Please pardon the obviousness but this dodges moi. I was wondering if one of you guys could help me out here:
I have a 600 VA Toroid with dual 55v secondaries. I have it hooked up to a pair of 10,000mfd caps via a 25amp bridge and get about 77vdc.
When I test load the capacitor terminals with a 500watt Halogen Lamp (about 28 ohm load) The DC sags from 77 to about 62 volts (which me think is a huge drop but still fathomable) but the AC side (measured before the bridge) drops only from 55vac to 52vac.
Question why this huge drop in the DC voltage and negligent drop in the AC side?
Second question would be, assuming the Toroid is holding on, what do I need to do to get at least 72 vdc under load using the same AC voltage i.e. 55vac?
Sorry about beinga little vague. Any info is appreciated.
thanks!
I have a 600 VA Toroid with dual 55v secondaries. I have it hooked up to a pair of 10,000mfd caps via a 25amp bridge and get about 77vdc.
When I test load the capacitor terminals with a 500watt Halogen Lamp (about 28 ohm load) The DC sags from 77 to about 62 volts (which me think is a huge drop but still fathomable) but the AC side (measured before the bridge) drops only from 55vac to 52vac.
Question why this huge drop in the DC voltage and negligent drop in the AC side?
Second question would be, assuming the Toroid is holding on, what do I need to do to get at least 72 vdc under load using the same AC voltage i.e. 55vac?
Sorry about beinga little vague. Any info is appreciated.
thanks!
The available voltage goes to 0V 120 times per second, it's AC.
I generally figure the DC voltage under load to be only about 10% higher than the no load AC voltage. In your case 55VAC + 10% = 60.5VDC, which is about what you have.
"try adding some more capacitance to the power supply so that they are not drained as much between the charging pulses they recieve from the transformer."
A 1F cap (1,000,000µF) cap puts out 1A for 1S. The point of diminishing returns is quickly reached. A 25A bridge rectifier only has about 300A surge capacity, filter caps above 30,000µF will generally require soft-start circuitry to avoid blowing the rectifiers. 'Fast' rectifiers have less surge rating than normal types. Consult the data sheets if you intend to use these types. People using stupidly large amounts of filtering may require a two-stage soft-start, it is possible to blow the rectifiers running at low powered normal use if the filter caps are too large.
The A40 article at the Pass Labs site has a power supply tutorial in it. Summary: use no less than 3,000µF per channel for an 8 ohm load, no more than 30,000µF. For 50hz use 4,000µF minimum and 40,000µF maximum, you will probably have rectifier problems near the maximum.
A Carver M1.5T puts out 600W per channel from only a single pair of 3,400µF filter caps.
A bigger amplifier does not need any more filter capacitance. If you double the power supply voltage for a larger amplifier the energy stored in the same sized filter cap quadruples.
Sonically, a regulated supply for the low current front end of your amplifier makes more sense than to try and psuedo-regulate the high current output stages with large filter caps.
I generally figure the DC voltage under load to be only about 10% higher than the no load AC voltage. In your case 55VAC + 10% = 60.5VDC, which is about what you have.
"try adding some more capacitance to the power supply so that they are not drained as much between the charging pulses they recieve from the transformer."
A 1F cap (1,000,000µF) cap puts out 1A for 1S. The point of diminishing returns is quickly reached. A 25A bridge rectifier only has about 300A surge capacity, filter caps above 30,000µF will generally require soft-start circuitry to avoid blowing the rectifiers. 'Fast' rectifiers have less surge rating than normal types. Consult the data sheets if you intend to use these types. People using stupidly large amounts of filtering may require a two-stage soft-start, it is possible to blow the rectifiers running at low powered normal use if the filter caps are too large.
The A40 article at the Pass Labs site has a power supply tutorial in it. Summary: use no less than 3,000µF per channel for an 8 ohm load, no more than 30,000µF. For 50hz use 4,000µF minimum and 40,000µF maximum, you will probably have rectifier problems near the maximum.
A Carver M1.5T puts out 600W per channel from only a single pair of 3,400µF filter caps.
A bigger amplifier does not need any more filter capacitance. If you double the power supply voltage for a larger amplifier the energy stored in the same sized filter cap quadruples.
Sonically, a regulated supply for the low current front end of your amplifier makes more sense than to try and psuedo-regulate the high current output stages with large filter caps.
Re: Ok will try that..
