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Old 12-02-2010, 06:46 PM   #1
wuertele
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Default SCR recommendations?

I am building an electric RIMS, but I only have one 120V circuit and that circuit has other loads on it (lights, motors, etc). I want to avoid tripping the breaker with the RIMS. I could use PWM to control an SSR, but since the SSR is either "on" or "off", and not "in-between", in the "on" state my heating element could trip the breaker.

I believe that SCRs might be the answer. They can control the load power from 0%-100%, so I could program my software to only use 100% if there are no other loads (my software is also controlling the pumps). If the pumps are on, the software will tell the SCR to only use some percentage of the max, say 80% (or whatever I determine is safe to avoid tripping the breaker).

But there are things I don't understand. I read somewhere that SCRs only pass current in one direction, which would only give me half of the power from my 120V circuit. Is that correct?

Do SCRs need a separate power supply, or is it just two terminals for voltage in and two terminals for voltage out (like an SSR)?

How efficient are SCRs? Will an SCR controlling the same load as an SSR require a heatsink that is larger than what the SSR would require?

Where are the cheap SCRs? I found some expensive ones on omega.com. I did some googling with wild variations in price, and I could not understand what the tradeoffs were.

Is anyone on HBT using an SCR in this kind of application?



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Old 12-02-2010, 08:03 PM   #2
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An SCR is a Silicon Controlled Rectifier, that means it allows current in only one direction and is controlled (turned on only, not on and off. Requires the current to go to zero to reset) I think you need to look up the definition of Triac and see if it meets your needs.



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Old 12-02-2010, 10:43 PM   #3
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I've been looking for triacs, but they all seem to be rather small compared to SCRs, so I'm not sure I'm finding the right thing.

I did broaden my search some and found this:

http://www.alliedelec.com/search/productdetail.aspx?SKU=6823038

It looks like it might be exactly what I need. But it is hard to tell if it passes AC in both directions.

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Old 12-03-2010, 06:13 PM   #4
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The specs all call out RMS values, so yes, it will pass AC.
Many high power triacs @ Allied http://www.alliedelec.com/search/searchresults.aspx?N=0&Ntk=Primary&Ntt=triac

Newark
http://www.newark.com/triacs?isRedirect=true

Digikey...

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Old 10-20-2011, 08:03 AM   #5
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I purchased an SCR from Anacon Power and Controls. it has a built in 4-20, 2-10, 0-10, 0-5 card on it and requires a 24VAC power supply.
I control it with a Honeywell Spyder 0-10VDC output on a PID, works tits. soft start, all that. you could also limit it from your controller if you wanted to, max output like 7.5VDC/75%.

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Old 10-20-2011, 06:13 PM   #6
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you could also do it the old fashoned way- wire in a properly sized shunt between the SSR and the heater to cause a drop in voltage (and thereby, amperage as well), and then have an additional SSR to bypass the shunt when you want full power.

it wouldnt be continuously variable, but would allow you to easily switch between full power and some predetermined percentage of it.



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