The one on the right looks wiser and more experienced and the one on the left reminds me of hockey. Hmm...I don't know John that's a tough one.
See, it made me think of donuts...uummmm...dooooonuts...🙂
The one on the right definetely has more experience. The toroid has a slight buzz that is not scoring any points with me.
Looking for relays, it's tough to find high current / high voltage DC at a reasonable price. I did find THIS ONE, cheap enough but only 35VDC. I'm curious how far beyond its rating that relay will operate. Will it break 52VDC at 6A?
Thinking about this from another angle, would it be better to use a pair of larger relays to cut both power supply rails, after the smoothing caps? To me, this makes more sense.
Speaking of smoothing caps, I think this new version of the power supply deserves new caps. I have searched through Digikey and Mouser for the best deal, and found Cornell Dubilier 39,000uF 80V at a reasonable price. I will provide space on the board layout for 4 of these (2 per rail) but start with 1 each for now.
Another possibility for the regulator on the main supply is the type I used on my lab power supply. The advantage over the one I've shown here is that it has feedback from the output and compensates the voltage according to load. The disadvantage to that scheme is that it can be difficult to stabilize (as seen in my lab power supply build thread). Another advantage is that, as it works to maintain a steady voltage, it also reduces ripple via the same mechanism. It would be possible (from a smoothing standpoint) to reduce the amount of smoothing capacitance. If I had a higher VA transformer, I think I would easily pick this method of regulation. Given that my transformer choices are very close to the design limits for amperage, I think a large amount of capacitance would give the best results.
Thinking about this from another angle, would it be better to use a pair of larger relays to cut both power supply rails, after the smoothing caps? To me, this makes more sense.
Speaking of smoothing caps, I think this new version of the power supply deserves new caps. I have searched through Digikey and Mouser for the best deal, and found Cornell Dubilier 39,000uF 80V at a reasonable price. I will provide space on the board layout for 4 of these (2 per rail) but start with 1 each for now.
Another possibility for the regulator on the main supply is the type I used on my lab power supply. The advantage over the one I've shown here is that it has feedback from the output and compensates the voltage according to load. The disadvantage to that scheme is that it can be difficult to stabilize (as seen in my lab power supply build thread). Another advantage is that, as it works to maintain a steady voltage, it also reduces ripple via the same mechanism. It would be possible (from a smoothing standpoint) to reduce the amount of smoothing capacitance. If I had a higher VA transformer, I think I would easily pick this method of regulation. Given that my transformer choices are very close to the design limits for amperage, I think a large amount of capacitance would give the best results.
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I can't answer that Q.
Why is the relay needed?
To protect the amplifier, or protect the speaker or ?
If the idea of the relay is to cut off the supply when a fault/abuse occurs then what is the fault current? What restricts it to <=6Apk?
If it is 6Apk from 52Vdc, that's >300W, how long can the supply keep pushing 300W into the arcing relay contacts, if the relay cannot break that fault current?
My suggestion is to think about how the relay could be abused and then see if there are ways to solve those problems.
A mosFET might be better for breaking a DC current flow. Both quicker and no risk of arcing nor wearing out. irf540 is 100V, 28A It should be full on or full off, no in between stage, so power dissipation is not an issue. One in each power rail with a trigger that can be set off by excess DC or excess current or excess temperature or loss of AC power or.....
Thanks Andrew,
I have been thinking about replacing the relay with solid state but was looking at TRIACs. A high current, high voltage MOSFET might be a better choice.
Breaking the rails (both simultaneously) would be triggered by DC detection only (the only protection that I'm really interested in - to protect the speakers without destroying the amp). This would not address my desire to have on/off muting of the speaker outputs though. I will probably order some of those relays and actually test the amount of current/voltage they can break. I know that sometimes ratings on these parts are on the conservative side.
I have been thinking about replacing the relay with solid state but was looking at TRIACs. A high current, high voltage MOSFET might be a better choice.
Breaking the rails (both simultaneously) would be triggered by DC detection only (the only protection that I'm really interested in - to protect the speakers without destroying the amp). This would not address my desire to have on/off muting of the speaker outputs though. I will probably order some of those relays and actually test the amount of current/voltage they can break. I know that sometimes ratings on these parts are on the conservative side.
you could use a triac across the speaker terminals to force the line fuses to blow. But it may need 2 triacs or a clever circuit to ensure both fuses actually blow.
The line fuses can be surprisingly low value. I have run some enormous test power through fuses that should not have survived.
The line fuses can be surprisingly low value. I have run some enormous test power through fuses that should not have survived.
I have seen this too and that's why I have never put much faith in fuses on the output. Right now, I have crowbars across the midranges in my speakers (I didn't get around to making them for the woofers and the tweeters have a large 11uF cap in series that will kill any DC), but no fuses in the output. This may be the best solution. Output relays for on/off muting and fuses on the same board to blow if the crowbar trips. Maybe a better place for the crowbars is on the output boards, with the relays and fuses.
