108 Relays

image of relay contacts, active






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image of relay contacts, inactive

Last update:
July  2021 (74)


Operating and Setup Tips

Here you will find hints on setting up connections and care and handling of sensors and circuit boards.   This page will be updated periodically with additional tips.   

About static electricity...  
If you live in a dry (low humidity) environment, static electricity is a fact of life - especially in Winter. Putting on or taking off a sweater, walking across carpet or linoleum can generate 35 -50000 volts of static. This can easily zap a sensor, or the microcontroller inputs. When handling sensors, cables, or the circuit board, it's best to wear a grounded wrist strap or, touch a ground point (eg: a metal water pipe, or the ground point of an electrical outlet) before handling, and to minimze physical movement as much as possible. This will bleed off any static you're carrying.  If walking on carpet or lino, lift your feet, rather than sliding them. 
About electrical interference...  
  Heavy duty electric motors have large start up currents, which can cause large emi (electromagnetic interference) around them.  If a sensor or sensor cable is in the vicinity, it may be impacted by the emi.  I know this can cause "noise" on the data signal, if the cable isn't properly shielded. But I'm not sure if it could actually stop a data signal. See this article about data cabling and shielding.    
About relay life, and load capabilities...   

1.  The small relays used on these boards may have a typical switch life of 100,000 operations (see here for an example of a relay datasheet).  If your project involves turning a pump on/off every few hours, that's quite a long lifetime.  If you're driving a flashing signboard that blinks on and off every few seconds, that will be a short lifetime.  It's a good idea to do a bit of math to see what the expected life might be.  This will help you decide whether the relay board you've chosen is suitable for the intended application. 

2.  The relays used in the kits vary from circuit to circuit, some are rated at 15 amps, some less.  The K108 board uses 15 amp relays, but the relay switching circuits are actually rated for only  5 amps. 


The limiting factor is more often the copper traces on the circuit board, rather than the relay contacts themselves.  The traces are not heavy enough to carry a great amount of current.  It may be possible to either build up the solder along a trace or solder a heavy jumper wire on top of a trace.  But this will extend current capabilities only so far.  It's better to use the onboard relays to switch a second set of relays with heavy duty contacts.    

3.  Relay contacts usually have 2 load ratings -- resistive and inductive.  A resistive load would be something like an incandescent light bulb or an electric heater.  An electric motor would be an example of an inductive load.    

In a general way, you can say an inductor is a storage device (similar but opposite to a capacitor), whereas a resistor is not (doesn't store energy).  When power is removed from an inductor (eg: motor), the stored energy reacts by creating what's called back emf.  This back emf (aka voltage surge) can cause arcing across the relay contacts, which will eventually lead to contact burnout.   

Since resistors don't store energy, there is no "reaction" when power is removed from a resistive load. 

Relay contacts usually have a lower inductive load rating because of this reactive effect.  If you are going to be controlling a pump or compressor motor, keep this in mind.   

USB - RS232 Adapters   (Feb/09) 
A user has reported successful connection to a K145 with the Aten brand (www.aten.com) of USB-Serial converter.  

At June/19:  As designed/wired, the K145 requires a true RS232 port.  Connections are at the 12V level. This 12V is used to supply the onboard 5V regulator (through the DTR line).  Many usb-serial adaptors operate at TTL (5V) levels.  At 5V, the regulator will be starved, and the K145 wont function.  Some FTDI adapters (not sure about all) provide true RS232 levels (12V). The K145 should function reliably with these.  

Clock Time shift   (Feb/09) 
If your system is configured to periodically update the system clock from the Internet, system time may jump by a few seconds, depending on the stability and accuracy of the system clock.  This time shift may appear in log timestamps for the K145, K190, and VK011.   

Note that Daylight Savings time change will also affect log timestamps. 

Put a Capacitor at the Sensor on Long Cables   (Feb/09)   
Quozl's Community Project page has a good discussion of the benefits of placing a capacitor across the power connections at the temperature sensor.  This is especially important with the K190 because of the low resistance value of the pullup resistors used.  The low resistance means higher current levels being switched by the temperature sensor when sending data.  The combination of higher current draw and resistance in the cable can cause enough of a voltage drop so that the sensor might be starved for power, resulting in data errors.  As Quozl noted, a capacitor at the sensor acts as a reservoir to provide extra oomph when the sensor is sending data. 


Lightning Protection  (Mar/09) 
If you have sensors (or K108 Inputs) on cables running outside, there may be the possibility of damage to electronics from lightning strikes.  One fellow has suggested using fiber optics as a coupler between the board and the cable.  The fiber would act as a super insulator, preventing the high voltage charge from zapping the electronics.  The fiber needs driver electronics to function properly, so this gets fairly complicated.  You can see his circuit here: 
    http://home.hetnet.nl/~thomas_7/1wire/1wire_isolating.html   **link is dead at June/19
A simpler alternative is perhaps to disconnect all external cables when a storm is approaching.  
K145 interrupted by new USB connection  (Mar/09)  
A user has reported that plugging in a USB stick caused the K145 to disconnect.  The K145 reconnected when the stick was removed.   
K190 as Temperature Alarm system  (May/09)  
Some alarm dialers can be triggered by a relay contact closing.  Such a dialer could be used in conjunction with the K190 to send a high- or low-temperature alarm message by phone.  Rather than using the relay contacts to turn a heater on/off, connect the contacts to the alarm dialer.  To function as a high-temperature alarm, set the channel for Cool mode, and the opposite for a low alarm.  Some dialers are multi-channel, allowing a different message to be programmed for each one.  This can be useful for temperature monitoring at remote locations, where it's important to be notified if there's an alarm condition.  Note that channels used for alarms couldn't be used for control, though.  (Thanks to Greg Mitchell for the idea.)  






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