Switching of Solid State Relay with PWM

Thread Starter

ankur_gajjar

Joined Nov 24, 2014
3
Dear all,
I am designing PID temperature control using Siemens PLC. In which, I need to control the heating element by means of PWM as the output of the PLC is in PWM, for which I want to use SSR as a switching element to heater. Now, my question is that : if I want to use SSR of appropriate rating for the switching of the heater, do I need to use heat sink along with it and is it mandatory? Right now, I am only having SSR with appropriate rating, nothing like SSR based control unit or whatsoever. Thank you.

Regards,
Ankur Gajjar
 

MaxHeadRoom

Joined Jul 18, 2013
28,831
Generally a heating element uses burst firing of a SSR, i.e. ON-OFF for several cycle periods, heating elements do not respond in a fast manner to phase angle control or PWM.
Also there is the possibility of electrical interference with rapid switching.
It is always best to use a heatsink when using power SSR's.
Max.
 

crutschow

Joined Mar 14, 2008
34,854
Whether the SSR needs a heat-sink depends upon the current rating of the heating element and the rating of the SSR and you haven't stated either of those values.
 

alfacliff

Joined Dec 13, 2013
2,458
we have oven controls here that use both the on off, and phase controlled firing methods, but none with pwm control, the high frequency [pwm would likely burn out the elements or cause them to vibrate till they broke.
 

MaxHeadRoom

Joined Jul 18, 2013
28,831
If using a PWM signal on the input to a SSR AC output type, the PWM is really going to be ineffectual, as once the initial conduction takes place it is on until the end of each half cycle?
Max.
 

alfacliff

Joined Dec 13, 2013
2,458
and how are the ssr's rated for frequency? I have never seen a rating for pwm service. some are even built for zero crosing switches for turning on and off without spikes.
 

crutschow

Joined Mar 14, 2008
34,854
A slow on/off control of an SSR (many 60Hz cycles on and off) is actually low frequency PWM. There's no valid reason to use a high frequency PMW to control typical heating elements since they have such a long thermal time constant.
 

MaxHeadRoom

Joined Jul 18, 2013
28,831
I assumed the OP meant control the SSR input with PWM which as per P#6 will be ineffectual on the AC output.
If by SSR it is meant a complete commercial unit such as Opto22 etc.
Max.
 

Thread Starter

ankur_gajjar

Joined Nov 24, 2014
3
Dear all,
Let me explain the application in more detail : As I have said earlier, I want to implement PID temperature control using Siemens PLC. Now, the output is given by the PLC in the form of "train of pulses" with the certain time period using pulse duration modulation. Now, I want to use this output for the switching of heater (which is in milliseconds) by means of SSR. I have read on forum of the siemens that SSR can be used for this type of application (Couldn't get more info from them though). However, I wanted to make it sure. So, is it advisable to use SSR for this application? If not, then why. And what is a suitable alternative?

Regards,
Ankur Gajjar
 

crutschow

Joined Mar 14, 2008
34,854
What is the frequency of this "train of pulses"?
The minimum ON duration of the SSR must be at least one cycle of the mains frequency.
 

Thread Starter

ankur_gajjar

Joined Nov 24, 2014
3
What is the frequency of this "train of pulses"?
The minimum ON duration of the SSR must be at least one cycle of the mains frequency.

Each pulse is of 100 milliseconds. In which the on and off time will be changing depending on the manipulated variable. In short, on time will be in terms of milliseconds.
 

crutschow

Joined Mar 14, 2008
34,854
Each pulse is of 100 milliseconds. In which the on and off time will be changing depending on the manipulated variable. In short, on time will be in terms of milliseconds.
In that case you can't use a standard AC SSR. The best way to do the PWM is to figure out how to reduce the frequency of the PWM signal so that the minimum on time is not less than one cycle of the mains frequency. That could be easily done with a microprocessor.
 

Bernard

Joined Aug 7, 2008
5,784
I see no reason that 10 CPS PWM should not work with 50- 60 Hz line. 0 crossing SSR preferred. Temperature kept to SSR specs.
 
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