25A Solid State Relays Keep Failing

Grumpy_Mike:

Does this also include the core or magnets?

Magnets and cores are not part of an electrical circuit

...seems like the danger is when you cut all power to the inductor/coils(while the coils/windings are charged). This releases like a burst of current?

This is well known it is called back EMF but is nothing to do with the problem here because a triac only turns off when the voltage across it is close to zero. It contributes to the out of phase current and voltage but again that is not the problem.

So could you say that metal or ferrite cores or magnets have an influence on the circuit then? Wrapping a core with copper winding (depending on how many turns, etc..)increases it's power or output (or like a transformer can decrease as well) ? Sorry for these questions Mike :) ...Inductance was the one thing I had a hard time grasping in electronics (Capacitance & Impedance...was much easier for me to understand). I understand your second point.

Also, if you have a charged coil, you shouldn't touch it with your hands, right? Is this because by touching the coil you're creating impedance?

Thanks again Mike

Also, if you have a charged coil, you shouldn't touch it with your hands, right?

Right

Is this because by touching the coil you're creating impedance?

No it is because you can get an electric shock.

A magnetic field is not like a capacitor the moment current stops flowing it starts to collapse. It is the collapsing magnetic field that induces the voltage in the coil opposite in direction to that which caused the field in the first place. Where as a capacitor can hold significant amounts of charge for days, a magnetic field collapses at speeds approaching the speed of light.

So could you say that metal or ferrite cores or magnets have an influence on the circuit then?

The core of a coil can effect its inductance in a similar way that the dielectric (stuff between the plates ) of a capacitor can effect the value of a capacitor.

increases it's power or output

Not quite.

or like a transformer can decrease as well

I think you are mixing up power with voltage. A transformer can not do anything with the power, the power (voltage times current ) is b and large the same. However, if you get more voltage out of a transformer than you put in you will get less current than you put in. And the other way round.

kslagerman: Hello, Wondering if someone can offer me some guideance on a exhibit that I have created. I am using an Arduino Uno to intermittently turn on and off a high power industrial fan. The fan draws about 12-13 amps when on. I am using a solid state relay to control the fan. The exhibit is a high traffic area and is activated several hundred times per day. This is the relay I am using:

This is a high power shaded pole induction motor? I didn't know they made them that powerful for single-phase mains!

If it doesn't have a centrifigal starter switch the starting current might be hundreds of amps - you should consult an electrician to see if the building wiring and fusing is adequate for this beast, it might be overloading the circuit and risk starting a fire.

Grumpy_Mike:

Also, if you have a charged coil, you shouldn't touch it with your hands, right?

Right

Is this because by touching the coil you're creating impedance?

No it is because you can get an electric shock.

A magnetic field is not like a capacitor the moment current stops flowing it starts to collapse. It is the collapsing magnetic field that induces the voltage in the coil opposite in direction to that which caused the field in the first place. Where as a capacitor can hold significant amounts of charge for days, a magnetic field collapses at speeds approaching the speed of light. Ahh..got it :)

So could you say that metal or ferrite cores or magnets have an influence on the circuit then?

The core of a coil can effect its inductance in a similar way that the dielectric (stuff between the plates ) of a capacitor can effect the value of a capacitor. Ok, kind of understand. Sounds similar to transistors and ICs. The one thing throwing me a bit off is how the metal, ferrite, etc.. cores are not part of the circuit. Looking at a schematic of some circuit that has for example a metal core transformer, it's shown in the schematic with it's symbol (coils separated by 2 lines).

increases it's power or output

Not quite.

or like a transformer can decrease as well

I think you are mixing up power with voltage. A transformer can not do anything with the power, the power (voltage times current ) is b and large the same. However, if you get more voltage out of a transformer than you put in you will get less current than you put in. And the other way round.

Actually now remembering as a kid, we lived in europe for a while and had to use a step up/step down transf. I vaguely remember my father explaining that to me hehe. Yeah, I shouldn't say power (Watts), I guess I keep thinking they go hand in hand (Volts and Amps). Ahh..one last question. All voltages contain current (although sometimes the current may be very small..), right?

