said that the instantaneous unloading of a transformer by disconnected the primary (un-keying a large plate supply etc.) can cause very large transients. So if you plan to run any power through that at all, protection on either side or both is certainly a good idea. It's cheap insurance.

[KA2DZT]

Not sure that any surge protection is necessary for any old tube equipment.  When this old stuff was made, probably nobody ever heard of MOV's.  The Stancor isolation xfmr can't hurt and is a plus when using equipment that doesn't have a power xfmr.

The best way to adjust the line voltage is to use a Variac

[w3jn]

Surge protectors are a waste of time and money and, if installed incorrectly, create a fire hazard.  Because they permanently short when they clamp, the device needs to be fused upstream.  Often, in surge protector outlet strips, they aren't fused at all and rely upon the service breaker to blow when they short.  That's a recipe for disaster.

Additionally, if you plug in a surge protector outlet strip into a 220 to 110 auto transformer, chances are 50-50 that the voltage from one side to ground will be 220V which exceeds the clamp voltage.  They'll eventually short, and burn up the autotrasnformer causing a fire. 

My employer has banned all surge protector outlet strips in its facilities.  We deal with a LOT of delicate electronic equipment at work, including outdoor CCTV cameras, and it's been over 15 years since I've seen a failure attributable to a power surge - and that was on a single CCTV camera that failed during a lightning storm.  IMHO these things are an insidious "feel good" waste of money.  I'm sure something, somewhere, has been saved by a MOV though 

[K5UJ]

Hi,

I think the isolation transformer is okay (I have always called the thing you seem to have a buck transformer but maybe I have it wrong) but periodically check the v. to the gear just to make sure it is 115 because there can be seasonal variations with customer loads like air conditioning.  I usually tap mine so that the v. to the equipment is lower, like around 110 to 113 so if the supply v. goes up it is still not awful.   Some vintage gear is designed to handle 117 or even 120 (Hallicrafters seemed to be rated for that).  

I don't think you need a surge suppressor for vintage vacuum tube gear.   If I were dead set on using one, I'd do the following:  avoid using MOV equipped suppressors.   Go with the Price Wheeler "Brickwall" line of professional grade suppressors, that do not employ MOVs.  What's wrong with MOVs?  They have a shelf life.  They also have a limited number of cycles.   But anyway, secondly, I'd install it before the buck transformer and not between it and the equipment.  But again, vacuum tube gear is so robust, you probably don't have to worry about using one.  

What may be a concern if the equipment is a transmitter, is in-rush.  In a 1997 ER article, (ER 99 July 1997 p. 26) W8ZR recommends using current limiting thermistors in series with the primaries of the supply transformers in transmitters.  Insert in the hot line(s) to the primary in a location where it has air around it because it will run hot.  In his article he focused on the Johnson 500.  He measured a plate supply current draw of 4.5 A on the line when modulating 100% but the in-rush measured 23 A!  (which is why the fuses would blow.)   I advise reading the article but it says you can get thermistors from Mouser and Digi-Key.  Based on fuse ratings, he recommends Digi-Key part nos. KC014L-ND (1 A), KC020L-ND (4 A), KC007L-ND (6 A).
If you can find these (if they still list them) you can study their spec's and extrapolate to your own fuses what you need I hope.

[WQ9E]

I don't like the outlet strip surge protectors for the reasons John noted.  But out here in the rural area surge protection is needed.  I installed entry types at the main distribution box and then each breaker box (house, barn, both garages) has an additional unit installed at the breaker box.  The ones that I use are multi-stage units installed in a metal case and are fed through a dedicated breaker on each panel.

Prior to installing these I had two separate major loss events in the first two years I lived here.  After installation (1992) I have lost nothing due to lightning.  There is additional light flickering during thunderstorms after I installed them and a continuous buzz from the box when running off my backup generator (which doesn't say much for the generator cleanliness).  

http://www.iceradioproducts.com/impulse2.html#6

[KE6DF]

Hi,

I think the isolation transformer is okay (I have always called the thing you seem to have a buck transformer but maybe I have it wrong) but periodically check the v. to the gear just to make sure it is 115 because there can be seasonal variations with customer loads like air conditioning.

The transformer I have isn't a bucking transformer. Nor is it an autotransformer. It's a full blown isolation transformer with two separate windings. The only slightly unusual feature is that it has three taps on the primary.

