FAQ - Questions and Answers

Do you have questions about the selection and use of transformers for resistance welding and heating technology?

Here we will continuously collect new questions and answers and a lively page is to be created. We will be happy to take your suggestions on board.

Which diodes are being used by Expert?
Welding diodes - with and without housing
Welding diodes - with and without housing

Basically, we are using diodes from the manufacturer ABB. Exceptionally, we use for the so-called 6000 diodes, diodes from the manufacturer Semikron.

For the purpose of distinction, we have a special naming in the product types:

Naming IFAVm IFRMS IFSM Reverse voltage Type
B 7.110 A 11.200 A 55.000 A 200 V 5SDD 71X0200
C 11.350 A 17.800 A 85.000 A 400 V 5SDD 0120C0400
D 9.244 A 14.520 A 64.000 A 400 V 5SDD 92Z0401
E 13.526 A 21.247 A 85.000 A 400 V 5SDD 0135Z0401
G 10.502 A 16.496 A 74.700 A 400 V 5SDD 0105Z0401
H 10.266 A 16.125 A 57.700 A 400 V 5SDF 0103Z0401
J 13.058 A 20.512 A 70.000 A 400 V 5SDF 0131Z0401
Why are diodes subject to wear and limited life time?

Diodes are subject to wear due to the thermal alternating load. This means that with each current pulse the internal temperature of the diodes is raised from e.g. 20°C to 80°C. This is called the temperature swing. This is called the temperature swing. In this case 60K.

This means a mechanical stress due to thermal expansion. Each stroke causes tiny cracks in the crystal structure of the silicon. The number and intensity of the cracks causes wear in the diode, so that after a time x it breaks down and becomes conductive on both sides.

Together with ABB and the University of Magdeburg, Expert has developed corresponding lifetime models for this wear and depending on the temperature range and cooling, so that we are able to define corresponding load diagrams for the respective requirements.

As standard, the load diagrams are calculated for 10 million cycles. In the automotive sector, this is 15 million cycles. For requirements such as pulsed roll seam welding, we also calculate the diode load for 300 million cycles.

You will find the load diagrams in the data sheets of the transformers. You can carry out calculations for the duty cycle and also determine the operating points in our calculation tools.

Load diagram - MFDC transformer
Load diagram - MFDC transformer
What is capacitor discharge welding?
Welding pulse of a CD welding machine
Welding pulse of a CD welding machine

EXPERT has been developing and building pulse transformers for capacitor discharge welding for many years. The process is characterised by the fact that large amounts of energy can be introduced into the workpiece within a relatively short time ( typically 5 ms to 20 ms).

The energy for the welding process is stored on capacitor banks.

By selective discharge via a matching transformer, a current pulse is generated which welds the components together. The welding process itself is a projection welding process in most applications.

To store high amounts of energy in the capacitor, relatively high charging voltages are used. Typical charging voltages are 900 - 3300V.

Due to the high energy input, the process enables the welding of even large-sized components and critical material pairings. For example,  gear wheels for gearboxes are welded with this process for the automotive industry.

Another characteristic is the optimal energy utilisation. During the welding process, only very small heat-affected zones are created in the component. The energy used is almost exclusively  used to melt the material directly at the joint.

In systems with EXPERT transformers, welding currents of up to 1,000,000 A have already been achieved.

Another advantage of capacitor discharge welding is the relatively low mains load, as the capacitors act as an energy buffer and can be recharged evenly in the pauses between welds.

In this process, the matching transformer (impulse transformer) not only takes over the function of the optimal energy feed into the joining process. With its properties, it also decisively influences the temporal course of the welding current (pulse shape) and thus decisively influences the welding process.

How do you determine the duty cycle?

For the duty cycle calculation, one considers the ratio between the actual current time and a total cycle time. For transformers with rectifier diodes, a distinction is made between the duty cycles for the transformer and the diodes. This has to do with the thermal behaviour of the two components. As a rule, only the ratio of the current time to the point-to-point time is considered for the diodes. For the transformer, the time to the next workpiece is added, i.e. the consideration of the cycle time.

