Need help on relay for 1200W Trafo soft start - diyAudio
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Old 9th January 2011, 04:26 PM   #1
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Default Need help on relay for 1200W Trafo soft start

I've blown many fuses lately, so I'm Thinking of building a soft start circuit, but I don't know much about relays yet, this being my first project using one. I got a free 1200 watt 20-0-20 transformer for my 5 channel surround sound gainclone amp, and I bought This hoping it might work, but I'm sure it won't after looking at the little plastic relays.

So what would be a good relay for this? I'm kinda shooting in the dark here, but will one of these for each 20V output work (the High Current General Purpose Power Relays at the bottom)?

I'm looking for a price efficient unit. I could also use a little info in English (i.e. easy to understand, I'm stupid) on relays, but I think I get the jist of how they work.

Do I want a 30 amp relay for 12vdc (separate 12V trafo to turn it on), I think thats cutting it too close, tho.

Thanks

Last edited by caxxxxxx; 9th January 2011 at 04:31 PM.
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Old 9th January 2011, 04:55 PM   #2
AndrewT is offline AndrewT  Scotland
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The relay duty is not particularly onerous.
The relay is open when the transformer starts up and stays open for hundreds of ms. No damage, no wear, no erosion.
The transformer is drawing current through the resistive element when the relay does close. There is an increase in current at that moment when the transformer is connected direct in line. I think any relay that is rated for 250Vac and Irelay > Iprimary would survive a long time.

The biggest wear/erosion will come when the relay opens on shutting down. This should be addressed with a switch snubber to minimise the arc across the opening contacts of the main power off switch. By the time that the relay contacts open there should be little or no current flowing.

Single pole single throw is sufficient to bypass the resistive element. SPST 250Vac 10Aac (normally open = NO) would do in my opinion.
The coil voltage is chosen to suit the voltage you are using as "control".
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Last edited by AndrewT; 9th January 2011 at 05:00 PM.
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Old 9th January 2011, 05:35 PM   #3
tatus is offline tatus  Germany
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Just a side remark: even when the transformer was for free I would not use it for a gainclone. Your electricity bill will sum up very quickly, and the efficiency of your amp will be very little. I'm not that much into transformers, but I would guess that a 1200VA unit will draw at least 50W idle power. I would rather spend the money and buy a much much smaller unit.
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Old 9th January 2011, 07:53 PM   #4
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Andrew, I'm confused.

According to Soft-Start Circuit For Power Amps, "The relay contacts must be rated for the full mains voltage, and at least the full power current of the amplifier. The use of a relay with 10A contact rating is strongly recommended".

He is basing this on a 500VA trafo with 230V mains, I have a 1200 watt and 120V mains. Also on the same website, he describes how to select the correct resistance, but states they should handle "twice" the expected Amperage, (I=VA rating of transformer/mains)*200%. I realize this is for a different method, but such an inrush of current I'd like to be safe.

In my case, I=VA rating of transformer/mains is as follows:

I=(1200/120)*200%

So, expected inrush of current could be as high as 20 Amps.
Again, this is based on the resistor method, which I don't plan to use. But I wanna be safe.

So is a 20 Amp relay overkill, safe, or....any ideas? I know you know alot more on this topic than I do so I'd love to hear your input.

Quote:
Originally Posted by tatus View Post
Just a side remark: even when the transformer was for free I would not use it for a gainclone. Your electricity bill will sum up very quickly, and the efficiency of your amp will be very little. I'm not that much into transformers, but I would guess that a 1200VA unit will draw at least 50W idle power. I would rather spend the money and buy a much much smaller unit.
I have tossed this idea back and forth, but decided to go with my 1200 watt for the following reasons:

Quoted from another website:

"On paper to get full power you should have about 150VA per LM3886" (@ 44VCT per the example), "but in practice it has been proven that as low as 90VA or so has worked..."

If that is the case, a 750 watt trafo should work for a 5 channel amp @ 44VCT. Mine measures 40VCT, so around 700 watts should suffice. Giving it a little headroom, 800 watt should be plenty. But it seems even that would draw a decent amount of power @ idle from my mains, plus the cost of finding and purchasing such a trafo would be in the neighborhood of around $100 AND I would still need a pretty hefty soft start circuit to be safe. I think I'll opt for what I have right now, and I'll still have plenty of headroom for additional amps (7 channels in the future, lol). If I do notice the idle watts used is on the high side, I can still downgrade the trafo and I will have an overkill soft start.
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Old 9th January 2011, 08:03 PM   #5
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Or....perhaps I'm not fully underastanding how this soft start circuit is supposed to work, as in where exactly the trafo and mains wiring will be placed in line with the relay..... I'll read some more and see what I come up with.
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Old 10th January 2011, 06:37 AM   #6
tatus is offline tatus  Germany
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Quote:
Originally Posted by caxxxxxx View Post
I have tossed this idea back and forth, but decided to go with my 1200 watt...
I missed the point that it was a 5-ch. Makes it (a bit :-) more reasonable for 5x75 watts rms output power (@4 ohms, would that be right?). I go the other way though and have good results. My (class D) stereo amp drives 250 w rms into each (8 ohm) speaker and is supplied with an SMPS capable of 100w continuous (600w short time peak). Plus large buffer capacitors. I don't manage to exceed 50C at any heat sink no matter how loud I "listen".
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Old 10th January 2011, 07:09 AM   #7
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Quote:
Originally Posted by caxxxxxx View Post
Or....perhaps I'm not fully underastanding how this soft start circuit is supposed to work, as in where exactly the trafo and mains wiring will be placed in line with the relay..... I'll read some more and see what I come up with.
Have a look at Soft-Start Circuit For Power Amps

Basically the circuit puts a resistor in series with the mains input at power on, after a short delay the relay shorts across the resistor so that the full mains is applied to the transformer.