It good to hash these issues out and get some informed input. The value of a forum like this is certainly not lost on me. 🙂
Yes, talking about it helps a lot in pulling out the issues and the solutions.
I found that a lot in my two professional careers.
I'm slightly at odds with the "no faith in fuses" statement.
eg. 100W into 8ohm amplifier tested to full power will run on a pair of F2.5A fuses. The amp will drive any 8ohm speaker to very high volumes and not suffer nuisance blowing. You may find that others who recommend a slightly lower fuse rating say T2A will have based this selection on exactly the same criteria.
Now look at the manufacturer's data for an F2.5A fuse.
How long will it last @ 5A continuous? and @ 10A continuous? Would durations of those values of current damage a woofer or it's inductor in that time period?
In the meantime you have DC detect and isolate.
Similarly, if one accidentally shorts the output terminals and draws an instantaneous peak current near the maximum short term rating for the output devices, say our output stage can easily survive 30Apk for 100ms. How long will that F2.5A last @ 30A continuous last?
How long will the PSU hold up that amount of current delivery into a dead short? Will the VI limiter come in and protect the outputs from destruction?
I found that a lot in my two professional careers.
I'm slightly at odds with the "no faith in fuses" statement.
eg. 100W into 8ohm amplifier tested to full power will run on a pair of F2.5A fuses. The amp will drive any 8ohm speaker to very high volumes and not suffer nuisance blowing. You may find that others who recommend a slightly lower fuse rating say T2A will have based this selection on exactly the same criteria.
Now look at the manufacturer's data for an F2.5A fuse.
How long will it last @ 5A continuous? and @ 10A continuous? Would durations of those values of current damage a woofer or it's inductor in that time period?
In the meantime you have DC detect and isolate.
Similarly, if one accidentally shorts the output terminals and draws an instantaneous peak current near the maximum short term rating for the output devices, say our output stage can easily survive 30Apk for 100ms. How long will that F2.5A last @ 30A continuous last?
How long will the PSU hold up that amount of current delivery into a dead short? Will the VI limiter come in and protect the outputs from destruction?
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Ok, so here's what I see:
- Main power supply as shown a few posts back, with no current limit of DC detection to cut the rails.
- Output boards that have on/off muting for the output, fuses in series with this and a crowbar across each output. Local DC detection keeps things simple but doesn't sacrifice performance or speaker protection (most important). The output fuses may provide some short circuit protection as well.
- Input boards that mute the input to each amp during start up and shut down. On startup, the input will be muted for a second after the outputs come on - thereby avoiding the possibility of the output relays switching on a load.
- A separate 12VAC transformer to power the softstart, input and output boards.
Fairly simple and quite an improvement over the current state of the amp. A few RC timers, some relays (I already have some for use in the softstart and input muting) and away we go.
- Main power supply as shown a few posts back, with no current limit of DC detection to cut the rails.
- Output boards that have on/off muting for the output, fuses in series with this and a crowbar across each output. Local DC detection keeps things simple but doesn't sacrifice performance or speaker protection (most important). The output fuses may provide some short circuit protection as well.
- Input boards that mute the input to each amp during start up and shut down. On startup, the input will be muted for a second after the outputs come on - thereby avoiding the possibility of the output relays switching on a load.
- A separate 12VAC transformer to power the softstart, input and output boards.
Fairly simple and quite an improvement over the current state of the amp. A few RC timers, some relays (I already have some for use in the softstart and input muting) and away we go.
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It doesn't do what I need, unfortunately.
Worth looking at for accurate power supply current monitoring is the ZXCT1050. Configurable for high side and low side current sensing. If I were to go the current limit route (I'm not going to on this supply), I'd use that.
Since I won't be trying to switch a loaded output with the output relays, I believe the ones I have will probably work. They are rated for 12A @ 120VAC, 7A @ 240VAC and 7A @ 28VDC. They are board mount miniature type that I bought a gross of a few years ago, so I have plenty of spares if something happens to one.
I have used them in a few circuits with great results.
I spent some time prototyping the delay circuit for the input / output muting relays and I think I have it nailed. The output relays need a shorter delay time than the input, to have the output relays closed before the input relays open (input relays short each amps input to ground). Likewise, a short delay is needed to extend the closing time of the output relays, until the input relays have closed, grounding the input. Tricky for a rookie like me but I think I have a good reliable solution. I'll post that schematic next.
I have used them in a few circuits with great results.
I spent some time prototyping the delay circuit for the input / output muting relays and I think I have it nailed. The output relays need a shorter delay time than the input, to have the output relays closed before the input relays open (input relays short each amps input to ground). Likewise, a short delay is needed to extend the closing time of the output relays, until the input relays have closed, grounding the input. Tricky for a rookie like me but I think I have a good reliable solution. I'll post that schematic next.
