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Power Relay on Transformer Secondary?

post #1 of 11
4/30/10 at 2:23am
Thread Starter 
I remember reading somewhere (perhaps on this board) that using a relay to cut power on the secondary side of a transformer was bad practice. I'm wondering if (assuming that's true), it's simply not the best solution or if it actually is a problem.

The situation is that I have a custom-wound 22V+22V+7V shielded/encapsulated transformer (SumR does beautiful work) for a Gainclone. I need the 7V to stay on at all times (to power the microcontroller controlling the power relay ans such) so putting the relay on the primary side isn't much of an option.

The idea is to use dual relays on the secondary side, cutting the top of the first 22V winding and the bottom of the second 22V winding (22V and -22V respectively), leaving the two in the middle that make up 0V intact. Is this correct/safe?
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post #2 of 11
4/30/10 at 1:22pm
I don't know about your particular application, hopefully someone that does will chime in. In a tube amp, the reason this is frowned upon is that it puts the high voltage secondary across a switch or relay. Commercial switches and relays rarely have contacts that are rated for the B+ of a typical tube amp. A potential safety and equipment hazard. The low voltages you're talking about shouldn't be a problem as long as the relay contacts are rated for the relevant current. Again, someone with specific knowledge of the Gainclone should comment.
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post #3 of 11
4/30/10 at 2:24pm
I would get a small 7v transformer in addition to the 2*22v transfomrer and keep them separate.

Hardwire the 7v transformer to the IEC (with a switch in the IEC ) and run the "brain" off of that. Switch the primary/mains side of the 2*22v transformer.

Im also not sure if there is a problem with switching the secondary of the transformer (although the currents may be quite high) BUT if the primary is not switched the transformer is always using at least some power.
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post #4 of 11
4/30/10 at 2:37pm
Thread Starter 
Yeah, running a second trafo is an option I was considering, but I'd prefer not to because of the extra space required and because it will add noise (I'm planning on a single box solution) unless I have one made with shielding/encapsulation, which would be overkill.
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post #5 of 11
4/30/10 at 4:45pm
I'd be concerned about having your primaries always on, if it's a big trafo. I know the bigger units will buzz, indicating there is some amount of physical stress on them, as long as the primaries have mains voltage on them. This may slightly reduce the life of the trafo.
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post #6 of 11
4/30/10 at 6:20pm
Thread Starter 
Quote:
Originally Posted by Iniamyen View Post
I'd be concerned about having your primaries always on, if it's a big trafo. I know the bigger units will buzz, indicating there is some amount of physical stress on them, as long as the primaries have mains voltage on them. This may slightly reduce the life of the trafo.
That's more along the lines of what I was thinking. Anyone have more info on how much of a problem this is? Google isn't my friend today. For what it's worth, the transformer is very well made and potted, so physical movement (buzz) shouldn't be as much of a problem, but I don't want to do any damage internally. And the 7V winding will still be connected so that should present some load for it work with.
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post #7 of 11
5/1/10 at 11:50pm
Thread Starter 
Well if there's no further comment I guess I'll go ahead and give it a shot (very carefully) once I get all my parts in.
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post #8 of 11
5/2/10 at 1:02am
Probably another issue with switching on the secondary is you will have a higher instant on current draw if you have large caps to fill. With the switch on the primary side the transformer itself will act somewhat as a slow start so that you do not have a high inrush current.
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post #9 of 11
5/2/10 at 1:26am
Thread Starter 
Yeah. What I'm thinking now (which isn't much different) is to attach the relays after the power supply (after rectification and snubberizing) to +/- Vcc. That way the psu caps will always remain full and the transformer will have a little more of a load (the power supply with it's resistors, etc).
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post #10 of 11
5/2/10 at 10:03am
Quote:
Originally Posted by Juaquin View Post
Yeah. What I'm thinking now (which isn't much different) is to attach the relays after the power supply (after rectification and snubberizing) to +/- Vcc. That way the psu caps will always remain full and the transformer will have a little more of a load (the power supply with it's resistors, etc).
I am not familiar with the B22 boards, but I know with the M3 this is very bad thing to do. Because the inrush current to charge the rail caps would be very high.
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post #11 of 11
5/2/10 at 3:24pm
Thread Starter 
I'm building a Gainclone (using the kit from chipamp.com). The only caps of significant size are on the PSU, which is why I was thinking of leaving the PSU always connected and doing the relay between the PSU and the amp boards, which only have a couple 100uF caps. That way, the psu's huge 10,000uF caps will always remain charged up as long as it's plugged into the wall (like they would be in a gainclone that did not have any relays). This way there won't be any significant inrush current, except the initial when you plug the unit in, which is normal for any gainclone that doesn't have fancy power switching.
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