Last Friday I ordered [u]The Art of Electronics[/u] used on Amazon. What is your opinion on this book? *Thanks again Mike for all the good info. *

Last Friday I ordered The Art of Electronics used on Amazon. What is your opinion on this book?

It is excellent, I wished it was around when I was learning the stuff it contains. I used to recommend it to my students.

There are a few possible reasons for the failure:

  1. Fan start current too high for the relay. Measure the fan startup current, then choose an SSR that can cope with that. The current surge rating quoted on the SSR datasheet is normally for a single cycle of AC mains, not for the second or two that the fan may take to start. If this is the problem, then using a 40A SSR instead of 25A may help.

If the startup current is very high, you can reduce it by connecting a suitable NTC thermistor in series with the fan, for example http://www.farnell.com/datasheets/87334.pdf.

  1. Voltage spike too large when the fan turns off, due to an inadequate snubber. You can add an external snubber to help with this. Again, the 40A relay may be more reliable, because it probably has a snubber designed for higher current loads than the 25A one.

  2. SSR overheating. But if it barely gets warm, that is unlikely to be the problem.

dc42: There are a few possible reasons for the failure:

  1. Fan start current too high for the relay. Measure the fan startup current, then choose an SSR that can cope with that. The current surge rating quoted on the SSR datasheet is normally for a single cycle of AC mains, not for the second or two that the fan may take to start. If this is the problem, then using a 40A SSR instead of 25A may help.

If the startup current is very high, you can reduce it by connecting a suitable NTC thermistor in series with the fan, for example http://www.farnell.com/datasheets/87334.pdf.

  1. Voltage spike too large when the fan turns off, due to an inadequate snubber. You can add an external snubber to help with this. Again, the 40A relay may be more reliable, because it probably has a snubber designed for higher current loads than the 25A one.

  2. SSR overheating. But if it barely gets warm, that is unlikely to be the problem.

DC42 - quite a nifty solution to use a thermistor inline.

Someone else has already stated that the SSR (supposedly) has Zero Crossing detection inbuilt - wouldn't that therefore reduce the voltage spike to nearly nothing ?

Craig

craigcurtin: DC42 - quite a nifty solution to use a thermistor inline.

NTC thermistors are routinely used to reduce the inrush current in switched mode power supplies. They were also used for the same purpose in valve (vacuum tube) TVs for many years. I haven't heard of one being used to reduce motor inrush current, but it should be viable if the thermistor has a high enough steady current rating and thermal time constant. Some of the fans used in warm air hand dryers start up very slowly, so I'm wondering whether they use NTC thermistors.

craigcurtin: Someone else has already stated that the SSR (supposedly) has Zero Crossing detection inbuilt - wouldn't that therefore reduce the voltage spike to nearly nothing ?

It doesn't matter whether the SSR is a zero-crossing type or not, because that only affects when the SSR turns on. AC SSRs use triacs as the switching elements, and a triac doesn't turn off until the current through it is near zero. There is a residual spike, because the current will still be a few tens of milliamps when the triac turns off, however under normal circumstances the built-in snubber should take care of that. It's mechanical relays that need really big snubbers when they are used with inductive loads, because the relay can disconnect at any point in the mains cycle.

dc42:

craigcurtin: DC42 - quite a nifty solution to use a thermistor inline.

NTC thermistors are routinely used to reduce the inrush current in switched mode power supplies. They were also used for the same purpose in valve (vacuum tube) TVs for many years. I haven't heard of one being used to reduce motor inrush current, but it should be viable if the thermistor has a high enough steady current rating and thermal time constant. Some of the fans used in warm air hand dryers start up very slowly, so I'm wondering whether they use NTC thermistors.

craigcurtin: Someone else has already stated that the SSR (supposedly) has Zero Crossing detection inbuilt - wouldn't that therefore reduce the voltage spike to nearly nothing ?

It doesn't matter whether the SSR is a zero-crossing type or not, because that only affects when the SSR turns on. AC SSRs use triacs as the switching elements, and a triac doesn't turn off until the current through it is near zero. There is a residual spike, because the current will still be a few tens of milliamps when the triac turns off, however under normal circumstances the built-in snubber should take care of that. It's mechanical relays that need really big snubbers when they are used with inductive loads, because the relay can disconnect at any point in the mains cycle.

Thanks for the clarification

Craig