The basic delay circuit:
suggestions for improvement welcome.
The top 6 relays switch the outputs of each amp, and there is a ~3 second delay before they close. When the power is switched off, C1 provides enough charge to keep the relays closed for an extra fraction of a second.
The lower back of relay is for input and are tripped after a ~4 second delay. On power down, D1 blocks the charge from C1 and the input relays close quickly, muting the inputs to each amp, before the output relays open.
The relays draw a considerable amount of current (30mA each, total of 360mA for all 12) so the regulator will need to be on a good heat sink. The 2 MJE15030's don't dissipate much so I could get away with MJE340's in their place.
suggestions for improvement welcome.
The top 6 relays switch the outputs of each amp, and there is a ~3 second delay before they close. When the power is switched off, C1 provides enough charge to keep the relays closed for an extra fraction of a second.
The lower back of relay is for input and are tripped after a ~4 second delay. On power down, D1 blocks the charge from C1 and the input relays close quickly, muting the inputs to each amp, before the output relays open.
The relays draw a considerable amount of current (30mA each, total of 360mA for all 12) so the regulator will need to be on a good heat sink. The 2 MJE15030's don't dissipate much so I could get away with MJE340's in their place.
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The scheme above gives an overview but isn't a practical layout. In fact, that will be split into 3 boards - an input board and 2 output boards. Here's one of the output boards, the simpler one:
3 relays for the three channels on one side of the amp. There are 3 fuses and 3 crowbar circuits as well. This can be place close to the output binding posts, under the amp shelf. It requires a 2 conductor connection to the other output board (which has the delay circuitry).
3 relays for the three channels on one side of the amp. There are 3 fuses and 3 crowbar circuits as well. This can be place close to the output binding posts, under the amp shelf. It requires a 2 conductor connection to the other output board (which has the delay circuitry).
yes, mute the input before the output relay is asked to open.
The order of switch on is not so important. But "unmute" first would allow DC detect to recognise a faulty source and hold the output relay OFF with a big red flashing light and a Klaxon telling you something is up !
Klaxon, eh? Might be a bit excessive. 😱
Monitoring DC on 6 channels would be a lot more involved than the fuse/crowbar solution. I'd like to do it, but stricktly speaking, I don't think it is nessassary. In 2 years of daily operation I've not had a fault, so the chances are good that this trend will continue. The amps are properly AC coupled to the source which kills the likelyhood of DC on input.
The right channel output relay board, includes the delay circuit for the left relay board (posted above) and input board:
The 12VAC, .45A transformer that powers this circuit will be on the board as well, though not shown in the drawing.
Monitoring DC on 6 channels would be a lot more involved than the fuse/crowbar solution. I'd like to do it, but stricktly speaking, I don't think it is nessassary. In 2 years of daily operation I've not had a fault, so the chances are good that this trend will continue. The amps are properly AC coupled to the source which kills the likelyhood of DC on input.
The right channel output relay board, includes the delay circuit for the left relay board (posted above) and input board:
The 12VAC, .45A transformer that powers this circuit will be on the board as well, though not shown in the drawing.
The input muting board:
Very simple, the relays are normally closed, grounding the inputs until the output relays close. This board will be placed at the input RCA jacks.
All that is left is to do some layout, build the boards and test them. One problem is that I only have 4 TRIACs (originally had 6 to make 6 crowbars for the 6 drivers), 2 are currently being used in crowbars on the midranges, I should have 4 DIACs as well, but I can locate them. These are not expensive and will not impede the building and testing, so I`ll get some more with my next Digikey order.
Very simple, the relays are normally closed, grounding the inputs until the output relays close. This board will be placed at the input RCA jacks.
All that is left is to do some layout, build the boards and test them. One problem is that I only have 4 TRIACs (originally had 6 to make 6 crowbars for the 6 drivers), 2 are currently being used in crowbars on the midranges, I should have 4 DIACs as well, but I can locate them. These are not expensive and will not impede the building and testing, so I`ll get some more with my next Digikey order.
Can you arrange that your groups of three relays are connected in series?
30mA @ 36V is nicer than 90mA @ 12V
with four banks of 3relays (mute & output for 6ch) the total relay supply becomes 120mA @ 36V.
30mA @ 36V is nicer than 90mA @ 12V
with four banks of 3relays (mute & output for 6ch) the total relay supply becomes 120mA @ 36V.
The transformer I have for this is a 12V that also supplies the softstart (single) relay. Good idea though.
Need to put this project back on the shelf for a bit. My guitar amp speakers came, so I`ll be devoting much of the weekend to that.
Need to put this project back on the shelf for a bit. My guitar amp speakers came, so I`ll be devoting much of the weekend to that.
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