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LDmicro Forum - Relays are sticking

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Relays are sticking (by Tim Buttars)
Hi,
I am using a PIC16F628A and 5vdc coil relays to control 24VDC and 120VAC devices. I am currently using 3904 transistors to activate the coil from a PIC output since coil load is too much for the PIC. But I have had problems on three different applications were my relay contacts are sticking both open and closed. Does anyone know why this is happening and is there an easy solution.

Thanks,
Tim
Wed Apr 25 2012, 23:23:09
(no subject) (by Jorge Jacome)
Depends on many factors as

What is the current rating of the contacts of your 5V relays, and what type of load is connected to them; if the current is high eventually you will get the contacts melted or closed permanent due the inrush current not when the contacts are closing is more likely when the contacts going from closed to open.

Also depends on the switching frequency of those relay some of them have a million operation ( ON and OFF ) which it is easy
achievable on a certain period of time.

I have seen those faults on a various numbers of PLC, that use
the internal relays to drive heavy loads, sometimes had to replace the PLC ( no more outputs available or use the spare output )and redesign the external circuitry to not make the same mistake.

My suggestion in this case; the small relay to drive and external relay with a greater current rate of the contacts.
Any other ideas or better suggestions folks.

Jorge
Thu Apr 26 2012, 02:19:02
(no subject) (by Tim Buttars)
Hi Jorge,
I appreciate your responce but current load should not be a factor for the project I am now working on. My relay is rated for 10amps at 120VAC and I am using it to control a small 24VDC air valve that only draws 5.4 watts.

Thanks,
Tim
Thu Apr 26 2012, 02:49:00
(no subject) (by Jonathan Westhues)
Can you post the complete circuit that you're using, with part numbers for all components?
Thu Apr 26 2012, 07:27:12
(no subject) (by Dat Tran)
Also your program.
Thu Apr 26 2012, 12:35:41
(no subject) (by Ciprian)
You could use the GTO thyristors. Or try to put a varistor in parallel with valve coil to cut off the current generated from the inductive component of the coil. Another way is to mount a rectifier diode in series with a resistor and all this in parallel with the valve coil (the rectifier diode should be mounted with the cathod at +24V).
Try this: http://en.wikipedia.org/wiki/Contact_protection, http://www.industrologic.com/mechrela.htm ...

Best regards.
Fri Apr 27 2012, 07:01:15
(no subject) (by Tim Buttars)
Heres the program and its very simple.
Mon Apr 30 2012, 17:12:22, download attachment Merch Pop Up.ld
(no subject) (by Tim Buttars)
Heres the schematic
Mon Apr 30 2012, 17:12:57, download attachment img-427121321.pdf
(no subject) (by Jonathan Westhues)
In the schematic, there's no power supply decoupling capacitors drawn. There's also potential for a high-voltage spike when the transistor switches, because of the inductance of the relay coil.

I'm not sure if those are your problem, but they should be fixed. For example, you could put a 100 nF capacitor and a 10 uF capacitor in parallel across both the input and the output of the voltage regulator, and you could put a 1N4007 diode across the relay coil, with its cathode to Vdd.
Mon Apr 30 2012, 20:55:59
(no subject) (by SEC_SUZUKI)
To the relay contacts when the inductance of the DC load will produce a spark.
Surge will not occur like the CR-surge absorber mounting.
Will not like wearing contacts and surge can be taken well.
Wed May 2 2012, 09:54:24
(no subject) (by Ronaldo)
Dear Tim Buttars,
Make the changes suggested in the modified circuit and
Put a good heatsink in 7805, because the input voltage is too high.
Wed May 2 2012, 17:01:08, download attachment imagem.jpg
(no subject) (by Tim Buttars)
Hi Ronaldo & Jonathan Westhues,
Thanks for your advise and I have read some about decoupling capacitors on Wikipedia. Can you explain why I should use both a 100nF and a 10uF on the 5VDC output instead of just one capacitor? Also you both recommended Capacitors on the 24VDC input side of my voltage regulator Why? I have another project were I made my own car alarm. Should I put capacitors on the battery +12VDC input side of that 5VDC voltage regulator?
Wed May 2 2012, 22:17:12
(no subject) (by Ronaldo)
Tim Buttars,
Your circuit can operate without the capacitors, but not without the diodes, When the relay and solenoid coils are switched off, creating a reverse polarity voltage spike of several hundred volts. The diodes eliminate this problem.
http://www.autoshop101.com/forms/hweb2.pdf - page 11,12

The capacitors are used to filter, read this link.
Always use capacitors, they help reduce noise and stability and transient response.
http://www.fairchildsemi.com/ds/LM/LM7805.pdf
EMI and EFI
http://en.wikipedia.org/wiki/E...nce#EMI_in_integrated_circuits.

The your circuit is working well?
Thu May 3 2012, 07:52:43
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