SSR DRIVER (AC)
Context
● You are developing an embedded application using one or more members of the 8051 family of microcontrollers.
● You are designing an appropriate hardware foundation for your application.
Problem
How do you ‘switch on’ or ‘switch off’ a piece of mains-powered (AC) electrical equip- ment using a microcontroller?
Background
We introduce DC SSRs in SSR DRIVER ( DC ) [page 144]. Please refer to the previous pattern for general background information on SSRs.
The main differences between the two devices is that, while in DC SSRs, the output stage typically consists of a MOSFET, in AC SSRs, the output stage is usually a TRIAC. Very briefly, a TRIAC is a semiconductor switch that allows current to flow in both directions: this is, of course, precisely what we require with AC loads.
Solution
Use of SSRs is generally straightforward: the inputs are directly compatible with microcontroller port values, and – because of the built-in opto-isolation – there is gen- erally no need to add additional gates between the microcontroller and the SSR. The examples that follow will illustrate the use of these devices.
Pull-up resistors
When using this pattern, you may need to incorporate pull-up resistors in your hard- ware design. See NAKED LED [page 110] for further details.
Reliability and safety issues
See the start of this chapter for general reliability and safety issues.
See also SSR DRIVER ( DC ) [page 144] for basic SSR guidelines.
A key difference between AC and DC SSRs is that, while MOSFETs have very low losses, the TRIAC output stages used in AC SSRs typically have a voltage drop of up to 1.5V when they conduct: this translates directly into a power loss of up to 1.5W per Ampere of current. If using a high-power device (greater than around 4A), you will require a heat sink.
When you fit a heat sink, keep it isolated from the case of your application. Never
bolt it to the chassis: this can have deadly consequences.
Portability
SSRs can be used with any processor type. However, there are other portability issues to consider.
The most important (already briefly mentioned) is that an AC SSR cannot be used to switch DC. The reason for this is that the AC SSR contains zero-crossing detection circuits. Because the DC supply never crosses zero, the SSR will never switch on.
Similarly, most DC SSRs are based on MOSFETs. Using a MOSFET to switch AC is ineffective: at best, the device will act as a form of rectifier for a short period, until it overheats.
Overall strengths and weaknesses
SSRs do not exhibit switch bounce. SSRs do not wear out (in normal use). SSRs do not generate acoustic noise.
Many (AC) SSRs incorporate ‘zero-crossing detection’ circuits. This helps to greatly reduce EM emissions.
SSRs are resistant to shock and vibration. SSRs have a high switching speed.
SSRs have high levels of isolation between the ‘control’ and ‘switching’ circuits.
EM relays can usually switch higher voltages and currents.
The on-resistance of AC SSRs is much larger than EM relays. This means extra heat is generated and you must plan for a heat sink or other forms of cooling.
Unlike an EM relay, the ‘switch’ in the AC SSR (usually a TRIAC) exhibits some leakage current in the off-state.
Most AC SSRs will not switch DC, partly because of the zero-crossing detection. SSRs can be irrevocably damaged almost instantly by excessive voltage and / or
current: EM relays are more forgiving.
The typical failure mode of an SSR is an output short circuit: this can be dangerous.
The ‘switched’ side has a minimum operating voltage and current: these may be quite high, which can make it more difficult to perform initial tests on small-scale prototypes.
Related patterns and alternative solutions
See the other patterns in this chapter and Chapter 7 for alternative approaches.
Example: Controlling a central-heating pump with an SSR
In EMR DRIVER [page 149] we present an example of a small reed relay used to control a central-heating pump. Here we consider an alternative solution using an AC SSR.
Recall that the pump we wish to control was a Grundfos Selectric UPS 15-50: this required 240V AC (50 Hz), and 0.17–0.42A. In this case, we will use a Crydom MP240D3 SSR. This is suitable for direct interfacing to a microcontroller and has a current capacity of 3A continuous (80A surge) at up to 280V AC. Figure 8.8 shows a suitable circuit.
Overall, control of central heating in this manner is probably best carried out using an EM relay. The heating is switched on a small number of times per day, so relay life will be long. Maintenance is possible, in the event of a relay failure. The occasional EM spikes from the system are unlikely to be troublesome in most domestic environments.
In this case, the EM relay is a better solution, and costs around 20% of the price of the SSR.
Further reading
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