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AC Relay Wildcard Users Guide

Instructions for easily controlling AC devices from your microcontroller based instrument.

This document describes using the AC Solid State Relay WildCard™. It provides instructions for the Wildcard's connection and software, as well as a full schematic.

The AC Solid State Relay Wildcard controls up to four, 5 amp AC loads. It features 8 screw terminal connectors so you can connect to a wide variety of instruments. Up to eight AC Relay Wildcards can stack onto a Mosaic embedded controller or you can mix and match any of the growing family of Wildcards. This wildcard is the perfect instrumentation choice when your application requires the control of standard 120V or 240VAC mains electricity. The following sections show you how to use the AC Relay Wildcard’s hardware and software.



AC Relay Wildcard Specifications
Channels: Four independent, optically isolated solid state AC relays (SSRS)
Voltage: Controls 12 to 280 VAC at 47 to 63 Hz
Current: Switches from 0.06 up to 5 amps continuously
Max. Surge Current: 250 A at 16.6 msec. (one AC cycle)
Isolation: Optically isolated to 4000 Vrms
Switching Mode: Zero voltage switching
Turn On/Off Times: Max turn on/off time of ½ AC cycle
Connections: 8 easy-to-connect-to screw terminals
SSR Used: Crydom PX240D5 AC solid state relay
Current: FIXME mA from 5V
Weight: 55 gram
Size : 2" x 2.5" x 1.10"1) (50.8mm x 63.5mm x 28mm)



The AC Relay Wildcard provides easy control of external AC devices. Each AC Relay provides:

  • Drive 12 to 280 VAC loads operating at 47 to 63 Hz and up to 5 Amps.
  • Opto isolated control to 4000 Vrms.
  • Zero current – zero voltage switching.
  • Max turn on/off time of ½ of the AC Cycle.

The next sections show you how to connect the AC Relay Wildcard to the Wildcard Bus on a Mosaic controller and how to configure the module for operation in your instrumnet.


Connecting to the Wildcard bus of any Mosaic controller

To attach the AC Relay Wildcard to your controller:

With the power OFF, connect the Wildcard Bus header of the AC Relay Wildcard to Wildcard Port 0 or Wildcard Port 1 on your controller or the Wildcard Carrier Board (These may also be called Module Port 0 or 1). The Wildcard Bus on the AC Relay Wildcard is located opposite from the screw terminals. If you are using a Wildcard Carrier Board, connect it to the QED Board as outlined in the "Wildcard Carrier Board Users Guide". The standoffs on your controller should line up with the Wildcard's corner mounting holes.

Caution -- Wildcard bus connectors are not keyed!
The AC Relay Wildcard Bus does not use keyed connectors. So, you must be sure to connect the AC Relay Wildcard so that all pins on the connector are in fact connected. Mosaic controllers and the AC Relay Wildcard might be damaged if the connection is not made correctly.

Selecting the Wildcard Address

Once you have attached the AC Relay Wildcard to the controller board, you must set its address with jumper shunts at jumper positions J1 and J2.

The Wildcard's address selection jumpers, J1 and J2, set a unique address of the Wildcard port. Each wildcard port on Mosaic Controllers accommodates up to four attache Wildcards. Wildcard Port 0 provides access to Wildcard addresses 0 to 3 while Wildcard Port 1 may host Wildcards at addresses 4 through 7. Two Wildcards mounted on the same port should not have the same address (jumper settings). These are the addresses corresponding to the possible jumper settings:

Address Jumper Settings
Wildcard Port Wildcard Address Installed Jumper Shunts
0 0 None
0 1 J1
0 2 J2
0 3 J1 and J2
1 4 None
1 5 J1
1 6 J2
1 7 J1 and J2
Note:<block indent>Address 0 isn't available on the Handheld or QScreen controllers. Instead, use addresses 1 through 7.</block>

Once you have attached and addressed the Wildcard, you can use its software drivers to control AC loads.



You control the AC SSRs on the Wildcard using very simple software – essentially just simple reads and writes to particular addresses of the microcontroller's memory. In the following software description we show you how the Wildcards are addressed, then how the relays are controlled, and finally present a software example that initializes and controls the AC relays.


