Felix2Fingerz
Active Member
Wondering if anyone has come across or used a device to control a RIMS heating element with an analog 0-10 vdc or 4-20 ma signal rather then pulsing the relay with a 3-32 vdc binary signal?
There is indeed a reason for this and I would also like to explain a little more what I meant because I don't think Doug quite understood what I was looking for. Sometimes I have a hard time wording things.
I'm in the control business, building automation and been doing it for 21 years. Over the years I have accumulated many programmable controllers and all of which are very easy for me to program, wire and just plain old work with. I have one in particular that not only has all the I/O I need but also allows me to build a custom graphical interface that I can easily control via web browser. So for me, that is "KISS".
The only issue I have is the digital outputs on the controller. They will provide a 24VAC signal to a relay and all the SSR's I see want a DC voltage for the input. When it comes to programming and setting up the loops, I have always worked better with proportional outputs, analog signals and hardly ever any pulsed duty cycle type outputs.
So in the HVAC world, they have what are called SCR's. These devices were used to control electric parameter heat or air handler heating coils. The input side was analog, (0-10 VDC range). The input voltage determined the pulse rate of the output. 0 VDC on the input would be no heat and 10VDC would be full heat and anything in between would pulse the heat proportionally, just like you would want to control a RIMS heater.
So........the question is me simply wondering if anyone has come across this, maybe seen a solid state relay with a 24VAC input or what would work better for me, something with an analog input and a pulsed high voltage output. The ones I'm used to working with are huge, expensive and impractical.
Thanks. I should have worded that in more detail first time around.
After reading some of the stuff here, I believe I am going to do what you suggested and pulse one of my analog outputs into the input of the common SSR's I see.
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Many SSR's are zero switching devices, meaning they will only switch on when the AC waveform is crossing zero. They switch off when the trigger signal is removed AND the output current waveform crosses zero. Thus most SSR controllers are set for time proportioning (pulse width) or pulse count modulation.
A straight TRIAC (basically back to back SCR's) can be switched on at any point in the AC waveform, and will also shut off when the trigger signal is removed AND the output current crosses zero. So, TRIAC's can be used for a form of pulse width control by varying the turn on point in the AC waveform. For full power, you turn it on at the zero crossing, and you get a full half wave of power before it shuts off. Turn it on at the peak of the AC waveform, and you get half power. Turn it on past peak, and you get less than half power. The problem is, the power percentage is not linear with the fractional trigger point in the 8.33 msec half cycle. You could linearize this in software.
If you have an analog control signal (4 - 20 mA, or 0 - 10 VDC) you have to run that into pulse width or pulse count modulator to run an SSR or TRIAC. If you have DC digital outputs, you can do the modulation in software.
Brew on
You can buy proportional SSRs. Crydom makes them, around $100 for 40A. There is another brand on eBay but I don’t recall the brand.
A thyristor only conducts in one direction, a TRIAC can conduct in both directions, so a thyristor can only deliver power on one polarity of the AC cycle. Full wave SSR's use TRIAC's. Both thyristors and TRIAC's can be turned on at any point in the AC half cycle (but thyristors only on the half cycles with the correct polarity), and both turn off at zero crossings in the absence of a trigger signal (thryristors shut off at the opposite polarity half cycle even if the trigger signal is still on.) Additional circuitry is used in the trigger circuit in order to get zero crossing turn on. Delivering a precise number of cycles is all done in the processing of the input signal.Yep and these are the Triac (phase angle control) and Thyrister (delivers a particular number of half wave cycles) based methods.
Triacs are good for very even power delivery, but very noisy, audibly and electrically, since they have a sharp rise. The thyrister method would likely be preferred since it turns off at zero crossings, although it has lower resolution (120 increments per second).
On a side note, SSRs that are designed to deliver AC loads are thyrister based too. But they lack the control circuits to precisely delivery a particular duty cycle.
like this?You can buy proportional SSRs. Crydom makes them, around $100 for 40A. There is another brand on eBay but I don’t recall the brand.
Those are all standard SSVR's, which as @BrunDog said are controlled by potentiometers. They use firing phase angle control to chop up the AC waveform as described earlier in the thread.like this?
https://www.ebay.com/itm/Douglas-Randall-SSR-Proportional-Controller-Crydom-RPC4840-Resistive-Loads-NOS/130761827918?epid=1083256454&hash=item1e7203264e:g:4N4AAMXQZdFRGZW9:sc:USPSPriority!14120!US!-1&_sacat=0&_nkw=proportional+ssr&_from=R40&rt=nc&_trksid=m570.l1313?
I bought a kyotto ssr a while with a pot knob for one of my builds which worked gret too..I always thought it was analog control like the ssrs found in these kits, No?
http://stilldragon.com/index.php/diy-small-controller.html which uses this ssr..
http://stilldragon.com/index.php/solid-state-relay.html
Those are all standard SSVR's, which as @BrunDog said are controlled by potentiometers. They use firing phase angle control to chop up the AC waveform as described earlier in the thread.
Here is the other brand I referenced above.
http://r.ebay.com/7NPsdb
Resurection of a dead thread. I built the calibration/verification tester for these.If you need a *really * big one... these are a good deal, but very big and industrial... a friend turned me on to them, great if you are electrically heating large volumes.
$2500 MSRP , 65A "WATLOW PC91-N20B-1000 POWER SERIES CONTROLLER" for cheap on eBay...
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