Linear SSR?

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BetterSense

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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?
 
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.
 
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.
 
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.
 
I looked this one up - it does not use a resistor across the input. This one uses 4 - 20ma control current.
 
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.
 
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.
 
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.
 
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...
 
It says in the datasheet input should drop 6v at 20mA at max power. Input resistance should then be:

6v/0.02A=300Ohms.

It should just turn on at 4mA. i.e. 0.004Ax300Ohm=1.2v
50% power approx: (20mA+4mA)/2=12mA ==> 0.012Ax300Ohm=3.6v
And max power 6v according to datasheet.

So you'll need a variable voltage source that regulate fairly accurate up to about 6volts. Everything below 4mA, which should drop 1.2v, is considered faulty signal in case of a brake in the control line and should turn the output off.

The best and most acurate is to use a variable current source that can source at least 6volts. You can build one using a LM317 set up as a constant current source with variable current out. Supply power to the circuit needs to be 6v plus whatever the LM317 sinks at 20mA. Say 12-15v and you should be covered.

For reference:
http://en.wikipedia.org/wiki/4-20_mA
http://users.telenet.be/davshomepage/current-source.htm
 
What datasheet did you find? I looked on their website and the only datasheet I could find was very watered-down.

I see what you did there with your calculation, but the device does not behave linearly/resistively. So your ohm's calculation is only valid at 20mA, and a voltage source will not work, or at least it will not be linear at all. In particular, the device does not operate until the voltage is over 5V; so the resistance is infinite. I measure 5.00V across the input when I hook up my power supply; there is no current flowing.

I could set up a current source, but I only have 5V available via USB, so I can't even turn it on, so it's worthless. I gave up an bought an Auber SSR. I will save this for some future project where I have 12V or something available.
 
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