The relay must be rated for the current that the transformer is expected to draw. Too small and the relay will fail, if it fails the resistor(s) will overheat and fail.
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Old 10th January 2011, 10:51 AM   #8
tatus is offline tatus  Germany
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Another option could be using a triac based soft start circuit. More complex but has the advantage that it does not contain bulky resistive parts with high dissipation. I have briefly looked at http://www.atmel.com/dyn/resources/p...ts/doc4712.pdf, and the typical application diagram looks just like what you are searching for.
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Old 10th January 2011, 11:59 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by caxxxxxx View Post
Andrew, I'm confused.

According to Soft-Start Circuit For Power Amps, "The relay contacts must be rated for the full mains voltage, and at least the full power current of the amplifier. The use of a relay with 10A contact rating is strongly recommended".
indeed you are.
we give you the explanation and we give you the answers and you are confused.

Do not be confused when working with mains electricity. Your life depends on you understanding exactly what you are doing. More research and more questions.

Now back to my answers.
I said 250Vac for the relay. That is higher than any of the domestic voltages available anywhere in the world which range from 100Vac to 220Vac
I hope there is no confusion in there.

I said 10Aac. This equals the maximum rated current for your 1200VA 120Vac transformer. I hope there is no confusion with this bit.

Now to soft start and why we need it for inductive loads (eg. motors and transformers).
At start up the inductive load has no current flowing and no flux in the magnetic field (this is a simplification but does not significantly alter what follows). If you were to measure the resistance of the primary and obtain an estimate of the mains supply impedance and add on the resistance of your internal household circuit to feed the transformer you will find a total resistance of <<10r.
Apply 120Vac to say 5r and the AC current could be as high as 120/5 ~ 24Aac.
But at the instant of start up the voltage available at the socket may be anywhere between -170Vpk and +170Vpk with reference to Neutral.
The worst case peak transient current at start up is ~170/5 (substitute your mains circuit resistance) i.e. <=34Apk

I have done this calculation for smaller and much smaller transformer and find that worst case instantaneous peak start up currents can approach 100Apk.

I use soft starts to reduce this potential worst case current.
Add a resistive device in series with that 5r household resistance. say 30r.
the worst case start up current is now 170/[5+30] <=5Apk
While this current is flowing the flux in the magnetic circuit builds up and the inductive load starts to develop an inductive impedance that adds to the 35r seen by the mains. The current starts falling towards the running current of the transformer, (this occurs in the first few cycles of the mains supply).
During this time or very soon after, the relay clicks over to bypass the 30r. The 5r + transformer impedance now resists the mains voltage at the moment of contacts closing. If the transformer has no load or very little load then that inductive impedance is likely to be around 1500ohms for this fairly large transformer. The current will increase slightly from the before relay bypass to after relay bypass. But it will never reach 10Aac, when the transformer is lightly loaded.

If the transformer is heavily loaded (eg. the smoothing caps are discharged) then we are in a completely different ballgame.
I cannot estimate the increased current that will flow after the relay bypasses, but I suspect it will not exceed 10Aac for long, if at all.
The damage to the relay contacts is not when they close.

Does this get rid of the confusion?

Now to switch off, when damage/wear can occur to the relay contacts.

The bypass relay is closed.
The mains switch is closed.
The transformer is feeding the lightly loaded amplifier (caps fully charged).
The mains switch opens and is susceptible to damage and wear. This mains switch MUST be RATED for the duty of switching OFF the transformer.
After the power has been switched off there may be an arc across the opening contacts, but this should not last longer than half a cycle of the mains frequency. As the relay driving circuit loses it's hold in voltage the relay drops out. Provided the relay drops out AFTER the arc has extinguished the relay contacts cannot suffer damage/wear. They will simply wipe themselves as they part.

Is this bit clear?


I will repeat my safety message.

Never guess when working with mains electricity. Understand what you propose or get professional help if you have doubts.

To ALL,
this is the first time I have gone through this version of the extended discussion.
Can anyone see any errors that must be corrected, or even a better way of explaining why and how the soft start works?
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Last edited by AndrewT; 10th January 2011 at 12:09 PM.
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Old 10th January 2011, 12:47 PM   #10
em2006 is offline em2006  Romania
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Quote:
Originally Posted by tatus View Post
Another option could be using a triac based soft start circuit....
In my opinion, phase-control circuits used in such applications should be avoided, because they generate noise.
Spectrum generated is very large, these frequencies are radio noise.
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