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Old 13th February 2006, 08:32 PM   #1
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Default caps for relays

I want to uses 4 sdt 12volt relays to turn on and off a dual mono preamp,that two relays per channel(one for -rail and one for + rail). Im going to use one 12 regulator per channel.

I question is, how much capacitance before and after regulator do i need(if any)
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Old 13th February 2006, 08:57 PM   #2
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Why don't you consider put the relays on the mains side?

Relays don't need to have regulated voltage BTW.
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Old 13th February 2006, 10:04 PM   #3
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Quote:
Originally posted by peranders
Why don't you consider put the relays on the mains side?
Outboard power supply.

Quote:
Originally posted by peranders

Relays don't need to have regulated voltage BTW.
I need to drop a few voltage to use the relay. I cant used the series resistor because the same power supply will power the power amps,so the voltage might be all over the place.
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Old 13th February 2006, 11:25 PM   #4
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Hi Jaudio,

A typical regulator set up will have about 100uF before and about 10uf after it plus bypass capacitors. This assumes adequate filtering of the main power supply.

You should not run the relays off the regulator and instead run them off the raw DC supply. If you run them off the regulator all you are doing is increasing the load on the regulator and creating unnecessary heat.

Lets use this example with a raw DC supply of 20 volts.

If the 12 relay coil is 300 ohms its operating current should be 40 mA. So 20 volts / 0.04 amps = 500 ohms total resistance.

500 ohms - 300 ohms (relay resistance) = 200 ohms series resistor. Use 180 ohms (nearest available value).

Power dissipated by the resistor will be about 320mW.

You can see from the above that the regulator will dissipate 1.3 watts of unnessary heat if it supplied 4 relays. If the voltage is higher, then the heat situation gets worse.

Mr Peranders is right you do not need a regulated supply for the relays. Your amplifier power supply should not vary by more than 10% (there is something wrong if it does) and relays have way more tolerance than this.

Cheers
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Old 8th March 2006, 01:03 AM   #5
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Default safe dc voltage

I have no experience with relays. Not thinking,I tested the relay with 28volts and the relay heatup. Now I know I shouldnt have gone above 24volt.

Can anybody tell me what is the max. and safe dc voltage for the 12volt version of this relay.



http://relays.tycoelectronics.com/datasheets/SDT.pdf
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Old 8th March 2006, 01:38 PM   #6
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I lower the switching voltage to 16volt(+/-),there is a lot less heat(40c well within the relay's operating temp.). I didnt think the relay would heat at all,drawning only 1amp.

What temp should the relay be? It is a 12volt relay with 3amp dc current,operating with 9volt on coil.
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Old 8th March 2006, 02:10 PM   #7
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The coil can take up to 160% of rated volatge. Page 475 at the bottom, left.
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Old 8th March 2006, 06:15 PM   #8
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Thanks for the reply

The coil part I understand but the contact and switch dc voltage I dont completely understand.

Now the contact is rated at 3a at 24volt,and the relay get to about 40C,is that normal?

What is the difference bewteen the contact(10A resistive and 3A inductive) and switched(10A 24volts)?
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Old 8th March 2006, 06:42 PM   #9
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The relay heats up because of the coil power dissipation. This is independent of the current through the contacts. You will find that even with nothing connected to the contacts it will heat up. 9V across the coil of 300 ohms = V squared / R = about a quarter of a watt?

The contacts ideally will not heat up because they will have a very low resistance so the power in the contacts being I squared * R will also be very low.

BTW, are you putting these relays before or after the supply capacitor?

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Old 8th March 2006, 06:51 PM   #10
Electrons are yellow and more is better!
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Quote:
Originally posted by janneman
The relay heats up because of the coil power dissipation. This is independent of the current through the contacts.
This is not quite true. Most relays get heated up contacts and the contact springs. A warm relay get also a bit worse performance when it comes to switching but in his case I don't think it's going to be any problems with this.
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