Isolation Transformer Use???

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Bench safety

If it is what I think it is, it is used when working on electronic equipment when the the design of the equipment is such that the mains current isused directly. I.e., the equipment being worked on has no transformer of it's own. Working on such a piece of equipment is hazardous because anything you couch is directly connected to the mains voltage. This is rare in currently maufactured audio gear, but I understand it was common in the past, especially with regard to old radios and TV's. (New TV's may still be like this -- I'm not familiar with their insides.)

Thus is is a piece of safety equipment that should be used in the circumstances described.

However, it posses a danger if misused, namely by plugging test equipment into one. Never, plug test equipment especially scopes in to an isolation transformer as there will be no ground (it will be "floated"). I once worked at a place where this was such a strong rule, doing so was a first time firing offence.
 
An isolation transformer is used to remove the DC ground connection to the AC power supply lines. It is simply a 1:1 transformer (usually same output voltage as input voltage). These are used when working on AC/DC type radios and TVs Which have no transformer and therefore can have a "hot" chassis depending on how you plug them in to the power outlet on the wall. In this situation, the isolation transformer is used to protect the technician doing the work. Without the transformer, if he were to simultaneously touch a "hot" chassis and a grounded piece of test equipment, he would receive a nasty shock.

Isolation transformers are also used on test equipment when you need to make a measurement that is not referenced to ground. Sometimes a circuit has a DC bias on the "ground" side and if you connect the ground of a scope probe to it, it will short the DC supply and maybe damage or destroy the circuit under test. It will certainly screw up the measurement. If you plug the scope into an isolation transformer, you break the DC connection from scope ground back to AC power system ground, and the measurement can be made safely.

NEVER use a variac in place of an isolation transformer. They are autotransformers and provide NO isolation from the AC power system at all.

MR
 
I use a 5000va isolation trannie for my whole audio system. It actually improves the sound a lot and I have little trouble with ground loops as well. Depending on what you want to do, you might try your hammond on a cd transport or any amplifier that draws under 500W from the transformer. The design of an ac isolation transformer is quite effective in absorbing high frequency transients in the ac line and thus may clean up the noise floor on your system. If you had a saturation core transformer, it would do an even more effective job of eliminating noise and it would also regulate the voltage during surges and brownouts. These saturation transformers are, however, difficult to find, especially if you want one capable of high power.

Experiment a bit and see what you like most about your hammond, perhaps you'll find just the perfect piece of audio equipment to use it with.
 
MRehorst said:
Isolation transformers are also used on test equipment when you need to make a measurement that is not referenced to ground. Sometimes a circuit has a DC bias on the "ground" side and if you connect the ground of a scope probe to it, it will short the DC supply and maybe damage or destroy the circuit under test. It will certainly screw up the measurement. If you plug the scope into an isolation transformer, you break the DC connection from scope ground back to AC power system ground, and the measurement can be made safely.

This can be a very dangerous practice as you may find that the scope is suddenly floating thousands of volts above ground. DO NOT attempt this unless you are suitably qualified and understand all of the risks involved.
 
It is very true that using test equipment on an isolation tranny can be quite dangerous if done improperly. It really depends on what you are trying to do. It seems that in the case of audio equipment, there aren't many risks as long as proper grounding and isolating schemes are employed. I found that with my iso tranny in place, there were less dangerous voltage potentials from ground to certain points in general equipment. This may not apply though when things like scope leads are left to float.
 
Duo said:
I use a 5000va isolation trannie for my whole audio system. It actually improves the sound a lot and I have little trouble with ground loops as well. Depending on what you want to do, you might try your hammond on a cd transport or any amplifier that draws under 500W from the transformer. The design of an ac isolation transformer is quite effective in absorbing high frequency transients in the ac line and thus may clean up the noise floor on your system. If you had a saturation core transformer, it would do an even more effective job of eliminating noise and it would also regulate the voltage during surges and brownouts. These saturation transformers are, however, difficult to find, especially if you want one capable of high power.

Experiment a bit and see what you like most about your hammond, perhaps you'll find just the perfect piece of audio equipment to use it with.
Try it on the digital gear first, esp if it's an EI core, as the limited bandwidth of most EI trannies will reduce the amount of rubbish being returned to the mains sytem in your house by your digital components, which can get back into other components.

