scr vs ssr Whats the difference

I'm trying to figure out the electric for my rims tube and can't figure out the difference between ssr's and scr's. I always thought ssr was just an electronic version of a mechanical relay. on or off I thought. But reading through the different rims threads it looks like the ssr's are being "modulated" with a pid controller. I'm looking at using one of the auber pid's with the ramp/soak features so I can still do step mashing if I want. How are the pid's doing the modulation with the ssr's? Are they just switching them on/off really fast, or is the variable voltage output of the pid allowing the ssr to vary it's output voltage. Can someone give me a clue and set me straight?
Thanks,
Walter

they control them with duty cycle. on time vs off time.

SSR = On/Off, like bad pulse said.

Not that fast, though. Maybe every second or two they switch. The PIDs aren't set up to do it any faster.

Attempting to lower the voltage would require a VERY serious pot or autotransformer.

I run a 1 second duty cycle

So at 50% duty, it is on for .5 seconds and off for .5 seconds.

SSRs are ON/OFF, but they are very fast

Ok so are ssr & scr the same? so much of the literature seems to make it seem that way, or do the scr's do something differently?

http://www.branom.com/literature/scr.html

http://www.chromalox.com/catalog/resources/4001.pdf

a scr is a component that can go into dimmers, ssr(s) or other drive electronics.
ssr is a module that can be made form scr(s) , mosfet(s) but there is other things in the package that make them easly integrated, like optical isolation of the control and load terminals.

ssr is a solid state relay, does the same thing as a mechanical relay with no moving parts and no mechanical contact wear or arcing. will switch dc or ac current if designed to do so..

scr is a silicon controlled rectifier. basically a controllable diode. works with dc only.

triac is similar to an scr but works with ac only. this is what light dimmers use.

but to answer your question: the pid controller sends on-off pulses to the ssr to control the process temperatures, usually slow pulses until the set point is reached and then fast to very fast pulses to maintain the setpoint temperature. passedpawn is incorrect, any decent pid controller that is properly set up will be able to vary pulse duty cycle to 1:10 second switching times.

most pid outputs are digital, not analog.

marzsit said:

triac is similar to an scr but works with ac only. this is what light dimmers use.

Click to expand...

hmmm some one should tell that to ETC, i have a 24 channel pack next door that is most definitely src based

lincoln said:

hmmm some one should tell that to ETC, i have a 24 channel pack next door that is most definitely src based

Click to expand...

if your pack has flat 3-pin stage connectors for the lights it's a good chance that the pack outputs dc, not ac. this is to prevent the lights from flickering, not noticeable at full brightness but is visible when the lamps are dimmed....

cheapo homeowner single dimmers that replace a wall switch is what i'm talking about when i say triacs are used in dimmers, not pro stage lighting systems...

it puts out ac, they use two scr parallel each one only works on one half of the wave form.

anyway, at this point we are just pissing for distance and not answering the question.

I'm working with a Carlo Gavazzi SSR that acts like a triac stuffed into a controller with either a 1 - 10 VDC or a 4-20 mA control signal input. One of its modes is phase angle control, 120 controlled width pieces of current per second. Smooth power, easy to control and monitor. $200 (<$100 used) for a 50A, 480VAC device. All you need is a signal source and some feedback (thermometer and eyeballs, thermostat, etc...).