Relays versus SSR (solid state relays)
LoopyByteloose
Posts: 12,537
The December issue of Nuts & Volts has an article discussing how to properly use relays that is very interesting.·I went to Google and found that the industry is aware of these concepts.
(You might look up National Instruments for Reed Relay Protection and Matsushita Electronics Works for Zener Diode Protection of relays -- I searched RELAY PROTECTION and ZENER DIODE RELAY PROTECTION to find these two informative discussions.)
IN PARTICULAR, the Nuts & Volts article·mentions that the commonly inserted diode (to protect your Microprocessor) shortens the life of relay unless other measures are taked to enhance performance.· Matsushita confirms this ugly little fact and specifies Zeners along similar lines to the article in Nuts & Volts.·In sum, one must add a proper resistor or preferably a Zener Diode to the protection.· This eliminates problems of slow-down in the opening of the contacts (which causes the contacts to burn more at higher voltages).
We all know mechanical relays are relatively slow (though reed relays are about 10 times quicker and more reliable) and they are noisy (clickety, click, click).· The SolidStateRelay offers an apparently attractive alternative.
So, I· looked into SSRs but I find that they too have there design problems/personality.·
Though they are wonderfully silent and easy to interface with the microprocessor, they will likely·fail in an on position or short circuit, are not as fully isolated as a mechanical relay, and create heat.· And, they are somewhat limited in the kinds of loads they can tolerate.· Highly inductive loads (which include motors and flurecent lights) create irregular behaviors and premature failures.
Both devices may suffer from high INRUSH currents, so one should budget there current to a little as needed and maybe use several relays/SSR to distribute the current.
Mechanical relays have problems of Contact Burn that shorten their life when switching higher voltages, so using low voltage devices may give you a more failure resistant device.
So, who has anything to add to this?· I am very interested in finding a way to extend the life of relays for as long as possible and reducing their size to minimal packages.· I am also wondering if anyone has solutions for using the SSRs with inductive loads?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
G. Herzog in Taiwan
(You might look up National Instruments for Reed Relay Protection and Matsushita Electronics Works for Zener Diode Protection of relays -- I searched RELAY PROTECTION and ZENER DIODE RELAY PROTECTION to find these two informative discussions.)
IN PARTICULAR, the Nuts & Volts article·mentions that the commonly inserted diode (to protect your Microprocessor) shortens the life of relay unless other measures are taked to enhance performance.· Matsushita confirms this ugly little fact and specifies Zeners along similar lines to the article in Nuts & Volts.·In sum, one must add a proper resistor or preferably a Zener Diode to the protection.· This eliminates problems of slow-down in the opening of the contacts (which causes the contacts to burn more at higher voltages).
We all know mechanical relays are relatively slow (though reed relays are about 10 times quicker and more reliable) and they are noisy (clickety, click, click).· The SolidStateRelay offers an apparently attractive alternative.
So, I· looked into SSRs but I find that they too have there design problems/personality.·
Though they are wonderfully silent and easy to interface with the microprocessor, they will likely·fail in an on position or short circuit, are not as fully isolated as a mechanical relay, and create heat.· And, they are somewhat limited in the kinds of loads they can tolerate.· Highly inductive loads (which include motors and flurecent lights) create irregular behaviors and premature failures.
Both devices may suffer from high INRUSH currents, so one should budget there current to a little as needed and maybe use several relays/SSR to distribute the current.
Mechanical relays have problems of Contact Burn that shorten their life when switching higher voltages, so using low voltage devices may give you a more failure resistant device.
So, who has anything to add to this?· I am very interested in finding a way to extend the life of relays for as long as possible and reducing their size to minimal packages.· I am also wondering if anyone has solutions for using the SSRs with inductive loads?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
G. Herzog in Taiwan
Comments
Having said that, I've used a very low resistance SSR (PVN012 I believe) to power a solenoid. It allowed 4 to 5 amps, and worked fine with a shunt diode.
As far as reliability goes, I would ask yourself: how many times do I think this relay will be closed and opened? If the answer is 500, 1000, or even 5000, mechanical relays should be fine (as long as you use a shunt diode). If the answer is 100,000 or more, then you might want to look at failure issues further.
Dave
If you have an application that switches often...and you want it to run for years (or some long period) then go with SSR's!
We have some modulators that switch 100's of times a day and originally we had these small mechanical relays that failed after 100,000 or so cycles so we upgraded all to SSR's that have been going for a few years so far (knock on wood!).
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·
Steve
http://members.rogers.com/steve.brady
http://www.geocities.com/paulsopenstage
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."