Think around 2 volts * Current, when you calculate. You must have a heatsink to the rectifier bridge. 2-3 A is max without heatsink. The only way to reduce power is to use schottky diodes but I don't think it's worth the trouble.K-amps said:Also do you think there would be abnormally high losses in the Bridge rectifier? Would a hi-speed discrete diode help or would I gain just 1 volt or so?
The capacitance numbers suggested here for the A40 do not necessarily apply to all Class A electrical amps as I understand it. The amount of filter capacitance is highly dependent on the amount of current bias used in the amp. The Aleph-X built with 250 milliohm source resistors would need at least 100mF per channel to tame the ripple. Look here for more information on this topic. For corroboration, ask Nelson Pass how much capacitance is included in the AX series of amps?
Obtaining DC by rectifiying a 50Hz sine wave is an inefficient and crappy power conversion method
Current is only drawn from mains during 15..30% of time so the current drawn in this interval is 3..7 times bigger than the DC current you are demanding and so you will be getting 3 to 7 times greater voltage drop and 3 to 7 times grater heating in the transformer and bridge rectifier
Peak voltage drop in mains line will also be 3 to 7 times bigger tan expected [the sine wave will appear clipped]
In your case, the RMS rectified voltage you get on the *output* of the bridge rectifier with constant load is almost the same for 1.000uF or 1.000.000uF
In other words, adding more capacitance past some value won't reduce RMS voltage drop, only ripple will be reduced
If somebody doesn't belive this, just simulate it and cry [I have assumed 1 ohm mains resistance and 100uH leakage inductance]
You can reduce voltage drop by using a tranformer rated at 3 to 7 times the power you will be drawing, but this is obviously not practical
Fast recovery rectifiers are a nonsense solution since they would have almost double voltage drop when conducting [making a rectifier ultrafast allways means increasing its conduction losses]
What you have seen is the crude reality, as I've said, 50Hz rectified supplies are crappy and huge voltage drop is one of their fundamental properties [If there weren't voltage drop then it wouln't be a 50Hz supply]
Fortunately audio signals have high crest factor so maximum power consumption only will happen during short periods of time and this will reduce effective voltage drop to some extent and will also benefit to some extent of extra storage capacitance [actually due to the fact that a 750Wrms class B, AB or D amplifier driven just under clipping will usually draw 250W or less of average power]
If you don't like voltage drop nor ripple, think that SMPS have regulated outputs ...
PD: I don't understand why high quality SMPS units or kits of 0,5 to 2Kw with good shielding and filtering, symmetrical adjustable outputs and ready to use for a reasonable pice aren't available for DIY
Current is only drawn from mains during 15..30% of time so the current drawn in this interval is 3..7 times bigger than the DC current you are demanding and so you will be getting 3 to 7 times greater voltage drop and 3 to 7 times grater heating in the transformer and bridge rectifier
Peak voltage drop in mains line will also be 3 to 7 times bigger tan expected [the sine wave will appear clipped]
In your case, the RMS rectified voltage you get on the *output* of the bridge rectifier with constant load is almost the same for 1.000uF or 1.000.000uF
In other words, adding more capacitance past some value won't reduce RMS voltage drop, only ripple will be reduced
If somebody doesn't belive this, just simulate it and cry [I have assumed 1 ohm mains resistance and 100uH leakage inductance]
You can reduce voltage drop by using a tranformer rated at 3 to 7 times the power you will be drawing, but this is obviously not practical
Fast recovery rectifiers are a nonsense solution since they would have almost double voltage drop when conducting [making a rectifier ultrafast allways means increasing its conduction losses]
What you have seen is the crude reality, as I've said, 50Hz rectified supplies are crappy and huge voltage drop is one of their fundamental properties [If there weren't voltage drop then it wouln't be a 50Hz supply]
Fortunately audio signals have high crest factor so maximum power consumption only will happen during short periods of time and this will reduce effective voltage drop to some extent and will also benefit to some extent of extra storage capacitance [actually due to the fact that a 750Wrms class B, AB or D amplifier driven just under clipping will usually draw 250W or less of average power]
If you don't like voltage drop nor ripple, think that SMPS have regulated outputs ...
PD: I don't understand why high quality SMPS units or kits of 0,5 to 2Kw with good shielding and filtering, symmetrical adjustable outputs and ready to use for a reasonable pice aren't available for DIY
The market is limited. A 2 kW SMPS for industrial use cost 
Switchmode power supplies are generaly not something you want to try and build yourself, as generaly, the whole of the supply ciricut can be considered live at mains voltage. Therefore there is a huge potential for blowing stuff up, and injuring or killing people who might come into contact with it (unlike the one's that people build to use in cars, which don't have the mains connected directly to them and are a little safer).
