How SSR can be driven by analog?

Hello There, I am new with control system and electronic devices. My application require to control an electric heater bank with a temperature controller. All the electric heater banks shall be modulated via new solid state relays in response to a 0 - 10 VDC analog control signal from the PLC. I read previous post related to SSRs but they refere to digital control which is not my case. I would like to know how can this be achived /posible and what type of solid state relay shall ve selected (additional to the type suitable to handle the load of the heater banks). Kind Regards,

Hi - a Google search 0-10V Solid State Relay lead me to this website, http://www.crydom.com/en/Tech/Newsletters/Solid Statements - SSRs switching types.pdf.

Seems to offer a fairly succinct explanation.

Cheers

http://www.plctalk.net/qanda/showthread.php?t=84076
This one is about the same control.
Freak's answer was the one I was thinking about

"heater banks shall be modulated via new solid state relays in response to a 0 - 10 VDC analog control signal from the PLC".

Is the 0-10VDC signal INPUT TO the PLC or OUTPUT FROM the PLC? Typically the number of times the SSR turns on
and the length of time the SSR stays on is varied by the temp. controller (ON/OFF PID control) to maintain setpoint,
not the input voltage to a fixed input voltage SSR.

If your using a temperature controller, it has a adjustable setpoint, so, what is the 0-10VDC signal used for?
If the 0-10VDC is input to the PLC, from where does the 0-10VDC signal originate? If the 0-10VDC signal is output
from the PLC, what is it used for? It would not be used to vary load current to electric heating elements.

I've seen murcury contactors in use for electric heating elements for over 25 years, and never had to change one.

What PLC are you using? The Control/Compact Logix have a SRTP instruction that is specifically for your application.

Look a Carlo Gavazzi RN.F.. SST

There's a whole category of final control elements called SCR's or thyristors, after the electrical component switching mechanism used. Their intended use is driving heater elements.

These devices have an analog input, typically 4-20mA or 0-10V, which represents the percentage on-time.

There are single phase, 3 phase, zero crossing and phase angle fired units. Zero crossing generates far less electrical noise than phase angle. The phase angled fired SCRs with soft start are needed for some heater elements that vary in resistance. Any type of SCR may be used with nichrome wire heater elements.

Freak said:

Hi - a Google search 0-10V Solid State Relay lead me to this website, http://www.crydom.com/en/Tech/Newsletters/Solid Statements - SSRs switching types.pdf.

Seems to offer a fairly succinct explanation.

Cheers

Click to expand...

I didn't know that there are different kinds of SSRs; those take analog input and the other take digital PWM input. Thanks.

Tpo

sinbernal said:

Hello There, I am new with control system and electronic devices. My application require to control an electric heater bank with a temperature controller. All the electric heater banks shall be modulated via new solid state relays in response to a 0 - 10 VDC analog control signal from the PLC. I read previous post related to SSRs but they refere to digital control which is not my case. I would like to know how can this be achived /posible and what type of solid state relay shall ve selected (additional to the type suitable to handle the load of the heater banks). Kind Regards,

Click to expand...

The CV (Control Variable) output of a PID is an analog variable.
How do you turn on and off a digital switching device? Establish a period. Establish a time frame, example: Period established 5 seconds.

If the PID CV out is 100%, the digital output will be 5 seconds out of 5 seconds.

If the PID CV out is 50%, the digital output will be 2.5 seconds on, and 2.5 seconds off.

Google "Time Proportioned Output" TPO

danw said:

There's a whole category of final control elements called SCR's or thyristors, after the electrical component switching mechanism used. Their intended use is driving heater elements.

These devices have an analog input, typically 4-20mA or 0-10V, which represents the percentage on-time.

There are single phase, 3 phase, zero crossing and phase angle fired units. Zero crossing generates far less electrical noise than phase angle. The phase angled fired SCRs with soft start are needed for some heater elements that vary in resistance. Any type of SCR may be used with nichrome wire heater elements.

Click to expand...

Another issue has to do with the power involved. Often times in very high temperature heater systems, the heaters have a limited resistance so they require a lot of current flow. It is done at a low voltage via large transformers so that you don't have to handle and switch the high current. You put the control elements on the higher voltage side where current is low, then go through the transformer to drop down to something like 10VAC, but very high current, right ahead of the heater connections. When you use that method, you cannot use Zero Cross SCR controllers, you must use Phase Angle controllers.

You also have to truly and fully understand your heating element itself, you cannot just ASSume it is NiCr or you might get into deep doo-doo. Some heating elements have high inrush and/or high hot-to-cold ratio of the temperature coefficient of resistance, meaning the resistance of the circuit changes dramatically based on the temperature involved, sometimes by as much as 20:1. In those cases you ALSO need to use Phase Angle Control.

Plastic said:

The CV (Control Variable) output of a PID is an analog variable.
How do you turn on and off a digital switching device? Establish a period. Establish a time frame, example: Period established 5 seconds.

If the PID CV out is 100%, the digital output will be 5 seconds out of 5 seconds.

If the PID CV out is 50%, the digital output will be 2.5 seconds on, and 2.5 seconds off.

Google "Time Proportioned Output" TPO

Click to expand...

So, is the total time range ( 5 seconds in your example) determined by the PLC program then? If so, what about using temperature controllers, such as Omron E5CN eurotherm?

So, is the total time range ( 5 seconds in your example) determined by the PLC program then?

Click to expand...

If you are using a PLC instruction (PID for example) to calculate the output, then the time range is set by the user. The PID Control Variable output is scaled to match the user-set time period. If is is 5 seconds, then every 5 seconds (as measured by a second self-resetting timer 1), the CV Scaled Output (now ranging from 0 to 5) can be moved to the Preset value of a PLC Timer 2. The Timer 2 is then used to turn on the heater for whatever time (0 to 5) that the PID calculated to control the temperature to the setpoint.

Actually if you use RSLogix, it could be done with one self-resetting 5-second timer. You use a comparison statement that has a variable input that is set by the Scaled PID Output. Say the Scaled PID Output is N7:20. Then the logic for the heater output would be:

If T4:0.ACC <= N7:20, then Heater Output ON.
(where T4:0.PRE = 5 seconds and N7:20 ranges from 0 to 5 as controlled by the PID).

If so, what about using temperature controllers, such as Omron E5CN eurotherm?

Click to expand...

You certainly could use those controllers. Some temperature controllers do have a SSR output and use the same time proportional On/Off methods. Others have a variable current or voltage output.

Hi Lancie, so if I use a temperature controller for PID control, do most temperature controllers give me a parameter where I can enter the total time range of time?

What kind of hardware do you have accessible to complete the job? The best solution is using something that you have on hand if it is practical and suited for the job. There's no reason going out to purchase other equipment to do a job that can be done with the tools at hand.

unsaint32 said:

Hi Lancie, so if I use a temperature controller for PID control, do most temperature controllers give me a parameter where I can enter the total time range of time?

Click to expand...

Answering my own question, Omron E5CN has a paramter called Control Period, which is used when PID control method is chosen. The manual says 20 sec for relay output and 2 sec for SSR output. Makes sense.