One of the best investments you can make is to buy a big surplus 5kVA+ iso trans (try metal recycler) and have your entire system run through it from the main fusebox (check local regulations, and get an electrician to wire it up). Cost might only be a couple of hundred bucks total. Having done the tests and seen the difference in the mains spectrum, bith with and without a big iso trans on an HP spectrum analyser, its a hell of a good investment.

Cheers
 
Floating test equipment.

AudioFreak said:


This can be a very dangerous practice as you may find that the scope is suddenly floating thousands of volts above ground. DO NOT attempt this unless you are suitably qualified and understand all of the risks involved.

Well, this is done as needed. I've done it myself from time to time. However, you need to be aware of the risks involved.

Usually when floating test equipment, it best to measure the level of float voltage before you start, unless you know what it is. Somewhere on tektronix web site, they specify you can float scopes by +- 30V. (This is probably because 30V is considered safe). However, if done carefully, higher voltages can be floated. Usually, the equipment can be isolated, and then use the test equipment ground to move the ground potential where ever it is needed. Again, you need to be careful.

Also keep in mind that sometimes even an isolation transformer isn't enough. TV picture tubes usually run around 25 kilo volts. If your transformer is rated for 2kV isolation, it is possible for the 25kV to actually blow through the isolastion transformer. Most transformers are only rated for a couple of KV isolation. So there are still risks involved. (However, you shouldn't really be measuring the 25Kv with a scope anyway ;) )

FYI, Most modern TV tubes are also the capacitor used for filtering the flyback, and even when off can be charged to 25kV. Make sure the tube is discharged before removing the high voltage lead.

The bottom line when working on with ANY electrical equipment is if you don't know, don't!

-Dan
 
I would check with Hammond and find out the winding ratio and the number of primary and secondary windings.
If it's at least 1:1, it's great for audio gear!
As Brett suggests, try it on the grungy stuff, DIGITAL.
Do not connect the CT. This defeats the purpose of Isolation.
THEN, providing it's an EI, if it has dual primaries and dual secondaries, wire it up in Balanced Power for some real improvement.
 
Wheezer,

I am very interested in your comments!.

The transformer in question is a Hammond 171E. I could not tell from the website if it has dual primary & secondaries.

Any help would be appreciated on how to determine this, and also how to wire it for balanced power if it can be done.

Cheers

KevinLee:)
 
Hi KL,
You cannot get to the windings as it's sold as strictly an ISO and carries a UL listing.
It's a 1:1 which negates any possibility for BP, but great for an ISO on the front end of your equipment.
FIRST
Plug it in and check the voltages to determine the regulation %.
Also L-G and N-G voltages
THis is an overlooked issue and very important.
Second overlooked issue is tranny overshoot.
If you would like details, I'd be happy to elaborate further.
Regards,
Mitch
 
Wheezer,

I measured the voltage at the wall outlet and got 117volts. I checked the secondaries out of the transformer and got 120 volts.

I also checked L-G & N-G & got 70 volts & 23 volts. I am not sure which measurement is the L-G & which one is the N-G.

Any thoughts on these measurements? Also any more guidance would be appreciated on how the carry out the other tests you mentioned.

Thanks

KevinLee
 
A binge of lunacy

Hi KL,
Rather impressive 3% regulation.
The wacky voltages means the center tap is not grounded. - a good thing. If grounded boths would/should be equal 1/2 the total. 60VAC each. NOTE, one is floating to ground.
Measure them with a load-- power strip and a light bulb, then note the voltages. to each other and ground.
You won't know which is N or L, with an ISO tranny
As for the Hammond it's designed specifically for ISOLATION.
plug that pup in the wall and your gear into it.
Do check the LOAD of each piece of your gear-the back sticker for WATTAGE.
If you go over the rated VA, the voltage will start to drop and the Temp will start to rise.
A power strip would be good for testing as you can stick you probes into one socket and test for Vout.
You can get up to 6dB of noise reduction from just an ISO.
We can always add some small trannies(185) wired in BP, or filtered BP mode fed by the 171. It does not get better than this!!!!!

