Why ssr's? (Solid state relays)

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grumpy_grinder

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Why is everyone so inclined towards solid state relays? I'm just curious. You would think being about 2.5 times less expensive and not as suited to be in harsh and high temp locations that people would have more of a tendency to go for Electromagnetic relays. I don't know , is it just a trend?
 
Many types of relays don't do well with continuous cycling. An SSR can cycle millions of times with no wear or damage. They also can handle any load voltage up to the max range as well as run on a wide range of trigger voltage. Also, while smaller standard electromagnetic relays are cheap, the ones that carry large current are not. The higher the current, the more they do poorly with constant cycling. At 20 amps, you can arc out the contacts pretty fast. On the much lower current draw of a PID controlled forge, this isn't as much of a problem, but the SSR will still be the better choice, in my opinion.

They are pretty cheap now days.
I just took a look, and a 40 amp SSR and heat sink runs about $10. A 10 amp electromagnetic relay is twice that ... and a 40 amp EMR wasn't even listed.
 
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The biggest advantage SSRs have is that they can operate fast. The also usually switch at the zero-crossing point of the AC cycle which gives everything else in the circuit a pretty easy time.

I'm assuming you are thinking in terms of HT ovens here. For the speed issue, it helps to consider what is going on when at temperature.

The controller cannot usually modulate the wattage of the elements. If you have 3000W of elements, they are outputting 3000 Watts when powered and nothing when unpowered. The controller has an output cycle time and powers the elements for a variable proportion of this cycle time in order to achieve an effective output somewhere between zero and 100%. If the power needed is 50% (1500W in our 3000W example), the elements are on for half the output cycle time and off for the other half.

While the elements are powered and putting out 3000W, the temperature rises. While they are off, the temperature falls. A trace of temperature against time would show saw teeth.

If the output cycle time is long, the saw teeth will be large. If the output cycle time is short, the saw teeth will be small. The smaller the saw teeth, the more stable the temperature.

For pottery kilns, a 30-second output cycle is not uncommon and electromechanical relays have no problem cycling at this rate. For a HT oven though, a 2-second output cycle time is quite common and this is too fast to be continuously cycling a contactor (a contactor is basically a high-current relay)
 
Good point on the switching point in the wave with SSRs, Tim.
With EMRs, there can be pretty large fly-back currents that play hell with controllers and other electronic circuits. Any device with a coil has this problem unless choked and filtered properly.
 
Then my question would be "Why use an SSR instead of a PWM?" Is it an issue of the PID not being able to control the PWM?
 
In my experience, PWM isn't suitable for controlling large current loads. It would require sophisticated and expensive components (big Triacs?).

SSR is a switch - ON/OFF. It can be used to control the on/off cycle timing sort of like big pulse width waves. The controller can be ON from 0% to 100% and OFF from 100% to 0%. In a heating oven control circuit, that is more than sufficient. Controlled from a PID, the timing is learned and auto-programmed. This allows pretty close tolerance resolution.

For a standard solenoid, PWM wouldn't work at all.
 
OK total disclaimer! I'm a NEWBIE! I dont' make knives for a living only as a hobby. With that said my profession is a electrician. In my experience I've not had great luck with SS relay's! But maybe they are better now adays. With that said I used a standard 30 amp magnetic relay! I still use a cooling fan. However atleast in my PID instructions there is a option for use with a standard relay. Instead of constant on of on off turning on the relay there is a setting for using standard relays. It works GREAT! You can choose or adjust the time off between cycles and at 20 seconds I don't' see a problem. Your mileage may vary!
 
Time proportioning output using as SSR or a relay/contactor IS Pulse Width Modulation. It's just done on a long timescale with the pulse width being measured in seconds, rather than milliseconds.

The cycle time is the inverse of the pulse frequency: With a 2-second cycle time, the PWM switching frequency is 0.5 Hz. With a 20-second cycle time, it's 0.05 Hz.

For an oven, 0.5 Hz is plenty good enough.

Because the SSR switches AC at zero voltage (when the AC waveform is passing through zero Voltage), and there are only 120 zero crossings per second on a 60 Hz supply, there is a limit to how short you can usefully make the cycle time. If you make the pulse width 5 AC cycles (83 ms), there are only 10 AC zero crossings per output cycle and the output can therefore only change in steps of 10%. Stepping up to a 50-cycle output time (833 ms), there are 100 zero crossings per output cycle and the output can change in steps of 1%.

There is effectively a tradeoff between output resolution and cycle time.

Back when I built my first couple of ovens, I tested them with a number of thermocouples and a datalogger. Going to 2 seconds from 5 seconds made a noticeable improvement in the smoothness of the trace. Going shorter did not give any further gain as far as I could tell, though that may well have been because the logger would not log faster faster than 500 ms intervals.
 
Stacy E. Apelt - Bladesmith said:
Good point on the switching point in the wave with SSRs, Tim.
With EMRs, there can be pretty large fly-back currents that play hell with controllers and other electronic circuits. Any device with a coil has this problem unless choked and filtered properly.
Click to expand...

I'd say a diode to recirculate/dissipate would work. I guess it might be time for me to get with the times. I was set on using my omega cn101 and a EmR ... Which I may still. It might just need a bit more engineering. Then again it's not like I'll be using it everyday. A contactor with arc arresters might be better. I can get all my electrical parts for free besides SSR's. Only SCR's that are the size of a case of beer.
 
Might be because I was lucky in my build and my HT oven doesn't rely strictly on soft bricks for insulation but has a full 2" of unknown insulation that was on the pottery kiln I salvaged the brick from. But unless my K thermo is wrong or the reading of such on my PID the temp varies a couple degrees after auto tuning it to using a standard relay. Like I said I don't use it this oven everyday only as a hobby maker. Just being totally honest but my magnetic relays work just fine controlling fine temps for SS that I use.
 
My thoughts also , I'm not talking about an oscillating process but allow for a bit of dampening 20 degrees or so. I'm a noob when it comes to tempering and to be honest I haven't researched it much. I understand completely if you where to maintain a steady tempering the relay would be in oscillation , chattering like to maintain the exact temperature. why not use an SCR instead and have a complete modulating system? I remember when I worked in a motor rewind shop we had a huge VPI oven and all it did was run off a EMR , no issues but it had a 30 degree span.
 
For my part my HT oven looses so little temperature in the off cycles using a standard relay that I quit using it for a tempering oven. In fact it looses temp so slowly I built a separate tempering oven. I use a regular toaster oven with added thermal mass controlled by a standard relay with a PID for tempering. Like I said I can put my hand on the outside of my homemade HT oven when it's at temp of 1875 and hold there for a long long time. My door is not double insulated though and it gets very hot. My HT oven can go from a cold start to 1875 in 35 minutes. It then holds that temp with in a few degrees using a standard relay with no problems.
 
Oh my ... I should of stayed in the instrumentation dept. in the concentrator ... All I do now is rack breakers in the mine MCC's ... So long are my days of industrial control and automation ... I probably wouldn't even be able to programme an input or output on foxboro now if my
Life depended on it ...
 
Tim is spot on in both of his responses, as an electronics professor for the past 30 years I give him an A+. :D Your homework assignment for today is to go back and read his comments 2 more times. SSR's are superior to EMR because there are no moving parts, no arcing contacts, and can cycle more quickly without any wear and tear. However, if you simply are not comfortable with the technology, then by all means continue to use EMR's. They served the purpose for many, many years before SSR's came along.
 
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