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Home Brew Forums > Home Brewing Beer > Electric Brewing > Linear SSR?
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Old 02-15-2013, 04:07 PM   #1
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Default Linear SSR?

I have a ssr labeled '25A 4-20ma linear input'. An SSR is an inherently on/off device so does the SSR have built-in PWM? Is this common?

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Old 02-15-2013, 05:04 PM   #2
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It's probably a lamp dimmer under the hood. They are sold as SSRs but my understanding is that they are really triac based phase angle controllers.

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Old 02-15-2013, 06:00 PM   #3
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I guess it doesnt much matter how it works. Is there any problem with phase-chopping controllers?

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Old 02-15-2013, 06:20 PM   #4
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They also make the phase angle SSRs with a resistor controlled front end. Works great with a potentiometer as an analog power control for a heating element.

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Old 03-07-2013, 02:18 AM   #5
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Well it's a Continental RVMA 6V25. I ordered it brand-new for a different project years ago and never used it. According to the datasheet, it has built-in PWM including logic to ensure it switches at the zero-crossing point.

The problem is it doesn't work. I hooked up a 5V PSU with a 1kOhm pot+250Ohms which should give 4-20ma. However the light never comes on on the SSR and it shows 5.00 volts across the inputs, like no current is flowing at all. This is pretty strange, because I expect a current input to be very low-impedence, but it's acting like it's dead open.

Also strange is that when I plug a CFL light into the 120V outlet between the SSR'd phase and neutral, the light comes on at seemingly normal brightness. However, when I plug my heating element in across both phases, it does not seem to heat up, even though I measure 240V at the plug. So it's like the SSR has 'failed closed', but will only supply a few watts. On the other hand maybe the CFL is 'off' but the CFL is running off the leakage current.

This is a killer because I wanted to brew this weekend, and now I have to buy an SSR and make up some kind of PWM.

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Old 03-07-2013, 03:10 AM   #6
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I looked this one up - it does not use a resistor across the input. This one uses 4 - 20ma control current.

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Old 03-07-2013, 01:44 PM   #7
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I know. The problem is, mine doesn't seem to work. The input is apparently "open", because it's showing 5.00V across the inputs when I have it hooked up. That means no current is flowing at all. There's no way this should happen because I expect a "current-control" input to have nearly zero impedence, and it should never drop significant voltage across the input at all. I can only assume it was actually DOA (at $95 no less) and I just never knew because I never tested it.

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Old 03-18-2013, 01:29 PM   #8
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After further bench-testing, it appears this SSR does indeed work. However it's stupid. I expect current-control inputs to be low-impedence, basically like a short. However this one will not even turn on with 5V, which is why I thought it was dead. It takes a higher voltage to turn it on and then it drops 7.5V at 60ma. This makes it worthless for me because I only have 5V available, but it's a pretty cool part otherwise.

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Old 03-25-2013, 03:19 AM   #9
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I believe 4-20mA current loops are USUALLY powered around ~12v sources. They're usually used with very long control lines with unknown/high impedances.

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Old 03-25-2013, 01:42 PM   #10
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This part apparently has a microcontroller inside it; I wouldn't be surprised to find it has a built-in LDO regulator to feed the microcontroller 5V and that's why it doesn't work on 5V.

Most industrial control circuits are actually 24V so I guess they figure it's no problem to require 7.5V to turn on, but if it requires 7.5V to turn on, they should say that on their datasheet, and not just say "4-20ma" input. If they arbitrarily decide it's ok to require 7.5V minimum, then why not 13V or 43V. That information might be relevant...

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