# Isolation transformer

#### Silva

Joined Apr 25, 2007
12
Hi, i'm trying to measure amps with my Fluke 88V/A, but already blew a fuse by doing the test same way we test voltage which i came about to know that's the wrong way to do it,i never had the need of measureing amps so i forgot what i learned about it, i was trying to measure amps of an old AC tv tube transformer input 120, and 3 outputs 140v,20v,15v, so i am using this transformer as an isolation transformer to connect the equipment as a safty proctection form the line service, anyway i'm trying to find out how many amps is it rated, any help would be wellcome thanks in advance.

#### wegener

Joined Apr 25, 2007
2
Hi, i'm trying to measure amps with my Fluke 88V/A, but already blew a fuse by doing the test same way we test voltage which i came about to know that's the wrong way to do it,i never had the need of measureing amps so i forgot what i learned about it, i was trying to measure amps of an old AC tv tube transformer input 120, and 3 outputs 140v,20v,15v, so i am using this transformer as an isolation transformer to connect the equipment as a safty proctection form the line service, anyway i'm trying to find out how many amps is it rated, any help would be wellcome thanks in advance.

Make sure you break the cct!!

to measure amps at a certain spot, make sure you make your amp meter part of the cct right where you want it.

ex.

oooooooooooo-----------(A)-----------
ooooooooooo|oooooooooooooooooooooo|
ooooooooooo|oooooooooooooooooooooo|
ooooooooooo|oooooooooooooooooooooo|
----R1-------|------------------------ |---------R2--------------------

(the o's are just filler, for some reason it collapsed all the spaces)

(A) = an meter

you get what i mean?? dont measure around a resistor (or load) or all the current will go there and blow your stuff up!!

- good luck

#### Silva

Joined Apr 25, 2007
12
I luckily just blew the fuse on my Fluke

--------------------------------------------------------------------------------

Thankyou for your help. Let me see if i understood the amps measurements on the Transformer,Correct me if i'm wrong,

I may take this as an exemple, I have a RCA TV and i connect it to the transformer that i mentioned that i wanted to know how to measure amps in its output,and to use it as an isolation, to repair the tv for circuit protection,

OK Now i plugg the tv power cord in the transformer, and i unsolder the black wire (the hot) that is soldered in the hot side of the CHASSIS and connect it to the COM black lead of the meter, and the positive +lead to the Chassis side where the hot black wire was soldered before,after i set the meter for amps settings, now i plugg power cord of the tranformer in the wall out let, now is this the way to measure amps , forgive me if i'm slow to understand how things works, but have no JD i am portuguese emigrant in the USA, i took an electronic correspondence course, i've been studying by myself electronics for a long time, what is very dificult to learn besides i have not a chance to get my hands on the job, besides a few tvs and stereo stuff that i got to learn on my own ,and read the books as you see, to learn this way is tough. Thanks again for you time and patience

#### recca02

Joined Apr 2, 2007
1,212
i think u r doing it right as long as you connect the ammeter in series and not
in parallel as in case of voltmeter. connecting in series is like breaking a circuit and introducing ammeter in between.

#### alim

Joined Dec 27, 2005
113
With the TV open -back off-remove the input fuse from its holder ,and place your ammeter probes across the ends of the fuse holder.

#### Silva

Joined Apr 25, 2007
12
Thank you guys, if i insert the probes in the fuse case i will be able to know how meny amps does the transformer puts out?. Or is how many amps does the tv draws?

#### recca02

Joined Apr 2, 2007
1,212
if its tv fuse it shud be the current drawn by tv i believe.
this is current drawn at the transformer sec voltage.

if the supply is given with the help of a transformer then it wud also mean
the current (not max current capacity) the transformer is putting out at its secondary.

#### Silva

Joined Apr 25, 2007
12
Thanks Recca, I suppose that what i was trying to find out the transformer amperage,it could be 2 or 5 amps, so i can safely plugg in the tv sets for safely repair them, i already used it in a 27"set and it handled it pretty good,i guess i have to try higher sizes and see i hope without smoke.

