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 Post subject: Building a current limited, variable, isolation transformer
PostPosted: Feb Sun 04, 2018 3:53 pm 
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Location: Fairfax, VA
This is prep work for bringing up antique gear that I inherited from my Dad properly --

I understand it's a good idea to A) isolate gear from mains so you're less likely to zap yourself, and B) bring up gear slowly through the use of a variac/auto-transformer. I've read that using a tungsten light bulb with a similar wattage in-series with the device under test is a decent current limiter, and will also glow really bright when there's a short to ground/neutral.

So my idea is NEMA-15 cord -> light bulb in series -> 1A fuse -> variac -> isolation transformer w/ 115v + 230 output; so I have variable 0-100% of 115v and 230v output.

I'm looking at this transformer on digikey - the datasheet says "dual primary/secondary windings are designed to be used in parallel or series", I'm interested in having a max of 1A on the 230v side, and this transformer is rated for 2A @230v. If I de-rate this at 50%, wire up one of the dual primary windings as an 115v isolated output, would that be safe?

Assuming I'm only drawing 1A total (e.g. one 1A device plugged in at a time, on either the 115v output or 230v output)


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Sun 04, 2018 4:08 pm 
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You don't need both a Variac AND a dim-bulb. Personally, I would stick with a Variac + Ammeter.

Wire the transformer with primaries in parallel, and secondaries switched between series ans parallel. That gives you 115 at 2 amps or 230 at 1 amp.

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Sun 04, 2018 4:26 pm 
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pixellany wrote:
Wire the transformer with primaries in parallel, and secondaries switched between series ans parallel. That gives you 115 at 2 amps or 230 at 1 amp.


Yeah, I could do that; but I don't want to accidentally bump a switch and suddenly over-volt a device by 2x what it expects!


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Sun 04, 2018 6:41 pm 
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xabean,

Where are you located?
You might add that to your profile so it shows up on every post.

Are your mains 115 volts or 230 volts?
I __assume__ the radios are designed for that voltage. If that's true, design your isolated supply for only that voltage, unless there's a good reason to do otherwise.

The idea of bringing a set up at low voltage is only a preliminary test to check whether or not a short or similar problem exists in the set. Once that test passes, there's no reason to apply reduced AC voltage. Trying to run a set at reduced voltage can cause damage.

One simple option if you choose a dim-bulb tester is to have some cartridge fuses. Bring the set up initially with a bulb in the socket. If all is well, replace the bulb with a fuse and you will apply full line voltage to the radio.

I always recommend sourcing that line voltage through a Variac. That enables you to set the voltage to whatever value the factory used when reading the voltages on the schematic.

Note that the Variac should be fused at its nominal rated current on BOTH the input and output lines. That ensues that you won't draw excess current when it's turned down.

- Leigh

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Sun 04, 2018 11:14 pm 
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Quote:
So my idea is NEMA-15 cord -> light bulb in series -> 1A fuse -> variac -> isolation transformer w/ 115v + 230 output; so I have variable 0-100% of 115v and 230v output.

Put the light bulb as the LAST thing in the string right before the radio. That way it is only seeing the current from the radio and isn't being pre-loaded by the current needed to run the transformers. If you want two output voltages, you will need bulbs for both voltages.

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 12:42 am 
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here is a cool one, look homebrewed, add a DBT at the output.
https://www.ebay.com/itm/Vintage-Rack-M ... 2919794090
I like using a DBT, get a few wattage bulbs for different needs, you definitely know when there's a short!

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 12:47 am 
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never mind

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 1:32 am 
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Joined: Oct Sun 16, 2016 3:13 am
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Leigh: updated my profile, so it now shows I'm in Virginia, not too astronomically away from you. Anyway:

Re: 115/230v devices - I've got a lot of gear my Dad owned, some of it looks like it was removed from airplanes. Some is rackmount, some standard 115v electrical cords. Most of the power cords have gone brittle and are hazardous.

Long story short: I'm trying to be prepared for reviewing all this random gear my Dad had. I don't know if it wants 48vDC, 115vAC, 230vAC, etc.

Jim Mueller wrote:
Put the light bulb as the LAST thing in the string right before the radio. That way it is only seeing the current from the radio and isn't being pre-loaded by the current needed to run the transformers. If you want two output voltages, you will need bulbs for both voltages.


That .. that's a really good point. That's why I ask questions here :)


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 1:42 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Quote:
Wire the transformer with primaries in parallel, and secondaries switched between series ans parallel. That gives you 115 at 2 amps or 230 at 1 amp.


You are aware that this transformer is not a stocked item but has a 12-week lead time at Digi-Key?

It is rated for 500-VA. If the secondaries are wired in parallel, you get 115-VAC at 4.34 amps. In series you get 230-VAC at 2.17 amps. You can easily arrange to have one voltage output or the other with a DPDT toggle switch. Both primaries must be connected; put them in series if the input is 230-VAC or in parallel if the input is 115-VAC.

Always put a fuse on the output of a variac. Input fusing is optional. Variacs transform current in inverse proportion to voltage, so if the output voltage is set to a lower value it becomes possible to overload the winding without drawing enough current to blow a primary fuse.

Do keep in mind that an isolation transformer only makes you slightly safer in that it reduces the possibility of ground fault shock. There are plenty of other ways to get a shock, and they're still possible. Variacs don't do anything at all for your safety, but they are good for soft-starting equipment and matching the line levels given in alignment procedures and schematics.

