SSR DRIVER (DC)
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 high-voltage (DC) electrical equip- ment using a microcontroller?
Background
A solid-state relay is a semiconductor device that was designed to take the place of a conventional electromagnetic (EM) relay. We do not recommend the use of EM relays for switching DC loads and therefore do not consider EM relays until we discuss the switching of AC loads in Chapter 8. If you are unfamiliar with EM relays, you may find it useful to refer to Chapter 8 (and specifically to EMR DRIVER [page 149]) for background information on EM relays before considering this pattern.
Solution
Unlike EM relays, solid-state relays (SSRs) are purely electronic in nature: they have no moving (mechanical) switch contacts. Both DC and AC solid-state relays are avail- able: note that, unlike EM relays, a DC relay will not switch AC supplies and a DC relay will not switch AC supplies: we explain why this is the case later in the pattern.
SSRs provide very high levels of isolation between the ‘control’ and ‘switching’ cir- cuits by optical techniques. The ‘control’ inputs will be connected to one (or more) LEDs. These will then, without any electrical link, control a phototransistor or photo- diode array, which will, in turn, connect to further switching circuitry. In the case of a DC SSR, the switching circuit will typically be based on a MOSFET, and the current- and voltage-switching capabilities will generally be similar to MOSFETs.
Use of SSRs is generally straightforward: the inputs are directly compatible with microcontroller port voltages, and – because of the built-in opto-isolation – there is generally no need to add additional gates between the microcontroller and the SSR.
The examples below 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, at the input to the SSR. See NAKED LED [page 110] for further details.
Hardware resource implications
Every implementation of this pattern uses at least one port pin.
Reliability and safety issues
There are a number of general reliability and safety issues associated with the use of high-power DC loads: these are discussed in the pattern BJT DRIVER [page 124]. Please refer to this pattern for further information.
How robust are SSRs?
SSRs are less electrically robust than EM relays: in the presence of excessive voltages (due to back EMF from inductive loads, for example) or where the SSR is subjected to larger currents (possibly due to ‘inrush’), the device will fail. If in doubt, over rate the device: that is, use a 300 V device where a 200 V device would probably do.
One other issue: we have seen people try to use more than one SSR in parallel in order to increase the current rating. This will only rarely work and is never reliable. The problem is that you have no way of ensuring that both relays switch on at exactly the same time. When one relay has turned on, the supply voltage drop will usually mean that the second (and subsequent) SSRs do not turn on – until the first relay fails. Thus, the likely result is that, within around a millisecond, all the relays will blow, in rapid succession.
What’s in a name?
Despite the name, there are important differences between ‘solid-state’ and ‘electro- mechanical’ relays, particularly when it comes to circuit testing. If you are using an EM relay, you can check to make sure that the contacts are closing by using a multimeter to measure the resistance of the switch contacts: this resistance will be essentially zero when the contacts are closed and essentially infinite when they are open. This behav- iour will be observed without connecting up the high-voltage side of the application.
You cannot test an SSR-based circuit in the same manner: most SSRs will always show an infinite resistance when measured with a multimeter. To test an SSR, you need to operate close to the specified operating voltage. Initial tests are therefore best performed (carefully) using a high-voltage load (we usually use an ordinary household lightbulb).
What happens when it goes wrong?
The typical failure mode of an SSR is an output short circuit: this can be dangerous.
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 (see Chapter 8 for details). 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 wear out (in normal use). 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. SSRs generate only very low levels of electrical noise and generate no
acoustic noise.
SSRs do not exhibit switch bounce.
SSRs can be instantly and irrevocably damaged by excessive voltage and / or current.
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, complicating initial testing.
EM relays can usually switch higher voltages and currents.
Unlike an EM relay, the ‘switched’ side exhibits some leakage current in the off-state.
The ‘on’ resistance is typically much larger than that of an EM relays: this trans- lates directly into wasted heat and, hence, the need for heatsinks.
Related patterns and alternative solutions
See the other patterns in this chapter and Chapter 8, for alternative approaches.
Example: SSRs in telecommunication applications
Small DC semiconductor relays are commonly used in telecommunications equip- ment, such as modems, in place of larger EM relays. Indeed, the telecoms market is so important that special-purpose SSRs, intended to be used for telephone current line sensing, are also available (see, for example, products from Erg components).
In modems and similar devices, these SSRs provide 200-300V (DC) output ratings at around 200 mA. They provide a low on resistance (typically 10Ω) and an ‘off’ resist- ance of some 500 MΩ, at voltages up to 4 kV.
Example: Open-loop DC motor control
In MOSFET DRIVER [page 139] we presented an example of a MOSFET used for open- loop DC motor control (see Figure 7.34 for details).
If we use an appropriate SSR we can simplify this circuit considerably. For example, our motor required 2A (continuous) at up to 12V for correct operation. Here we can use an IOR PVNO12 SSR. Unlike the majority of SSRs, this can switch AC or DC loads, of up to 20 V, 4.5A. It has no zero-crossing detection. The control current is a maxi- mum of 10mA, which is compatible with our microcontroller. Figure 7.35 shows the required circuit.
Note that an important advantage of the SSR solution is that it provides a greater degree of isolation between the controller and the motor itself. Note also that, to con- trol the speed of this motor, pulse-width modulation may be possible: see HARDW ARE PWM [page 808] and SOFTW ARE PWM [page 831].