In general, integration times apply here. Formally, an integration time of 60s applies to the transformer. However, this depends on the weight of the transformer and also the internal losses. For the MF8 series, for example, we have a thermal integration time of approx. 120s.

For the diodes, an integration time of 2s applies in the literature and in the standards. We have already integrated this time into the load diagrams. Therefore, statements on the load capacity as a function of the duty cycle are also possible for current times >2s.


Duty cycle transformer:

Duty cycle diodes:

Definition of duty cycle - display of welding time (current time), pause time, cycle time
Definition of duty cycle - display of welding time (current time), pause time, cycle time
What is the permanent current of the transformer?

Transformers for resistance welding or also for continuous applications are designed, among other things, with regard to their maximum current load. This means that the balance of thermal losses and cooling is considered without components such as windings, insulation, casting resin ... suffer long-term damage. Thermal losses are caused by ohmic resistances and also losses in the iron core due to magnetisation processes.

The components of the transformer (primary winding, magnetic circuit, secondary winding) are dimensioned in the sense of the above-mentioned equilibrium.

In pulsed applications, such as resistance welding, one makes use of the inertia in the heating of the components and can therefore specifically overload the transformer. This overload is explicitly linked to the ratio between current time and times without current flow. This ratio is linked to the concept of duty cycle.

Due to the quadratic dependence of the losses on the current, it is easy to calculate the maximum overload as a function of the duty cycle. Since the user needs a certain current for his welding task and the transformer manufacturer has to design the transformer for the continuous current, it is necessary to calculate back and forth.

I2s - Welding current
I2p - Permanent current
X - Duty cycle

If you have a welding current and a duty cycle, you can calculate the required permanent current as follows:

If you have the continuous current of the transformer, you can calculate the maximum welding current for a given duty cycle as follows.

You are welcome to make this calculation yourself in our online calculator under the tab Welding and permanent current.

Diagram with operating point to maximum welding current
Diagram with operating point to maximum welding current
How to check an MFDC transformer in a built-in situation?


The suggested checks are only a tool for quick verification. They cannot replace a detailed fault analysis.
Required tools: Multimeter with diode test

Diode test

The welding gun must be open. A possible voltage measurement on the gun could possibly falsify the measurement. It should be disconnected.

Measuring from "minus" to "plus" with the multimeter in diode test mode (not resistance measurement)

  • If a voltage drop of approx. 0.25 - 0.45V can be measured, the diodes are OK.
  • If the meter shows 0V, at least one diode is defective.
    Only short-circuited diodes can be detected. This is the most frequent case of failure.

Measuring from "plus" to "minus" with the multimeter in diode test mode

  • If the meter does not show a voltage drop, both diodes are OK, at least in the reverse direction.
  • If a voltage drop or 0V can be measured, at least one diode is defective.

Primary winding

Measure the resistance between U and V with a multimeter. The connecting cables must be disconnected. The measured resistance value depends strongly on the contacting of the test probes.

  • If the resistance is in the mΩ range, the winding is OK.
  • If the resistance is infinitely high, there is an interruption in the winding.


Measurement of the resistance between a primary and a secondary connection. Due to the low measuring voltage, only rough statements can be made.

  • If a resistance in the Ω range can be measured, the insulation is severely damaged.
  • Statements beyond this - especially whether the insulation is actually OK - can only be made with a special insulation measuring device with a higher test voltage. Typical insulation resistances are >100 MOhm.

Temperature monitor

Measure the resistance between the terminals of the temperature monitor with a multimeter. The connecting cable must be disconnected. As temperature monitors bimetal switches as 'opener' are used.

  • If a resistance in the single-digit Ω range or smaller can be measured, the monitor is OK.
  • If the resistance is infinitely high, the monitor is defective.

Measuring coil

Measure the resistance between the connections of the temperature monitor with a multimeter. The connecting cable must be disconnected.

  • If the resistance is in the low Ω range (15 - 35 Ohm), the measuring coil is OK.
  • If the resistance is infinitely high or close to zero, the measuring coil is defective.