My voltage is a solid 123-124 vac all year around -- even with our AC and those of our neighbors on.

Our house was built 6 years ago and the whole area was farm fields 10 years ago so the whole electrical infrastructure is new and underground. I don't think there is a single above ground power pole within 5 miles.

The power transformer for 1/2 the block is in our front yard, so no significant drop between it and our electrical service.

For these reasons, we seem to have very stable line voltages summer and winter -- but they are high.

I plan to put a meter on my "power box" and also a switch so I can select the step down or 1:1 taps, so if the power to my shack does vary I can deal with it.

As to whether isolation transformers are better than variacs, I don't know about that. Variacs can have a voltage sag problem. But then so can isolation transformers. Isolation transformers do have the feature that neither output line is grounded so shock hazard is reduced a little.

I also plan to put in the box a beaker rated slightly above the max rating of the transformer, and a fast blow fuse rated a bit higher.

Dave

[KM1H]

Using a tapped isolation transformer or a bucking transformer is an excellent way to protect and extend the life of vintage gear. Much older equipment is designed to work at a "nominal" 110-117VAC and even if it is speced at 105-125 that high end is an invitation to a meltdown. Hallicrafters, some Nationals and others are already entering transformer saturation at ~123V from many tests Ive run here. That means core losses are now increasing rapidly on a new curve rather than linear. In addition old transformer insulation breaks down faster with elevated temperature even without a saturated core.

My line was 124-125V for at least 10 years and I had added full bench bucking transformers using oddball (hamfest cheap) voltages to bring the line down to 108-113V or so. Everything ran much cooler; panel lamps didnt blow, etc.

About 2 years ago the line dropped to 119-120 so I changed a few transformers even tho the gear seemed to run fine at 100-105 input.

The past 2 weeks the line has been 114-115 and Ive no idea why but Im not complaining. Perhaps the utility company was tired of complaints of CFL's causing fires

Id stay away from any SS suppressors mounted inside equipment including those input surge limiters mentioned above and sold under various names such as CL-90, etc. Those things run too hot to touch even in normal conditions.

Ive built my own MOV protected outlet strips using old all metal strips with built in fuses or breakers and the largest MOV's I could find at reasonable cost which were 275V, 20,000A peak rated with built in thermal fuses. There is no need for using a 130-150V device which is always going to be limiting at every little harmless spike. I let dual rated X-Y disc capacitors take care of the little stuff inside the equipment as well as in the outlet strips which also include LC lowpass filters out of QST/Handbook.

I recently picked up some new surplus mega modules to mount inside the main panel
but havent got to it yet but will before lightning season.  http://www.littelfuse.com/products/Varistors/Packaged/BA/V251BA60.html

Carl

[W3SLK]

Dave said:

My voltage is a solid 123-124 vac all year around -- even with our AC and those of our neighbors on.

Same here. When the power company was out her when voltages were 127~129, I asked if it could be tapped down to 117. Last I checked, it has creeped back up to 124. Its probably more responsible for my transformer faults than what I think.

[Steve - K4HX]

Do some searches on these.

Ferroresonant transformer

Motor Generator set

[k4kyv]

I pulled a bunch of RCA Iso-Tap isolation transformers from the dumpster at work a few years ago.  They were originally manufactured for use with bench work in TV repair shops. Mine came from a discontinued electronics lab at the local high school.  They are rated at 400 V-A, and have several taps for line voltages ranging from 105 to 130 volts.  The output side has two sets of receptacles each for 105, 115 and 125 volts, either tapped directly from the primary, or isolated, from the secondary winding.

I find them very useful; glad I grabbed every one I could find.  I gave one away, but kept all the rest. One goes between the mains and the TV and other home entertainment equipment, and I use the rest at various points in the station for isolation purposes and line voltage adjustment. They work great for adjusting the temperature of a soldering iron.

I don't use a surge protector with the transformers.  The isolation transformer itself, unless it is hugely over-rated for its load, should offer some surge protection of its own.  A transformer will handle only so much power or volts per turn; past that point the core magnetically saturates and further increase in primary voltage results in no more increase in secondary voltage. That should offer at least some protection against high amplitude surges and lightning spikes, particularly if the the isolation transformer is loaded close to its maximum power rating.

