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DC Controlled AC Load

fatman57

May 27, 2013
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The term isolation is used to describe the case where there is no connection between two parts of the system.
A transformer can have isolation if the input and output windings are not connected together.
A relay can have isolation as the coil pulls in the contacts with an insulating part.
Triacs, resistors or transistors have connections through them. They do NOT have isolation.
A solid state relay uses a light source (led) to activate the output. It does have isolation.

New electrical installations in the UK have earth leakage trips but there are houses without them. You should never rely on safety devices to save your life. Would you be happy to crash your car because you are wearing a seat belt?

Thanks Duke - that explains everything!

Is there a way to thanks members on this forum?
 

davenn

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Is there a technical term for a brief encounter with electricity that locks up your muscles but you manage to escape somehow (by chance or aid from an external source) to live another day?

yes ... an electric shock

Dave
 

KrisBlueNZ

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Yes, that SSR you've found looks suitable. If RS have a similar part that has zero-crossing switching, that would be better.

Zero-crossing switching means that the switch device will only turn ON or OFF at the instant that the AC mains sinewave crosses 0V, either upwards or downwards. This means that when the switching device turns ON, the load will see a voltage that starts from 0V and ramps up (the start of a cycle of the mains sinewave). Without zero-crossing switching, if the enable signal happens to occur when the mains sinewave is at a positive or negative peak, the load will see an immediate jump in instantaneous voltage, from 0V to the peak voltage (325V for a 230VAC mains supply).

So the zero-crossing switching feature reduces the worst case possible surge current in the load and is desirable.

The SSR that I suggested, and the one you have found, contain an LED, which illuminates a photodetector that drives the SCRs that switch the load. The LED is connected directly to the two "input" pins. So you need to drive the SSR as if it was an LED. This means that you need a current limiter of some kind; a resistor is the usual and simplest option.

The specification sheet for that SSR says that the LED has a maximum forward voltage of 1.4V at 20 mA and needs 8 mA to switch the SCRs ON reliably. I would run it at twice that current, i.e. about 16 mA.

If your Arduino is running at 5V, when the output is active you will have the following circuit:

Arduino output -------------/\/\/\/\/-------|>|--------0V

Excuse the ASCII art. That's supposed to be a resistor, and the LED inside the SSR.

There will be 5V (roughly) from the Arduino output to the 0V rail, and the LED will drop 1.4V, leaving 3.6V across the resistor. You want 16 mA (0.016 amps) to flow in the resistor, so use Ohm's Law to calculate R:

R = V / I
= 3.6 / 0.016
= 225 ohms
Closest preferred value = 220 ohms.

You could use a triac. Or you could use a bridge rectifier and an SCR. But in both cases you need electrical isolation between your Arduino and the switching device. It's a lot simpler to use one of these SSRs; these have a triac or two SCRs in them anyway, and are more compact and simpler to use.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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yes ... an electric shock

That's a bit colloquial!

Technically, if you survive an electric shock, the source of that shock can be measured in Duckworths* (D).

The actual computation is rather tricky, and so you normally give a broad estimate in sD's, vD's, eD's of fD's. These SI multipliers and submultipliers are well known terms of the art**.

At either end of the spectrum there are terms for the almost incalculable values asymptotic to 0 (less than 0.005sD) or approaching infinity (> 100fD). Because these can't be ignored in calculus, they are preserved and are Tna(s) at the zero, and efD at the pole(s).***

Often people who are new to the discipline are surprised by a sensation of Tna(s) and will react significantly. The person charged with teaching this person will often say "Tna(s)! This is a shock" And offer them something charged to a couple of kV. with a shock rating of just under 0.1 sD.

I hope this helps you in describing how lucky you were. A shock capable of locking your muscles would be rated by me up in the moderate vD's

*As in how worthy you are to be termed a Lucky Duck.

** s=slightly, v=very, e=extremely, f=something I can't quite remember,

*** "That's not a shock" and "extremely fortuitously"
 

(*steve*)

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And of course, the preceding post is complete and utter *******s :)
 

fatman57

May 27, 2013
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And of course, the preceding post is complete and utter *******s :)

Steve - your contributions are highly valued...keep them coming :) I would say I am a lucky duck - it was in France and in that country the plugs do not have safety guards, so being a kid (as they do) my finger went right in. I did lock up (lost control of my muscles and felt physically attached to the plug) and shook but managed to get away...wasn't exposed for too long...(just thought I would add to the story there).

KrisBlueNZ - GREAT! Learnt something new today - zero-crossing switching. Thanks so much for your help. I will have a look for a better SSR, zero-crossing switching makes the best sense. The Arduino can do 40mA on each pin so should be able to keep it going.

I still need to get my head around everything in a datasheet - I can identify what I need (after much head scratching) but still get lost in most of it - I guess the more exposure I have the better...and of course not being formally trained I have to be careful with these things! Constant wiki lookups is the call of the day usually, and I don't learn anything half as well as I need to that quickly!

Found this item:

http://uk.rs-online.com/web/p/solid-state-relays/6675221/

It has a MAX control voltage of 1.4V...workable (same as before) but not ideal...thanks again KrisBlueNZ for the resistor calcs...
 
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KrisBlueNZ

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Yes, that SSR looks good. A bit overkill, but for a one-off project the extra cost isn't important.

The 1.4V control voltage is not "not ideal". It's just the forward voltage of the LED. There's nothing unusual or problematic about it. You just need to drive the LED inside the SSR like a normal LED - using a current limiting resistor. Have a look in the tutorial section of this site for Steve's excellent tutorial on driving LEDs. It will explain things more clearly.
 

fatman57

May 27, 2013
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Yes, that SSR looks good. A bit overkill, but for a one-off project the extra cost isn't important.

The 1.4V control voltage is not "not ideal". It's just the forward voltage of the LED. There's nothing unusual or problematic about it. You just need to drive the LED inside the SSR like a normal LED - using a current limiting resistor. Have a look in the tutorial section of this site for Steve's excellent tutorial on driving LEDs. It will explain things more clearly.

Thanks KrisBlueNZ - I have used LEDs before so should be able to tackle this. I have not done an exhaustive search, but am happy to have a confirmed component. A nice device to find, I think SSR's are quite elegant.

Of course, a mechanical relay has no leakage (I presume)? Shouldn't a resistor help against leakage when using an SSR or would this not be a good idea?
 
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KrisBlueNZ

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Right. Some SSRs are probably approved by UL Labs and other safety organisations too.

True, a mechanical relay has practically no leakage. That SSR you just found seems to have a leakage of 100 µA which is pretty low, but you're right, it's a consideration.

You could add a resistor across the load, but of course that would waste power when the load is energised. You could use an optocoupler in series with the resistor, and switch the optocoupler ON when the SSR is OFF and vice versa. I don't know of any optocouplers that can handle mains voltages on their transistors, but they might exist. I'm off to sleep now so you can search for them if you want.
 

fatman57

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Right. Some SSRs are probably approved by UL Labs and other safety organisations too.

True, a mechanical relay has practically no leakage. That SSR you just found seems to have a leakage of 100 µA which is pretty low, but you're right, it's a consideration.

You could add a resistor across the load, but of course that would waste power when the load is energised. You could use an optocoupler in series with the resistor, and switch the optocoupler ON when the SSR is OFF and vice versa. I don't know of any optocouplers that can handle mains voltages on their transistors, but they might exist. I'm off to sleep now so you can search for them if you want.

Thanks KrisBlueNZ - another new one to me! ;)
 
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