Typical is 10% regulation on the average.
FYI, this would mean no load Secondary Vout would be 132VAC from 120 mains.
By adding some load say 250VA or 1/2 the tranny's rating would bring the Vout to 126. Some gear, say a 10va DAC might have some problems with 132V... So, it's important to choose the tranny according to ones load.
We'll deal with the overshoot issue later!
Regards,
Mitch
 
Losses.......

"One of the best investments you can make is to buy a big surplus 5kVA+ iso trans (try metal recycler) and have your entire system run through it from the main fusebox (check local regulations, and get an electrician to wire it up). Cost might only be a couple of hundred bucks total. Having done the tests and seen the difference in the mains spectrum, bith with and without a big iso trans on an HP spectrum analyser, its a hell of a good investment."

But the idling power consumption of such a transformer will keep you poor paying electricity bills.
In my experience isolation transformers more suitably rated for the load will give very nice sonic improvements.

Eric.
 
Hello Wheezer,

I now have the iso tranny in my place. Here is what I found out:

-Wall voltage, 120.6 volts
-Transformer out no load, 123.9 volts
-Transformer with CD player plugged into it, 124.0 volts
-Transformer with Amp plugged into it, 124.0 volts
-Transformer with Amp & CD plugged in, 123.7 volts

Amp is a Rotel RA930AX (30-40 watts per channel) Power consumption is listed at 180 watts on back of amp.

CD player is Marantz CD63. No power consumption listed. All that is listed is 120volt, 60 hertz, 0.19 amps.

The transformer is fairly warm when plugged into the wall with no load. Is this normal?

Also, there is a slight humm from the transformer, it is not passed to the music.

As before, any thoughts would be appreciated.

Thanks

KevinLee
 
Hi KL,
Very interesting, I have an RB930AX, that I have had sitting dormant for several years.I recently decided to use it as a mono bridged for the center channel. Have to ramp it up slowly on the Variac and check for any DC offset. Thanks for reminding me.
I have not played with this particular isolation tranny, but some do get warm, USUALLY when loaded almost to max.Not NO LOAD.
Some do hum, usually the big Topaz. As long as the hum is not passed on AND it does not ge that hot... no problem
Just enjoy it....keep adding more stuff but not to exceed around 400VA. safety margin.
Have you noticed any difference in sound?

If you check the specs you'll see the operating temp s/b pretty high. If you have a temp probe,now's the time to try it. Again another thing I must get, my DMM does C and F but did not come with one.
Regards,
Mitch
 
We can always add some small trannies(185) wired in BP, or filtered BP mode fed by the 171. It does not get better than this!!!!!

Mitch,

It seems the most objectionable noise on the hydro lines is spiking, often caused by local motors and switch mode power supplies
throughout the house. This seems to be very difficult to filter out.

Transformers with low capacitive coupling between the primaries and secondaries seem to filter out some of this spiking noise.
E core transformers with dual bobbins have low capacitive coupling.

Also, I've noticed that audio equipment can also put noise back on the hydro line. So if another piece of audio equipment is plugged into the same outlet,
noise will show up on its supply rails.
For example, if I look at the supply rails of my preamp on a scope, things are OK. But my power amp has a big old humming E core transformer.
If the power amp is switched on, spiking noise shows up on the supply rails of the preamp.

My Blue Ray play player (with a motor) , DAC and Preamp dissipate about 8 to 12 Watts each. I used a x10 rule and bought Hammond 185G230 transformers rated at 175 VA.
Low Voltage Chassis Mount (185 Series) - Hammond Mfg.
The intent was to have a separate isolation transformer for each piece of equipment.

But here is the issue, I realized that for reasons of safety, the voltage potential for each "Hot" and "Neutral" on the secondary side, must be the same for each outlet.

Any recommendations on how multiple isolation transformers can be safely wired - using unbalanced power ?

.
 
Last edited:

PRR

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Joined 2003
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> If the power amp is switched on, spiking noise shows up on the supply rails of the preamp.

That is a function of how much resistance is in the line from the hi-fi shelf back to the generator. (Mostly in your walls and the line to the street.)

Transformers won't fix it.
 
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