Capacitor voltage rating question...
Hi,
How to calculate that value ? Please see the attached schematic. The transformers T1 and T2 are having only 24V (with 300VA) secondary out. The bridge rectifier is of 35A - 700V.
Best regards,
Bins.
Hi,
How to calculate that value ? Please see the attached schematic. The transformers T1 and T2 are having only 24V (with 300VA) secondary out. The bridge rectifier is of 35A - 700V.
Best regards,
Bins.
Attachments
I had a quick look through the application list, and you'll be using two dual 24v secondary transformers wired in series, giving you the equivilent of 48-0-48v.
What I use as a general rule to work out the DC from the AC is simply multiply the AC figure by 1.404, so:
48 x 1.404 = 67.4 V DC
Hence 80v capacitors will be fine.
Unless you already have the transformers, it might be quicker, easier, and cheaper just to use a single 45-0-45v or 50-0-50v transformer.
What I use as a general rule to work out the DC from the AC is simply multiply the AC figure by 1.404, so:
48 x 1.404 = 67.4 V DC
Hence 80v capacitors will be fine.
Unless you already have the transformers, it might be quicker, easier, and cheaper just to use a single 45-0-45v or 50-0-50v transformer.
Transformer rating...
Hi,
I have not yet started collecting the parts. Thanks for you suggestion. What about the VA rating ? Will 250VA be enough ?
Best regards,
Bins.
Hi,
I have not yet started collecting the parts. Thanks for you suggestion. What about the VA rating ? Will 250VA be enough ?
Best regards,
Bins.
completely misleading.
The worst case voltage could be very different from what you have predicted.
Take a 230:48Vac 5% regulation transformer.
The open circuit output voltage on 230Vac mains is 50.4Vac.
Now apply the maximum tolerance mains supply voltage. The output in the UK is 254/230 * 50.4 = 55.66Vac.
The peak voltage is going to be 78.70Vpk and after passing through a diode at very low current give a DC voltage at the smoothing caps of ~78.2Vdc. This is OK with 80V rated capacitors.
67.4V predicted by your model is wrong!!!!
250VA can power an amplifier with a maximum output power of 125W to 250W, economically.
Thanks for that, I welcome the opportunity to learn from someone with more experience of these things. I never took into consideration the regulation, and I was only using the nominal primary voltage.
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Hi Bins,
I've attached a schematic for the power supply I would use, if I were to build the amplifier you described at the start of this thread.
You may feel that it's a bit overkill, but I really do think it's worthwhile to run the LME's on their own supply, in order to swing the mosfets from rail to rail.
This supply is just the basics, but you can feel free to pepper it with 0.1uF bypass caps all over the place.
Use your favorite bridge or discrete diodes, and also feel free to use the dual bridge setup that AndrewT described earlier in the thread (one bridge per secondary). You could also try lower forward voltage schottky diodes for the main supply for fun.
Make sure to use good local decoupling on all parts, including each mosfet, and especially the LME49830, where you should follow the datasheet closely.
If you don't want to bother with the dual supply, just build a single one using a 45-0-45 transformer in place of the 40-0-40 and scrap the second supply altogether.
Good luck!
Owen
(EDIT: I just realized that I changed the secondary voltage on the second supply, but didn't change the DC voltage... you'll see between 62-64 VDC there)
I've attached a schematic for the power supply I would use, if I were to build the amplifier you described at the start of this thread.
You may feel that it's a bit overkill, but I really do think it's worthwhile to run the LME's on their own supply, in order to swing the mosfets from rail to rail.
This supply is just the basics, but you can feel free to pepper it with 0.1uF bypass caps all over the place.
Use your favorite bridge or discrete diodes, and also feel free to use the dual bridge setup that AndrewT described earlier in the thread (one bridge per secondary). You could also try lower forward voltage schottky diodes for the main supply for fun.
Make sure to use good local decoupling on all parts, including each mosfet, and especially the LME49830, where you should follow the datasheet closely.
If you don't want to bother with the dual supply, just build a single one using a 45-0-45 transformer in place of the 40-0-40 and scrap the second supply altogether.
Good luck!
Owen
(EDIT: I just realized that I changed the secondary voltage on the second supply, but didn't change the DC voltage... you'll see between 62-64 VDC there)
Attachments
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Transformers...
