1. THYRISTORS
WHAT ARE THYRISTORS ?

2. THYRISTORS 1. What is a thyristor unit? 2. What do they do?
3. Why use them?
4. Why tell us about them?
5. What questions should I ask?

3. WHAT IS A THYRISTOR UNIT
It is a switch - It is either ON or it is OFF.
It consists of solid state power switching devices, controlled by an input signal from a controller, computer etc.
It can be driven by LOGIC or ANALOGUE input signals or by Profibus / Modbus comms.
Models are available for single or three phase loads.

4. WHAT DO THEY DO ?
1. They control the power delivered to electric heaters. Current ratings range from 1 to over 2,000 amps.
2. The load can be a simple resistance, one which changes resistance with temperature or a transformer coupled load.
3. Various firing techniques are available to suit the different applications.

5. WHY USE A THYRISTOR ?
1. Unlike mechanical contactors, they are virtually maintenance free.
2. Contactors have slow cycle times which can cause heater fatigue due to hot / cold stress.
3. Thyristors can provide soft start and current limit, thus reducing the current rating of associated cables, circuit breakers etc.

6. SO WHY TELL US ABOUT THEM ?
1. The most common outputs from EPA systems are 4-20 mA or digital comms. These can be used to control up to 2.5MW of power via a thyristor unit.
2. Model 394 chart recorders can have 4-20 mA control outputs which can be similarly used.
3. There could well be other temperature control loops within a system which could use thyristors - and help the sale.

7. WHAT QUESTIONS SHOULD I ASK ?
1. Are there any heating applications in addition to the process loops ?
2. If so are they electric heaters ?
3. If so are they resistive, high temperature coefficient (HTCL) or transformer coupled loads ? This determines the firing mode.