An isolation transformer is essential to develop and test mains off-line SMPS
Since commercial ones are expensive, I use one 2x60V 750VA and one 2x40V 470VA transformers to get 40V, 60V, 80V, 100V, 120V, 160V or 200V AC isolated from mains and up to 1,2KW in a cheap and very useful way
Also, to experiment with an SMPS design it's not recommended to use the maximum supply voltage until everything works right [when everything works right with the maximum isolated supply voltage then it's mains time]
I've never got shocked in years of experiments
In comparison, you can find 160V peak to peak or more in the supply rails of some medium to high power audio amplifiers, this is as dangerous as an SMPS powered from 120V AC so I see no additional risk here
Taking some stupid precautions, anybody can experiment with SMPS like it does with audio amplifiers
Since commercial ones are expensive, I use one 2x60V 750VA and one 2x40V 470VA transformers to get 40V, 60V, 80V, 100V, 120V, 160V or 200V AC isolated from mains and up to 1,2KW in a cheap and very useful way
Also, to experiment with an SMPS design it's not recommended to use the maximum supply voltage until everything works right [when everything works right with the maximum isolated supply voltage then it's mains time]
I've never got shocked in years of experiments
In comparison, you can find 160V peak to peak or more in the supply rails of some medium to high power audio amplifiers, this is as dangerous as an SMPS powered from 120V AC so I see no additional risk here
Taking some stupid precautions, anybody can experiment with SMPS like it does with audio amplifiers
That is only assuming us/canada mains, here in the uk, and a lot of the other countries around the world, we opperate at 240V mains (340V peak). I for one, feel far more comfotable not having to go near those sorts of voltages unless I have to.
What you are saying about the isolation transformers is correct to a point, it does remove the risk of shock from only one single wire, but the potential is still there from two wires shorting out. Also, why spend money on test transformers when you could just buy one in the first place and use it in your amp? Unless you were going to make large numebrs high power supplies I realy don't see the benifit for most DIY projects. But, then if that is what you are into and you have the knowledge and patience to do it then I would never want to stop you, but just make sure that others who might not fully understand them know of the potential dangers before they start.
What you are saying about the isolation transformers is correct to a point, it does remove the risk of shock from only one single wire, but the potential is still there from two wires shorting out. Also, why spend money on test transformers when you could just buy one in the first place and use it in your amp? Unless you were going to make large numebrs high power supplies I realy don't see the benifit for most DIY projects. But, then if that is what you are into and you have the knowledge and patience to do it then I would never want to stop you, but just make sure that others who might not fully understand them know of the potential dangers before they start.
In my country mains voltage was 220V, recenly raised to 230V to meet european standards
Also I've worked as an electrician from time to time since age 14 so I never developed the usual fear to mains found in most people [I learnt the profession very early, then my interests came to computers, then electronics and later acoustics]
Actually people does lots of things far more dangerous [for them and for other people] than working on live 230V AC wires [like : driving drunk, smoking, using drgus, having children and guns in the same house, ...]
PD: In my later prototypes I use active mains-AC to 450V-DC boost rectification [loading mains in a fully resistive way a.k.a. 'PFC'] to improve efficiency and I'm still alive, no deadly shocks, actually no shocks at all... [the worst thing that happened was an axial diode exploding like a firecracker due to a too slow reverse recovery characteristic]
Also I've worked as an electrician from time to time since age 14 so I never developed the usual fear to mains found in most people [I learnt the profession very early, then my interests came to computers, then electronics and later acoustics]
Actually people does lots of things far more dangerous [for them and for other people] than working on live 230V AC wires [like : driving drunk, smoking, using drgus, having children and guns in the same house, ...]
PD: In my later prototypes I use active mains-AC to 450V-DC boost rectification [loading mains in a fully resistive way a.k.a. 'PFC'] to improve efficiency and I'm still alive, no deadly shocks, actually no shocks at all... [the worst thing that happened was an axial diode exploding like a firecracker due to a too slow reverse recovery characteristic]
"Since I am new, would you send me a link to the A40 PSU article you mention?"
http://www.passdiy.com/pdf/a40.pdf
http://www.passdiy.com/pdf/a40.pdf
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