Hello AndrewT,
1. I need only 150W per channel. The speakers are of 8 Ohms. So, I hope, 250VA will be enough. Please confirm.
2. I am planning to use the transformer with the rating: 250VA - 2 X 50 (MULTICOMP|MCTA250/50|TRANSFORMER, 250VA, 2 X 50V | Farnell India). In that case, will the 80V capacitors be enough ?
Best regards,
Bins.
Hello AndrewT,
1. I need only 150W per channel. The speakers are of 8 Ohms. So, I hope, 250VA will be enough. Please confirm.
2. I am planning to use the transformer with the rating: 250VA - 2 X 50 (MULTICOMP|MCTA250/50|TRANSFORMER, 250VA, 2 X 50V | Farnell India). In that case, will the 80V capacitors be enough ?
Best regards,
Bins.
Possibly not.
You will have to model it based on your local mains voltage variations or buy it and check the worst case output voltage.
Note the data in the link.
It is described as a 250VA 250:50Vac transformer, but further reading shows it to be a 230:50Vac transformer.
This virtually guarantees that periodically the capacitors will be asked to withstand >80Vdc. It depends totally on your local voltage variations as promised by your mains supplier.
You must learn to read and understand what the data is telling you.
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the rectifier voltage must be ~ >3times the AC voltage. i.e >130V. Use >=200V rectifier. A 30Vac transformer might just work with a 100V rectifier, but it gets close to maximum rating particularly if regulation is high and/or supply voltage is high.
The current rating is not quite so important. I would use 25A or 35A rectifiers, but you will find that 15A or 16A rectifiers will work reliably if you check that they never become hot.
You might even get away with 8A rectifiers but that may need a slow charge circuit to reduce the peak current at the start of charge from zero volts.
Hi AndrewT / Owen,
So, I am going to design the power supply using the following elements:
1. 250VA - 2X45V Transformer.
2. 10000uF - 80V Capacitors (8 per channel).
3. 200V - 35A Bridge Rectifier.
Do we need to add the bleeder resistors in the power supply ?
Best regards,
Bins.
So, I am going to design the power supply using the following elements:
1. 250VA - 2X45V Transformer.
2. 10000uF - 80V Capacitors (8 per channel).
3. 200V - 35A Bridge Rectifier.
Do we need to add the bleeder resistors in the power supply ?
Best regards,
Bins.
Hi AndrewT,
As per your suggestion, I will change that to 20000uF and will try. Do you know how to integrate a thermistor into the primary for implementing a soft start mechanism ?
@Owen: Can you please let us know why you suggested 80000uF ?
Best regards,
Bins.
As per your suggestion, I will change that to 20000uF and will try. Do you know how to integrate a thermistor into the primary for implementing a soft start mechanism ?
@Owen: Can you please let us know why you suggested 80000uF ?
Best regards,
Bins.
Hi Bins,
I was merely suggesting that for the same price as the 10,000uF you had listed, you could buy 80,000uF of the other caps. As AndrewT said, 20,000uF is probably fine, but it certainly wouldn't hurt to have more. I think there's a price break on those caps at 10 pcs, so why not buy 10 of them and use two per rail in each monoblock for a total of 8, and save the last two for your next project.
The 250VA 45-0-45 transformer listed below would be perfect for what you're doing:
MULTICOMP|MCTA250/45|TRANSFORMER, 250VA, 2 X 45V | Farnell India
Use that, along with the 10,000uf 80V caps, and a bridge:
INTERNATIONAL RECTIFIER|GBPC3502A|Bridge Rectifier | Farnell India
Should do the trick.
Owen
I was merely suggesting that for the same price as the 10,000uF you had listed, you could buy 80,000uF of the other caps. As AndrewT said, 20,000uF is probably fine, but it certainly wouldn't hurt to have more. I think there's a price break on those caps at 10 pcs, so why not buy 10 of them and use two per rail in each monoblock for a total of 8, and save the last two for your next project.
The 250VA 45-0-45 transformer listed below would be perfect for what you're doing:
MULTICOMP|MCTA250/45|TRANSFORMER, 250VA, 2 X 45V | Farnell India
Use that, along with the 10,000uf 80V caps, and a bridge:
INTERNATIONAL RECTIFIER|GBPC3502A|Bridge Rectifier | Farnell India
Should do the trick.
Owen
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