Initializing the Wildcards

The functions IOSetBits, and IOClearBits, IOStoreChar, and IOFetchChar communicate with the AC Relay Wildcard. Each function requires an offset address and a Wildcard address. These functions are contained in the file QED_QCard_IO.4th for V4.xx kernels, and they are automatically included in the V6.xx kernel. This file QED_QCard_IO.4th can be found in:

Mosaic\Wildcard Drivers\Libraries\include\

The AC Relays on the AC Relay Wildcard are controlled by an onboard Xilinx CPLD (Complex Programmable Logic Device). The AC Relay control lines on the CPLD must be initialized as outputs (on power up, they come up as inputs). To initialize the Wildcard, execute Init_AC_Relay, as shown in the demo programs.

Qfter the Wildcard has been initialized, use Control_AC_Relay to turn the solid-state relays OFF or ON, and use Read_AC_Relay_Status to read in the relays' status. The control lines are all active low – to turn a relay ON, you write a 0 to its control line.


C demo program controls the AC relays

You can find a C language demo program in your installation directory, and it is also provided here for reference.


Forth language demo program

You can also find a Forth language demo program in your installation directory, or online here.
Download the demo to your board, then type:


to run it with the AC wildcard installed on port 0.


Connecting to the field header

The relay contacts and AC ground plane are brought out to a 9-position screw terminal block on the AC Relay Wildcard as shown in the following table.

AC Relay Wildcard
Screw Terminal Block
Signal Pin
R0+ 1
R0- 2
R1+ 3
R1- 4
R2+ 5
R2- 6
R3+ 7
R3- 8

The + and labels on the relay contacts do not actually imply any inherent polarization, though it is recommended for clarity of your instrument design to use the + side for the power line (hot) connection and the side for the load connection.

You should place the output relay contacts in series with the hot side of the AC supply rather than the neutral side. That is, connect the incoming AC hot wire to the + relay contact, connect the relay contact to your load, and connect the other side of the load to the neutral wire of the AC power source.

The AC_GND terminal connects to the printed circuit board ground plane beneath the AC side of the relays, and also is used as the path for dissipating excess energy if the protective varistors are installed (see below). In general, AC_GND should be connected to earth ground in an AC grid powered instrument, along with the instrument enclosure if it is made of a conductive material.


Installing varistors for transient overvoltage protection

Historically, controlling inductive loads such as motors with relays required incorporating methods for dissipating the voltage spike that occurs when an inductive load is abruptly disconnected. However, the solid-state relays on the AC Relay Wildcard take advantage of monitoring the AC waveform to only turn on at the zero crossing of voltage between the contacts, and only turn off at the zero crossing of current through the contacts. This synchronization with the zero crossings of the AC waveform effectively prevents any inductive transient voltage spikes. As such, no overvoltage protection is needed in most applications.

However, if the AC Relay Wildcard is used in instruments in which separate inductive loads are controlled by other means, such as a mechanical relay or a manual toggle switch, then transient voltage spikes will occur on non-synchronized disconnects of an inductive load. The relays on the AC Relay Wildcard provide 4000V of isolation from the AC contacts to the control inputs, which is sufficient for preventing damage due to voltage spikes in many cases. However, if you wish to provide a means of dissipating such voltage spikes, install appropriately-rated varistors such as the ERZ-V05D431 (rated for 275VAC) at positions VR1 through VR8.

If the AC Relay Wildcard is being used to simply turn on and off loads from a power supply, varistors are only needed on the line side of the relay, as the switching of the relays on the wildcard does not cause transient voltage spikes. See the schematic below for information on which varistor connects to which relay contact, and note that on the unlabeled first revision of the printed circuit board, the varistors are installed in line with the long side of the board between two holes 0.3 inches apart, not on top of the VR1..8 labels.



Now you are ready to start using your AC Relay Wildcard. All of the software routines listed in this document are also on the distribution CD that accompanies each module.


AC SSR schematic

Relay Datasheet

Height is from the bottom of the PCB to the top of the tallest component on the top side of the PCB.
This page is about: How to Use AC Solid State Relays (SSRs) in OEM Instruments, Controlling Crydom AC Relays, Users Manual – How to control or drive large AC loads reliably from your embedded controllers and microcontroller with this solid state relay card. Optically isolated Crydom AC SSRs are used for zero crossing switching of AC currents. Solid State AC Relay, SSRS, AC Relays, 5 amps, control, switch, AC Load, schematic, Crydom, active low