[WA1GFZ]

MOVs have been banned in aircraft for at least 30 years due to the end of life failure modes. MOV were used in Cubic power supplies and have taken out many an input bridge.
They have two failure modes dead short and explode or high leakage and make lots of heat.
An isolation transformer is good for breaking up ground loops. Also good for lightning isolation. Better to have lightning take out the ioslation transformer primary than your equipment. 

[KA2QFX]

I have to agree, MOV.s are more trouble than they're worth.  They do however keep the "power strip" market alive due to their inevitable failure mode.

Here's my solution to transient (and RF) line suppression.  I grab these chokes whenever I see them and have built several over the years. The one below is going together for my 20 amp 220V line in the shack. The chokes are 370uH and the caps are .5uF/600V I set them up as Pi filters.

I also have a pair of 5KVA Constant Voltage Transformers; they're great for line regulation but aren't worth the weight, core buzz and power losses for transient suppression alone. 

Nowadays these "Power Line Conditioners" are popular with Sound and Stage types. But 3KVA of conditioning can run $1500 or more. I don't know whats in them besides a bunch of toroids and caps but I'm told they behave much like CVTs.  I'm skeptical frankly.

[WA1GFZ]

CVTs have a nasty waveform but they do regulate.

[Opcom]

Surge protectors are a waste of time and money and, if installed incorrectly, create a fire hazard.  Because they permanently short when they clamp, the device needs to be fused upstream.  Often, in surge protector outlet strips, they aren't fused at all and rely upon the service breaker to blow when they short.  That's a recipe for disaster.

Additionally, if you plug in a surge protector outlet strip into a 220 to 110 auto transformer, chances are 50-50 that the voltage from one side to ground will be 220V which exceeds the clamp voltage.  They'll eventually short, and burn up the autotrasnformer causing a fire.  

My employer has banned all surge protector outlet strips in its facilities.  We deal with a LOT of delicate electronic equipment at work, including outdoor CCTV cameras, and it's been over 15 years since I've seen a failure attributable to a power surge - and that was on a single CCTV camera that failed during a lightning storm.  IMHO these things are an insidious "feel good" waste of money.  I'm sure something, somewhere, has been saved by a MOV though  

maybe. A surge protector outlet strip in an office at work caught fire. The MOV did it. The internal breaker did not trip. If the device had been a transil and had actually failed, the breaker would have tripped. Of note is that there was a 'burning electronics' odor for several days before the MOV fired up and the office occupant was not aware of the cause.

For regulation here, two G.E. Inductrols are waiting to be installed. Each is rated 200A. CVT are noisy and make distortion. Another option is the servo-controlled buck-boost autotransformer.  I got a couple of 6KVA ones. Maybe they will be OK for the equipment racks, i.e. making 115V for the gear instead of 125.

Is energy saved by running big stuff like air conditioning from 115V instead of 125V?  10% perhaps?

[flintstone mop]

I pulled a bunch of RCA Iso-Tap isolation transformers from the dumpster at work a few years ago.  They were originally manufactured for use with bench work in TV repair shops. Mine came from a discontinued electronics lab at the local high school.  They are rated at 400 V-A, and have several taps for line voltages ranging from 105 to 130 volts.  The output side has two sets of receptacles each for 105, 115 and 125 volts, either tapped directly from the primary, or isolated, from the secondary winding.

I find them very useful; glad I grabbed every one I could find.  I gave one away, but kept all the rest. One goes between the mains and the TV and other home entertainment equipment, and I use the rest at various points in the station for isolation purposes and line voltage adjustment. They work great for adjusting the temperature of a soldering iron.

I don't use a surge protector with the transformers.  The isolation transformer itself, unless it is hugely over-rated for its load, should offer some surge protection of its own.  A transformer will handle only so much power or volts per turn; past that point the core magnetically saturates and further increase in primary voltage results in no more increase in secondary voltage. That should offer at least some protection against high amplitude surges and lightning spikes, particularly if the the isolation transformer is loaded close to its maximum power rating.

I just got fired up to try to find another RCA Isotap. I have had one on my R390A ever since I bought the radio from a fellow Ham. Despite the advice of Rick Mish, I set my tap for 115 VAC. Pilot lamps last for years of 24/7 duty and I have never replaced a Ballast tube.
Fred