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Old 11th March 2009, 12:58 PM   #1
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Default Wiring up a 240V relay?

I've searched here and the internet, but I can't find a simple picture/explanation about how to wire up a relay using 240V.

I need to implement a basic soft start circuit (Figure 1 at this link ), but I don't know how to connect all the pins. Can someone please give an idiots guide to this?

I've been quite proud of myself up until now as I've managed to work out wiring schematics, chip and transistor connections, etc, but this one really has me stumped.

I think I need a Sinle pole normally on relay which has 4 connectors, but again advice on this would also be appreciated.

It's only to try and stop my 300VA torroid from blowing fuses at switch on time - I know it shouldn't need a soft start circuit, but mine does.
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Old 11th March 2009, 02:19 PM   #2
infinia is offline infinia  United States
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Default Re: Wiring up a 240V relay?

Quote:
Originally posted by westers151


I think I need a Sinle pole normally on relay which has 4 connectors, but again advice on this would also be appreciated.


No you need the normally off switch pins. After some time the relay will energize closing the switch bypassing the series R to the XFRMR primary.
Is the relay part marked showing the pole position? otherwise you need to show the relay part diagram you want to use.
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Old 11th March 2009, 02:33 PM   #3
AndrewT is online now AndrewT  Scotland
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yes,
you need the normally open contacts for the bypass function.
They close when power is sent to the relay coil.
A change over relay has this function, just leave the third pin empty.

Have a look at fig 4 for the power saving circuit.
It allows the relay to drop out more quickly.
Combine fig3 with fig4 to operate the relay coil from any higher supply voltage. eg the relay coil could be 12Vdc but run from 20 to 28Vdc and the timer circuit can be on the 15V or 12V supply.
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Old 12th March 2009, 05:42 AM   #4
infinia is offline infinia  United States
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There are much simpler/cheaper ways of doing what you want than the ESP site you referenced.


For example by using the charging of bulk caps on the XFMR secondary to power the relays coil. This has a built in delay that is usually good enough for the job at hand.
On a chip amp you could use a 48V/24V relay coil with a series resistor across 1/2 of the DC supply. The series resistor drops the excess PS voltage to the relay coils. I've done this successfully a number of times before.
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Old 13th March 2009, 07:24 AM   #5
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Infinia

Thanks for the supply - could you post up a diagram, or a link, to show what you've discussed. That would help me understand it a bit better.

Thanks
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Old 13th March 2009, 12:09 PM   #6
infinia is offline infinia  United States
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I did this for a GFA-555 amp in a past thread here.
Tell me what you want ie what transformer and good UK source for relay parts.


Basically you need 3 parts
1) 2 ohm, 5 or 10 Watt sand cast resistor (Rsurge) in series with the transformers primary winding. Rsurge is wired between the fuse and the XFRMR primary leg.

2) relay - 24 V or 48 V coil SPDT w/ contacts rated for 240V and current greater than VA. Way of mounting the relay for point to point wiring or PCB/perfboard with all three parts near the AC input fuse.

3) Resistor in series with relay coil , where
Rd = (Vout -Vrelay)/Icoil,
This will drop excess voltage from Vout to reach the coil turn on V.
Rd in series with relay coil both across one or both of your output caps whichever is close to your relay coils voltage.

Operation on start
Rsurge limits the current inrush in the transformer primary until the the relay energizes thus bypassing Rsurge.
The relay switch contacts (pole) and (normally open) will be wired in parallel with Rsurge.
The relay won't energize untill the outputs come up.





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Old 18th March 2009, 01:37 PM   #7
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Infinia

Thanks for the reply.

I've got a 240V AC, 300VA, 25V dual secondary torroid.

Parts supplier for me would be Farnell in the UK http://uk.farnell.com/

Of the relays they stock then here are the items that match your description Relays.

However, I can see only one that matches your specs and that's this one

As for Rd, can you give me a worked example to understand that calculation - basically I'm confused as to what Volts you're referring to (i.e, mains volts, or DC volts).

And the final question for now, Output cap? Where are those?
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Old 18th March 2009, 02:25 PM   #8
AndrewT is online now AndrewT  Scotland
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Hi,
here is a little Excel spreadsheet that does the calculations and allows for a power saving function that lets the relay run cooler and drop out more quickly.

The Zener voltage can be around the relay rated voltage.
The 22uF cap voltage must exceed the maximum DC supply voltage
Attached Files
File Type: zip relaysaver.zip (4.2 KB, 27 views)
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Old 18th March 2009, 02:48 PM   #9
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Andrew

Thanks for the spreadsheet and inevitably I have questions !!

Relay pull in and Relay drop out voltages - How do I determine those? i.e, how do I know what value to enter here?

I'm guessing that R1 is wired in series between the live terminal of the fused IEC socket and the live terminal on the relay?

I assume the value given for R1 at the end of the calculation will be enough to drop the mains 230V voltage down to the 24V rating of the relay - Correct or incorrect?

Thanks
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Old 18th March 2009, 02:57 PM   #10
AndrewT is online now AndrewT  Scotland
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NO NO NO.
It works off a DC supply!!!!!!!
Note the use of semiconductors and diodes. Mains AC would kill this circuit.

The relay should be measured to obtain the pull in and drop out voltages.
The coil resistance can be checked but the manufacturer's specification is likely to be quite close.

Most relays operate on DC supplies.
Some are made and specified to operate on AC supplies.
Your linked one is for 24Vdc.
Quote:
Voltage, Coil DC Nom: 24V
If you don't learn to read the data and spec sheets, you are putting yourself and others at risk of serious harm.
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