Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jan Tue 19, 2021 4:16 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 18 posts ] 
Author Message
 Post subject: Need help understanding my isolation & autotransformers
PostPosted: Nov Fri 27, 2020 11:58 pm 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
Hello.

I'm having difficulty understanding which of my items I should use, and how exactly to hook them up, when testing old tube equipment.

I apologize ahead of time if these questions are dumb. I'm a novice and I just cannot wrap my brain around this, and I want to be safe.

I've attached photos of what I have: One Brodhead Garret shop class power supply (variac) model P230/5, one Staco 3PN1010 variable autotransformer (variac), and one Triad N-48X isolation transformer.

Questions:

1) Isn't my Brodhead Garret unit both an isolation transformer and variac in one? I drew up the wiring diagram if anyone wants to take a look at it. If yes, can't I just use this unit by itself?

And if so, do I lift the ground pin of the item plugged into this unit if it has a three-prong cord, or do I lift the ground pin of the Brodhead Garrett itself at the wall?

2) If number 1 is no, then I can throw the Triad isolation transformer into a little project box, but I'm unclear how to wire it up and what order it versus the Staco variac go in, and who gets grounded and who doesn't.

Thank you!


Attachments:
group_variacs.jpg
group_variacs.jpg [ 855.57 KiB | Viewed 618 times ]
broadhead_garrett2.jpg
broadhead_garrett2.jpg [ 846.73 KiB | Viewed 618 times ]
broadhead_garrett3.jpg
broadhead_garrett3.jpg [ 747.69 KiB | Viewed 618 times ]
Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 12:06 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 1981
First of all, your diagram does not definitely assure you that the unit has isolation. To find out, unplug it and connect an Ohmmeter between the line input plug and the output plug, with the switches on.

If you see anything but infinite resistance, it's not idolated. If it shows infinity, it's isolated.

If it's isolated, you don't need anything else for use. If it's not, use your Triad transformer to isolate it.

As for the ground pin, leave them all connected.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 12:24 am 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
Hello,

Not sure if I'm following you completely, but when I put one probe of the ohmmeter on the unplugged hot lead of the unit's AC cord, and the other probe of the ohmmeter on the hot side of the variable outlet on the front of the unit (where you would plug something in to it), and turn it "on" and turn the variac up, I unfortunately get infinity when it's "off," and 2.8 ohms when it's "on."

So I guess that means it's not isolated.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 12:45 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 1981
Correct. You can use your isolation transformer to get isolation but observe maximum ratings.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 1:08 am 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1110
From your diagram the only thing isolated are the low voltage AC and DC. The 120 volt outlets are not isolated and one of them run off the variac. Your isolation transformer is only rated at 15 VA, not enough to run most radios.
Get a larger isolation transformer and use it with your variac or Garret

DM


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 2:12 am 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
You guys are good. I was going to ask next if the little Triad isolation transformer was strong enough, but it's not.

So who makes an affordable isolation transformer strong enough to use to help work on old tube amps?

I'm having trouble finding anything online. Does this mean I have to get on ebay and buy an old RCA Isotap or something like that?

Thanks!


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 2:12 am 
Member
User avatar

Joined: May Sun 10, 2020 2:55 pm
Posts: 780
Location: Bridgman, MI
not sure I can help.. others with more experienced might be more helpful. you do not want grd coming through ISO or varica to -> work on hand.

_________________
only the dead fish go with the flow


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sat 28, 2020 3:10 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10256
Location: Long Island NY
You are actually not that far off with your Broadhead Garrett unit. It contains a variac and a low voltage (24-V/12-V) power transformer. To convert this into an isolated variac, all you need is another 120-volt to 24-volt or 12-volt transformer that is capable of handling a couple of amperes on its output* side. Such transformers are quite common in low voltage garden lighting systems, pool lights, and computer UPS units. This new transformer can be fed back-to-back, i.e. put low voltage power into its secondary and it will step the voltage back up to 120 volts at its primary, and it will be isolated.

All you would do is connect the 12-volt or 24-volt wires of the new transformer to the 12-volt or 24-volt outputs of the Broadhead, as appropriate for the transformer you get. Use the 24-volt connection if you have a choice. Then the 120-volt primary leads of the new transformer become your output. Connect them to a receptacle so you can plug in the radios you are testing and you are all set.

Never defeat the grounded three wire plugs on devices like the Broadhead or the Staco variac. Those ground connections protect you in case anything goes wrong with the transformers or wiring and short circuits the AC line to the housings. The ground connections also bleed off the small leakage currents which are inherent in transformers. As for the isolated power receptacle on your new transformer, there are a couple of approaches you could take. One would be to simply use a two-slot receptacle that doesn't have a ground connection, if you can find one somewhere. If you use a common grounded outlet then by all rights the ground terminal should be connected to ground. That way if you someday plug something in expecting the ground to be there it will be. It does not diminish the isolation for radios which will invariably have two-prong cords on them anyway. There are not, and should not be any hot chassis devices with three-wire grounded cords, but if you did run across something like that, ground could always be lifted with a two-prong to three-prong adapter.

*Note (added later): Since the suggestion is to use the transformer backwards, i.e. feed power into the low voltage secondary and take the output from the line voltage primary, the term "output" here is used to refer to the primary. Therefore, a suitable transformer would be rated for considerably more low voltage current from its nominal secondary. For example, if we're talking about a 12-volt transformer and we wanted to get two amps out of it at 120 volts, the secondary would have to be rated for 20 amps. If a 24-volt transformer is used, then one with a 10 amp rating would be sufficient.