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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 2:12 am 
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Location: Fairfax, VA
Chris108 wrote:
Both primaries must be connected; put them in series if the input is 230-VAC or in parallel if the input is 115-VAC.


This is something I'm actually trying to figure out. (I've actually got an e-mail into the manufacturer asking of I can use one of the primary windings as an isolated secondary, if I only pull 1A through it ever)

Educated-enough-to-be-downright-dangerous guess: I'm assuming that the dual primary windings wired in parallel effectively doubles the current capacity on the input side of the transformer, so to say. So if I halved the current I was to try to pull through it (and never exceed it, by means of fuses on the input and output), would that be OK?

I don't know if the transformer is physically constructed in a way that one of the two dual primaries is like ... somehow magnetizing only half the core? Where wiring up the second primary takes care of the remaining half.


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 2:16 am 
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xabean wrote:
I've actually got an e-mail into the manufacturer asking of I can use one of the primary windings as an isolated secondary, if I only pull 1A through it ever.
...
I don't know if the transformer is physically constructed in a way that one of the two dual primaries is like ... somehow magnetizing only half the core? Where wiring up the second primary takes care of the remaining half.

Yes, you can use one primary winding (out of two) as a secondary.
The available current will be somewhat less than 50% of the total rated current.

I do this with one of my products.

It's not possible to magnetize only half of the core.
Reduced excitation magnetizes the whole core, but only at half the amplitude,.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 2:48 am 
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Joined: Oct Sun 16, 2016 3:13 am
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Location: Fairfax, VA
Leigh wrote:
Yes, you can use one primary winding (out of two) as a secondary.
The available current will be somewhat less than 50% of the total rated current.


Thanks. I wanted to have something near 1A but not too much more. 1A is plenty enough as-is. Okay, it sounds like if I get 3/4 amp fuses on the input and outputs (and again, only drive one device at a time through the transformer) it should either A) blow one or more fuses protecting the setup, or B) be safe from melting down due to too much current being drawn through it.

I was planning on using cartridge fuses, Dad had a small mountain of the case mount cartridge fuse twist-cap ones.


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 2:56 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Quote:
This is something I'm actually trying to figure out. (I've actually got an e-mail into the manufacturer asking of I can use one of the primary windings as an isolated secondary, if I only pull 1A through it ever)


They will likely tell you not to do it. Not that there's any physical reason, but it has to do with the way the safety ratings work. The primaries have a breakdown voltage rating to the frame of the transformer and to the secondaries. But it is not necessarily the same rating from one primary to the other. They're not going to recommend any mode of operation that has not been tested and sanctioned by UL or whoever does their compliance testing.

Quote:
Educated-enough-to-be-downright-dangerous guess: I'm assuming that the dual primary windings wired in parallel effectively doubles the current capacity on the input side of the transformer, so to say. So if I halved the current I was to try to pull through it (and never exceed it, by means of fuses on the input and output), would that be OK?


The transformer is rated for 500-VA output with both primaries in use. If you only apply power to one primary, the maximum output you can take from all the other windings combined cannot exceed 250-VA. Law of physics: you can't take more power out of a transformer than you put in, and the most you can put into one primary in this case is 250-VA.

Quote:
I don't know if the transformer is physically constructed in a way that one of the two dual primaries is like ... somehow magnetizing only half the core? Where wiring up the second primary takes care of the remaining half.


One core, so all the magnetic field the transformer has to be contained in it.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 11:02 pm 
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Joined: Dec Tue 23, 2014 6:51 pm
Posts: 686
Location: N. Palm Bch, Fl.
Chris made a statement. Isolation transformers and Variacs don't do anything at all for your safety.
How True--How True. Today I was taking some voltage checks and laid my hand on the chassis to hold
the probe steady. I reached over to turn my VTVM down and hit the metal case. Goch Ya. I don't think it
can kill you because you move to fast, but it sure says I wasn't concentrating.

I can't wrap my brain around what your trying to build, but I'd love to see a picture when complete.

Freeman.


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 Post subject: Re: Building a current limited, variable, isolation transfor
PostPosted: Feb Mon 05, 2018 11:09 pm 
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Location: Tucson, Arizona U.S.A.
Quote:
Re: 115/230v devices - I've got a lot of gear my Dad owned, some of it looks like it was removed from airplanes.

If so, it probably requires power at 400 or 800 Hz. Running it at 60 Hz will burn out the power transformers. The setup you propose won't work to power this equipment since it doesn't change the frequency. You need a frequency converter for that. Those are quite expensive unless you can find a surplus one. Even then, the price will depend on what the dealer thinks it is and how many customers he thinks are looking for one.

You can't simply replace the power transformers with ones made to run on 60 Hz. 400 or 800 Hz transformers are considerably smaller and there won't be room for a lower frequency unit. Filter chokes and capacitors for 60 Hz are bigger too. You will have to build an external power supply.

Some tube type aircraft equipment that ran on 28 VDC can be converted to 120 VAC 60 Hz. It requires removal of the existing power supply or dynamotor and using that space for the new power supply (built by you).

Since you got this equipment from an old timer, it may already have been converted one way or another. So you may have to look for two pieces that work together. And if it doesn't look original, don't trust the nameplate rating. You'll have to figure it out for yourself.

Quote:
I don't think it
can kill you because you move to fast,

An isolation transformer can supply enough current to paralyze your muscles so you can't move. You most certainly can be killed. If you aren't, you are lucky.

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Jim Mueller


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