How do we design
a 60VA - non-inherently short-circuit proof safety transformer
for halogen lamps in accordance with IEC 61558,
protected by a thermal cutout ?
Technical specification relevant only
Electrical data and diagram
||230V, +6%, -10%, sinusoidal
|Nominal output voltage
|Nominal output current
Ambient and operating conditions:
|Mode of operation
short-circuit proof, protected by a thermal cutout
- Safety transformer to IEC 61558
A non-inherently short-circuit proof transformer as per IEC
61558 is equipped with a cutout to protect against short-circuit
and overload. In this case, the transformer should be equipped
with a thermal cutout:
The procedure for testing is prescribed as per paragraphs 14.2,
15.3.1 and 15.3.5 as follows:
- Firstly, the transformer as per paragraph 14.2 is loaded
with nominal resistance and at 1.06 x the nominal input
voltage until permanent operating temperature is
achieved. In this context, the temperature of the
windings must not exceed the value of q
- Next, all output windings are short-circuited. At 1.06 x
the nominal input voltage, the integral thermal cutout
should actuate, before the temperature reaches the level
of q max
as per the following table.
- Finally, the transformer is loaded at 0.95 x the value of
the lowest current, which causes actuation of the thermal
cutout, until attainment of continuous operating
temperature. In this context, the thermal cutout should
not actuate and the temperature should not exceed the
value of q max.
temperature in test q max (° C)
temperature in test (° C)
temperature in nominal operation mode q nominal
temperature in nominal operation mode (° C)
Maximal winding temperature in nominal operation mode =
Maximal winding temperature in test mode = 215°C
Insulation class E is prescribed.
The thermal cutout (temperature-operated cutout, PTC, ..) is
located in the primary windings. We choose an actuation
temperature of between q nominal and
q max. In the case
of a transformer in a mobile installation, in which context the
surface temperature of the case must not exceed 80°C, the
insulation class of the transformer A and the actuation
temperature of the thermal cutout are selected lower than q nominal.
Lets choose a thermal cutout actuation temperature
of between q nominal and
Sheet quality, core form and winding technology
For pricing reasons, transformers for halogen lamps are
equipped with core quality 5.3W/kg and 1.5T, and 50Hz. The
distinction is drawn between two basic specifications:
- With a bobbin which has a cover or is potted in the case.
- Without a bobbin, "dry" pressed, with a cover.
In this specification, we use small cut forms with high
coating thickness. Windings are enveloped in a
"hose" and pressed into the winding space of
The core construction looks like that of a choke or of a
welded transformer. The result is a gap which is suitable
for operation with a dimmer which is envisaged for the
Lets choose the non-annealed, cold-rolled core
quality of 5.3W/kg for 1.5T, 50Hz, 0.5mm thick, sheet form EI
42/125, without holes in the corners, with pressed windings.
Normally, a double-chamber bobbin unit is made with a cover
such that the transformer can be manufactured "dry".
Our transformer is manufactured without impregnation.
These transformers are designed with an induction of between
1.3T and 1.5T in nominal operating mode.
Nominal output voltage
Normally, the nominal output voltage of transformers for
halogen lamps is 5% lower than the lamps' nominal voltage.
Lets calculate our design with 11.5 V nominal output
Procedure for design
- If you are not yet familiar with the Rale Design
Software, then you should read the text: "How do
I design a small transformer?". You should keep
a copy of this text within your reach whenever performing
- Fill in the input mask as follows. If you need any help,
press function key F1. There is extensive description for
each input field.
- Select the core family of core form EI 42. Copy any EI 42
onto the clipboard.
- Change the name and the coating thickness. You also have
to set up the "bobbin unit" for windings which
are enveloped by a "hose".
- Click on OK.
- Save your input data file. In this design example, the
input data has been saved in input data file CAL0004E.TK1.
This input data file was supplied together with this
document. Copy it into the directory where the RALE Demo
program is installed.
- Make a connection with your Rale design server.
- Load up your input data file and start designing.
- After completion of your design work, the following
design data is available, that can be printed on three
- This is followed by monitoring of the design data.
- We start by checking the max winding temperature in
nominal operation mode = ambient temperature + dTprim in
nominal operation mode= 40+72 =112 < 115°C
- Next, we check the winding data and the filling factor
- If the design data is not satisfactory, then there are
two ways to implement the desired correction:
- We can go back to the input mask (function key F2),
correct the input data and re-design the transformer, or:
- We can access the test program (function key F5),
manually modify the transformer design and re-design the
transformer by that means. Here are results at the
nominal input voltage:
- After completing our design work, we can print out the
design data on-line, or save it on the local PC and print
it out off-line. The output data file from this design
example CAL0004E.TK2 is supplied together with this
document. Copy it into the directory in which the Rale
demo program is installed.
The transformer is too full
This is often the case if we select our own core. Suppose we
want to use the core for 100VA, for example.
- Increase the overtemperature to the permitted value.
- Increase the induction. Ensure that the idling
temperature does not exceed the permitted value for the
The transformer is relatively empty
This is often the case if we select our own core. Suppose we
want to use the core for 100VA, f or example.
- Reduce the temperature rise
The bobbin unit chambers are filled asymmetrically, e.g.
70% : 90%.
Manually change the wire thickness in the test program:
- Select the next higher wire thickness in the chamber
which is less full.
- Select the next lower wire thickness in the chamber,
which is more full.
- Correct the number of windings of the secondary, in order
to arrive at the desired output voltage.
Nominal operating data
This transformer was designed for 6% over-voltage. In order
to arrive at the nominal data for this transformer design, test
the designed transformer with the nominal input voltage in the
test program. (Uin = 1 ).
Temperature in nominal operation mode is too high
- Reduce the temperature rise and increase the induction.
- Set a better core quality.
- Increase the cooling surface area. Choose a larger case.
The color of the case should be dark.
Is the idling current too high?
The idling current is not a criterion for design. The idling
temperature must not exceed the permitted limit.
The combined fuse
In the event of poor thermal connection between the thermal
cutout and the protected winding, a further fine fuse is employed
to prevent short-circuit. With this combined fuse, the thermal
cutout provides protection against overload and the fine fuse
provides protection against short-circuit.