#### awright

Joined Jul 5, 2006
91
Silva, I sympathize with the difficulty of learning electronics and coping with the complexity of the english language. But you are making it MUCH more difficult to understand what you are trying to do and to help you by apparently using Internet sentance construction, grammar, and punctuation. If you know how to construct a sentance, use correct grammar, and use correct punctuation, FOR GODS SAKE, USE IT! A language has conventions of use to allow people to communicate effectively. You are not writing poetry here. You are asking for technical help. To ask a technical question and ask others to expend the energy to assist you for no fee, but to then make your question and its background information as obscure as possible in order to be "hip," by omitting punctuation, running one sentances on through the entire question, omitting all capitalization, etc. is an insult to those you are asking to help you.

Please help us help you. For one thing, in addition to making the basic communication difficult, by using this horrendous writing style, you make yourself sound like you will not be able to understand the technical information anyway, so why should anyone bother?

OK. Got that off my chest.

How are you using this transformer? I hope you are not applying 140 volts to the equipment you are using. With the winding voltages you show you can get a variety of voltages that would be more appropriate for your bench testing of equipment. Perhaps you are already doing this, but you don't mention it.

If you put the 20 volt winding in SERIES AIDING with the primary you would end up with a transformer having a winding turns ratio of 145:140. You could use this to either slightly boost or slightly reduce the line voltage. If you put the 15 volt winding in SERIES AIDING with the primary and disconnect the 20 volt winding, you will have a 135:140 turns ratio, giving you slightly larger possible adjustments. If you use the 20 volt secondary in SERIES BUCKING with the 140 volt winding, you will have a 1:1 voltage ratio. To use the auxiliary windings this way they would have to be would with the same size or larger wire as the 120 volt and 140 volt windings.

You can't necessarily determine the power handling capability of a transformer by measuring the current drawn in its present circuit. You don't know whether the transformer was selected for conservative rating and long life or pushed right up to or over its limits for lowest initial cost. You also do not know the division of power dissipation between the three windings. Nevertheless, measuring the current drawn in the existing circuit can at least give you an approximation of power rating to start with.

A better way of determining power handling in the exact winding configuration that you intend to use is to set up a test with the input and output voltages you want, apply a load somewhat below the expected power capability of the transformer, and let it cook, checking it frequently by hand or with a thermometer. It will get warm, but should remain comfortable to touch. If it eventually starts to smell or gets hot enough to be uncomfortable to touch, you are drawing too much current.

You can load up the transformer secondary in the intended usage configuration with a space heater or one or more large light bulbs and a Variac (variable autotransformer). Apply the output of your transformer to the input of a suitably rated Variac and load the output of the Variac with the heater. Gradually turn up the variac while monitoring transformer current and stop somewhat below what you think the transformer is rated for. Let it sit with that load and check transformer temperature frequently. If the transformer core and windings remain too cool, turn the Variac up slightly and try again. You will eventually reach a transformer temperature that is a little uncomfortable to touch. Back the current off slightly and find the current that is easy to touch, but feels a little hot. That is the current you can probably use indefinitely. It will take quite a long time for the transformer to reach thermal equilibrium, so keep checking the temperature.

Be sure the space heater and lamps, if used as a load, do not heat the transformer directly.

If you do not have access to a Variac you can do a pretty good job with a handful of lamps and sockets. Just keep varying load by screwing lamps of various wattages into several parallel sockets fed by the transformer secondary until you get the current you want.

Then again, you can buy a handily packaged bench isolation transformer of precisely known rating for around \$100.00.

awright

#### Silva

Joined Apr 25, 2007
12
Thanks Awright for your response, about my bad Inglish i know it, and try hard to do my best,. Now about the transformer i'm using it to isolate the line to power the tv sets when repairing them, and i'm not using the two outputs 20 & 15v, just using secondary 140v, do you say that i shouldn't use 140v secondary? can you explane this a litle better,you wrote, If you use the 20 volt secondary in SERIES BUCKING with the 140 volt winding, you will have a 1:1 voltage ratio. To use the auxiliary windings this way they would have to be would with the same size or larger wire as the 120 volt and 140 volt windings.

#### awright

Joined Jul 5, 2006
91
If you are applying 120 volts to the primary and connecting the TV directly to the 140 volt winding, you are seriously overdriving the TV power supply, possibly leading to burnout of the TV transformer or components ;and possibly upsetting the proper voltage relationships inside the TV.

A low voltage winding usually (but not always) is designed for a larger current than a high voltage winding. This means that you can PROBABLY use the low voltage windings to adjust the output voltage.

If you connect the 20 volt winding in series with the 140 volt winding without knowing the polarity of the winding you have two possible outcomes. The 20 volt winding can SUBTRACT 20 volts from the 140 volts, giving 120 volts from end to end of the pair, or it can ADD 20 volts to the 140 volts, giving 160 volts - obviously way too much for a piece of consumer electronics. So just wire them up and measure the resulting voltage. 160 volts, STOP and reverse the winding! Do not apply 160 volts to any line powered equipment! If you read 120 volts, it is OK to use to power items under test.

Similarly, the 140 volt and 15 volt windings can be used to provide 125 volts or 155 volts. You might use the 125 volts to determine how an item behaves with high line voltage, but I can't think of a use for 155 volts.

Using the two low voltage windings in series with the PRIMARY will give you different output voltages, some useful to test how the device under test reacts to undervoltage. Unlike using the low voltage windings with the secondary, which is safe to wire up and try out, it is possibly hazardous to do trial and error using the low voltage windings with the primary. This is because you could apply 120 volts to a combined winding giving a nominal 100volt primary, resulting in 168 volts on the "140 volt" winding and probably burning out any connected load.

The proper method of checking the proper connections to the primary would be to arbitrarily connect the 20 volt winding to the 120 volt primary, apply 120 volts TO THE 120 VOLT WINDING, and measure the voltage from end-to-end of the composite primary. If you measure 100 volts, STOP and reverse the 20 volt winding. If you measure 140 volts, OK.

You now have a transformer with a 140 volt composite primary and 140 volt and 15 volt secondaries. You can use the two "140 volt" windings as a 1:1 ratio isolation transformer. This is probably preferrable to using two "120 volt" windings because you are using less of the available flux excursions in the core and probably generating less heat.

You can calculate the result of using the 15 volt winding in this manner to get yet more voltages.

These composite connections are AUTOTRANSFORMER configurations (although purpose-built autotransformers are always series AIDING).

The reason you want the same or larger wire in the low voltage winding relative to the high voltage winding it is paired with is to assure that the current rating of the composite wind is not compromized by having an undersized segment in the winding.

awright

#### n9352527

Joined Oct 14, 2005
1,198
Isn't that dangerous? I mean connecting the windings in series with opposite polarity. The substracted voltage has to go somewhere, and the most obvious answer is it would be converted to heat. It could possibly overheat the transformer, even melting it and letting the smoke out if there was enough power dissipated.

#### bloguetronica

Joined Apr 27, 2007
1,541
Thanks Awright for your response, about my bad Inglish i know it, and try hard to do my best,. Now about the transformer i'm using it to isolate the line to power the tv sets when repairing them, and i'm not using the two outputs 20 & 15v, just using secondary 140v, do you say that i shouldn't use 140v secondary? can you explane this a litle better,you wrote, If you use the 20 volt secondary in SERIES BUCKING with the 140 volt winding, you will have a 1:1 voltage ratio. To use the auxiliary windings this way they would have to be would with the same size or larger wire as the 120 volt and 140 volt windings.
Well, the problem you had is that, even measuring the amps with the ammeter in series with the load resistance, you may blow a fuse. Most ammeters have 200mA fuses. Also, you should use ammeters with care, since they have very low resistance.

Also, it seems by what you said that you are connecting the probes directly to the poles of the transformer to measure the amps it can give. You are doing two things bad, one is overating the ammeter, since the transformer is a power source with a low thevelin (series) resistance, and the amps measured will be a lot more than the max rated, which leads us to the second point: you are shorting the transformer.

Once I've measured about 7 amps from an old small wattage power supply (rated at 1A). It was completely unintentional. Fortunately, the only harm that was done was a spark and a small electrical damage in one of the probes, but that was because I was measuring using the 10A unprotected mode. If I used one of the fused modes, I would have my ammeter's fuse blown either.

More to add, if you want real isolation don't use an autotransformer configuration (primary and secondary must be isolated from each other). Just use your primary and your 140V and 20V secondary. Connect the 140V secondary with the 20V secondary, but mind that the 20V secondary should be reversed. You should get 120V AC. If you get 160V AC, just reverse the 20V secondary.

P.S.: Are you portuguese? Me too!