piggyback transformer

[lil-r]

Hi guys,

I came across the term "piggyback" transformer while doing a site inspection. I asked the site electrician what it is and he's got no idea. Can someone please tell me what exactly "piggyback" transformer is and what does it do?

thx in advance

[BrynRovers]

Could it be an isolating transformer ??

[lil-r]

i have no idea. I've tried to search google and all i can get is this

Quote:

http://www.freepatentsonline.com/4651075.html The circuitry shown in FIG. 2 as shown therein includes three iron core transformers identified as T1, T2 and T3. The transformer T1 can be characterized as being a power transformer which has its primary winding 31 connected to a suitable source of voltage such as 115 volts AC, 2000 Hertz. The transformer T1 reduces the voltage on its secondary winding 32 to a suitable voltage such as approximately 20 volts AC. Typically this voltage should be a voltage which can be utilized with standard transistor amplifiers and logic circuits. The secondary 32 of the transformer T1 is connected to lines 33 and 34 at its outer extremities and has a center tap connected to a line 36. The center tap line 36 is connected to a ground G2. The lines 33 and 34 are connected through high impedance resistors R1 and R2 to the primary 37 of the transformer T2 which can be characterized as a "piggyback" transformer. Transformer T2 provides unity transformation and thus supplies the same voltage which is supplied from the secondary of the transformer T1 to the secondary 38 of the transformer T2 as, for example, 20 volts. By the use of this second transformer T2, the secondary 38 is floating with respect to the primary 37 and makes it possible for the secondary to be utilized to control electronic devices as hereinafter described isolated from the ground G2.

If I may ask a dumb question, what's an isolating transformer?

[breezer]

thought you liked google?

an isolating transformer does what it says on the tin.

usually they have a ratio of 1:1

eg 230 in, 230 out

the idea is to still have the required power, but to isolate it from the supply

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[lil-r]

google just like other friends, they can betray u sometimes....

so basically, piggyback transformer is 2 transformers connecting in paralel and one of them (T2) is an isolating transformer (unity) as described in the quote above?
And the reasong behind this is instead of using 1 transformer (say 1000 kVA) can use 2 of 500 kVA (if for some reasong installing 1x1000kVA is not possible), would that be correct way to explain it?

[ricicle]

You can have, say, a 415/230V transformer rated at 1000VA but the secondary side will output 2 x 115V.If you wire the output connections in series you get the full 1000 VA at 230V but if you use one of the 115V outputs you will only get 500VA at 115V.If you parallel the two 115V connections up you then have 1000VA at 115V.

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[BrynRovers]

In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.

The voltage between either output point of an isolating transformer and earth is Zero (or near Zero).
Will still give you a nasty tingle but far safer tingle, especially on older AC/DC circuits where te chassis can be live.

[lil-r]

thanks guys, all make sense now.