Electromagnetic relay

[adrianvon]

Hi,

From where can i find a datasheet of an electromagnetic relay with a coil voltage of 12VDC and an output of 230-240VAC 40-50A ??

I want the datasheet to compare electromagnetic relays with SSRs.

 

[ronv]

https://www.mouser.com/Electromecha...0x1ecZ1yzsx4oZ1yzoqarZ1yzoqchZ1yzoqa5Z1z0x0w9

 

[adrianvon]

Thanks for the reply. So the coil current at 12V is 160mA, am i right?

 

[ronv]

Yes you are.

 

[adrianvon]

Hi again,

When using SSR, is there the need to connect a current limiter resister (1K) to its input? and can i connect it directly to the PIC or is it better to use a transistor to drive it?


Edit: I bought the SSR in the link below. It says that the Control Voltage is 3-32V DC and the current is 3-25mA DC. Does this mean that the higher the input voltage is, the lower the control current? and if yes, the power consumed will still remain the same regardless the input voltage?

**broken link removed**

Thanks is advance.

 

[alec_t]

Try the manufacturer's website to get a datasheet with the full spec of the SSR. That should help you.

 

[adrianvon]

I have googled the part number but couldn't find a datasheet. This product is made from china so i doubt if they have a datasheet.

 

[Sceadwian]

Does this mean that the higher the input voltage is, the lower the control current?

To understand better you have to realize what the first stage of an SSR actually is, it's nothing more than a optocoupler (for electrical isolation) that triggers the high voltage SCR's. So consider the input to an SSR to be nothing more than an LED. It won't start conducting until there's 3volts, it won't operate the high side until it reaches 5ma of current and will not tolerate a differential voltage over 32 volts or a current of more than 25ma. I say differential voltage only because these devices can be run on 50 volts DC or higher as long as the differential voltage was never more than 32 volts, consider the case of running it off a resistor on a 100 volt supply where the resistor is valued to drops 5 volts.

 

[dr peppers]

Originally you mentioned electromagnetic relays, if you go down this route then for that sort of current I'd reccomend a contactor, this is just a larger industrial version of a relay designed to take more current, they usually have 3 main contacts (for 3 phases), you can wire all 3 of these in parallel tripling the current rating.
Your probably better triggering a small relay with the pic micro via a transistor, then using the small relays contacts to pull in the contactor (dont forget a snubber accross the contactors coil).

An SSR can be better way of switching, esp if the SSR has zero voltage switching, there are various types, all the ones I've seen have been 24vdc input, but you can get 12v, 5v and direct to led versions, if you can get a 5v input and the current is less than around 20mA (depending on the pic you use) then you'll be able to connect it directly to the pic (allthough using a transistor is reccomended for protection).
For your particular application if you cant get a datasheet, then your best option would be to get a sample and test it using a variable power supply.

 

[Sceadwian]

An SSR can be better way of switching, esp if the SSR has zero voltage switching, there are various types, all the ones I've seen have been 24vdc input, but you can get 12v, 5v and direct to led versions, if you can get a 5v input and the current is less than around 20mA (depending on the pic you use) then you'll be able to connect it directly to the pic (allthough using a transistor is reccomended for protection).

It's not usually called zero voltage switching because obviously voltage has to be present on the switching side to function, it's refereed to as a zero crossing SSR. The bulk of all SSRs I've ever seen are all direct to LED I've never run across any with internal resistors, but perhaps you run across them in situations where they're designed as drop in relay replacements. The use of a transistor doesn't add much protection, if the isolation voltage of an SSR is surpassed to failure it will blow through most transistors like a rocket train, it does however offload the current required from the PIC to trigger the SSR as although I/O lines have current limits the entire port has a limit as well as there being a limit for the entire chip.

 

[dr peppers]

Yes the zero crossing switching reduces harmonics and rf emissions, and also means lower fuse values can be used in certain applications to protect the switched circuit.
I work in industrial control and 24vdc is very common, which would explain why I only come accross 24vdc versions.
I was thinking of a transistor for protection more for if the user connects the ssr up wrong or theres a short in the input wiring, not so much if somehow the input of the ssr manages to connect to the output, I've seen this happen once, the control box was in one piece still but it resembled a toaster as all the boards were toasted, theres usually good protection within a ssr against this sort of issue so unless its a mil design extra protection isnt warranted.
I came across the total port current limiting in a recent project using the pic micro, another foxhole I fell into, yes you cant pull the pins max current from all the pins, and that causes all kinds of other issues such as my favorite r-m-w, if I had a big mac for every time that got me I'd be very fat.

 

[gmphadte]

U can connect the specified input dc without a resistor in series. These SSRs have a transistor that bypasses the current flowing through the LED by sensing the LED current.

Don't worry, just connect anything from 3 to 32 Volts DC.

Gajanan Phadte

 

[adrianvon]

Thanks for your help.

 

[Sceadwian]

I thought about that while I was at work today gmhadte, you're correct, I briefly got my basic opto isolated triac/back to back SCR images mixed up in my head with SSR's.

One note about zero switching SSR's is they can't be used for phase angle dimming/current control in lighting or motors, you can at best drop a half cycle of the AC wave.