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

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sun 29, 2020 1:29 am 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
Chris108,

Thank you for your detailed response. I have a couple additional questions in regards to your comments:

1) Make no mistake, the following paragraph is over my head, and thus I am still confused about grounding in this situation. I have both read and watched videos which state that way back at the telephone pole, your home neutral and ground either are, or can be, connected, and thus this is why guys have damaged sensitive oscilloscopes plugging then into grounded, not-truly-isolated isolation transformers, and so I took this to mean that (and correct me if I'm wrong here) that while the variac and isolation transformer can be grounded at the wall outlet with a three-prong plug, the device plugged into the isolation transformer should have its ground lifted if it has a modern three-prong chassis-ground plug installed. Is this incorrect?

2) Otherwise, I understand what you're saying about using a 24/120 10amp transformer, plugged into the 24vac output of my Brodhead Garrett unit. Just to clarify, one secondary 24vac lead of this transformer would go into the 24vac output of my unit, and the other secondary 24vac lead would go into my common output?

Thank you again!


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Sun 29, 2020 11:30 pm 
Member
User avatar

Joined: May Sun 10, 2020 2:55 pm
Posts: 780
Location: Bridgman, MI
maybe you seen this... he has many good videos

https://www.youtube.com/watch?v=xaELqAo4kkQ

_________________
only the dead fish go with the flow


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 1:14 am 
Member

Joined: Oct Thu 29, 2020 10:17 pm
Posts: 42
Your Broadhead Garret unit is both an isolation transformer at the white outlet, but the black outlet is not. Simply use your multimeter on AC and check for AC voltage between the outlet and ground. You detect a little leakage, but these units were used in schools and I can assure you the white outlet is isolated, but the current is limited to something less than 2 amps. This should be adequate unless your working on old TV sets or high powered stereo.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 2:03 am 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
Ack now I'm getting conflicting answers, so now I don't know what to think.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 2:25 am 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1110
Bill Eccher Jr wrote:
Your Broadhead Garret unit is both an isolation transformer at the white outlet, but the black outlet is not. Simply use your multimeter on AC and check for AC voltage between the outlet and ground. You detect a little leakage, but these units were used in schools and I can assure you the white outlet is isolated, but the current is limited to something less than 2 amps. This should be adequate unless your working on old TV sets or high powered stereo.

Sorry but I have to disagree with you. One 120 outlet is fed from the incoming line, the other is fed from the variac which is a autotransformer (notice it only has three leads) There is no isolation on these circuits. As I already stated earlier the only circuits isolated are the low voltage circuits.
The suggestion by Chris to use another 24 volt transformer was a very good one, there should be room to mount it in the enclosure.

DM


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 2:37 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 1981
Loomis, you will need a bigger isolation transformer. They aren't cheap. I do have an extra one but shipping from Los Angeles may be expensive.

Stick with the comments I made and you will be okay.

In general you don't need an isolation transformer because most equipment was isolated. Exceptions were small radios, some TV sets, and various other units. In every case you need to check for isolation. The Ohmmeter test works most of the time but there are always exceptions. Modern gear is usually isolated also unless you are working on a switch mode power supply. These units have isolation of the outputs but the inner workings, the switch mode circuitry, does not.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 3:39 am 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
Thank you for your replies thus far.

The only thing I am still unclear on, and have read/watched conflicting answers, is the question of neutral and ground being possibly tied together at the telephone pole, and thus the need to lift the ground wire of the three-prong-chassis-grounded-unit that you're working on, which is plugged into the three-prong-grounded isolation transformer and/or variac.

Thanks


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 6:15 pm 
Member
User avatar

Joined: Oct Wed 14, 2009 6:36 am
Posts: 6072
Location: New York USA
Here is a 300 watt Isolation Transformer, big enough for testing any radio
https://www.amazon.com/dp/B00B886DGS


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Nov Mon 30, 2020 11:43 pm 
Member

Joined: Oct Fri 23, 2009 11:18 pm
Posts: 52
Location: Akron, Ohio
I looked at that one on amazon. Price is great; reviews are a mixed bag. Definitely no UL or any other certification on it anywhere. I believe it's 2.5amps. Is that enough?

Moreover, I see a lot of people asking the same question that I have, as far as that unit purposely being made with only a two-prong cord.

Thanks.


Top
 Profile  
 
 Post subject: Re: Need help understanding my isolation & autotransformers
PostPosted: Dec Tue 01, 2020 12:05 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 1981
If you are isolating a unit, don't worry about the grounding. If it has a 3 wire cord, you can ground the third green wire. If it has a two wire cord, don't bother. If you want to feel comfortable, ground its chassis to the primary side.

Isolation concerns the current carrying conductors, for the most part. The ground wire should never carry load current.

Two-wire cords are often what they call double insulated. There is an extra insulation layer for redundancy. But for a test bench one need not be concerned about it. Isolation is needed for units that have power line connected to chassis, mostly. Then we don't worry about orientation of the plug.


Top
 Profile  
 
Post New Topic Post Reply  [ 18 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Alan S, Google [Bot], stevebyan, thunderbird281 and 25 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB