Hi,
I'm considering building the dual stage regulated PSU and discrete buffer from Nuuk's site on this page:
http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre.html
...which requires a transformer with 18VAC secondaries. Nick recommends with dual regulation stages that the first stage of regulation be down to 21VDC then the second to 15V.
I have in my surplus an EI transformer which outputs 16.8VAC X2
(33.6V CT) - by its size and weight I guess it's around the 100VA mark.
My question is; will this transformer output be too low to work properly (being ~23VDC after regulation, without losses) given possible fluctuations in the mains voltage?
I was going to ask Nick direct, but I realise this is more of a general question and that he's probably sick of emails through his site!
Regards and thanks
Mark
I'm considering building the dual stage regulated PSU and discrete buffer from Nuuk's site on this page:
http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre.html
...which requires a transformer with 18VAC secondaries. Nick recommends with dual regulation stages that the first stage of regulation be down to 21VDC then the second to 15V.
I have in my surplus an EI transformer which outputs 16.8VAC X2
(33.6V CT) - by its size and weight I guess it's around the 100VA mark.
My question is; will this transformer output be too low to work properly (being ~23VDC after regulation, without losses) given possible fluctuations in the mains voltage?
I was going to ask Nick direct, but I realise this is more of a general question and that he's probably sick of emails through his site!
Regards and thanks
Mark
You could use Schottky diodes for less diode voltage drop. You transformer wont be very loaded, it will probably run high anyhow. Was ~23VDC out from actual testing?
If your not getting enough voltage, substitute low dropout regulators for the LM317 and LM337.
Another approach would be to use a voltage doubler on each rail instead of a bridge and heavy passive filtering to burn off some voltage to the first regulators.
If your not getting enough voltage, substitute low dropout regulators for the LM317 and LM337.
Another approach would be to use a voltage doubler on each rail instead of a bridge and heavy passive filtering to burn off some voltage to the first regulators.
Thanks for the response Tweeker.
No, the ~23V wasn't from testing, just a rough calc. I'm not sure what the losses would be through the diodes and smoothing/bypass caps.
I'm assuming, though, that so long as the drop of the first stage exceeds 3V as per the LM317/337 datasheets (which from my reading specifies a minimum 3V between Vin and Vout) there shouldn't be a problem. I guess it depends upon what is the minimum drop over the second (discrete) stage should be.
I may as well build it and see how it goes!
Cheers
Stuey
No, the ~23V wasn't from testing, just a rough calc. I'm not sure what the losses would be through the diodes and smoothing/bypass caps.
I'm assuming, though, that so long as the drop of the first stage exceeds 3V as per the LM317/337 datasheets (which from my reading specifies a minimum 3V between Vin and Vout) there shouldn't be a problem. I guess it depends upon what is the minimum drop over the second (discrete) stage should be.
I may as well build it and see how it goes!
Cheers
Stuey
Hi Stuey,
you don't need the two stage regulation. If you want to use your transformer (which is oversized anyway), a single regulator stage set at 15V will do. But in order to achieve the quality of Nuuk's idea, a pre-regulator is recommended.
A rectifier bridge made of Schottky diodes and two stages of low dropout (i.e. LT108x) regulators will allow you to build according to Nuuk's idea: The Schottky bridge will drop almost a volt less than regular silicon diodes (even under load) and the regulators allow as low as a volt of drop across them, which makes it possible to set the first stage at the recommended 21V.
Those regulators are of higher ripple suppression/lower noise (read: better quality) than the LM3x7 series, but cost a little. 😀
And it might get difficult to find a low drop negative regulator to match the easily available positive types...
Is the trafo really center tapped? You wrote about 2 x 16.8V.
In this case, a first stage of two times a positive regulator (preceded by two rectifier bridges and followed by the ground connection) could be a solution that uses only easily available positive low dropout regulators.
Cheers,
Sebastian.
you don't need the two stage regulation. If you want to use your transformer (which is oversized anyway), a single regulator stage set at 15V will do. But in order to achieve the quality of Nuuk's idea, a pre-regulator is recommended.
A rectifier bridge made of Schottky diodes and two stages of low dropout (i.e. LT108x) regulators will allow you to build according to Nuuk's idea: The Schottky bridge will drop almost a volt less than regular silicon diodes (even under load) and the regulators allow as low as a volt of drop across them, which makes it possible to set the first stage at the recommended 21V.
Those regulators are of higher ripple suppression/lower noise (read: better quality) than the LM3x7 series, but cost a little. 😀
And it might get difficult to find a low drop negative regulator to match the easily available positive types...
Is the trafo really center tapped? You wrote about 2 x 16.8V.
In this case, a first stage of two times a positive regulator (preceded by two rectifier bridges and followed by the ground connection) could be a solution that uses only easily available positive low dropout regulators.
Cheers,
Sebastian.
Hi there Sek,
Yes, I understood that I could get away with one stage of regulation. I was just trying to get maximum performance as the second stage is quite simple! 😉
I wonder, though, whether I could regulate the first stage to a lower voltage, then to 15V in the second stage? In other words, whether the second stage can have a Vin - Vout of less than 2V.
The transformer is from an old amplifier and has three leads on the outputs measuring 16.8 - 0 - 16.8.
Thanks for the input.
Stuey
Yes, I understood that I could get away with one stage of regulation. I was just trying to get maximum performance as the second stage is quite simple! 😉
I wonder, though, whether I could regulate the first stage to a lower voltage, then to 15V in the second stage? In other words, whether the second stage can have a Vin - Vout of less than 2V.
The transformer is from an old amplifier and has three leads on the outputs measuring 16.8 - 0 - 16.8.
Thanks for the input.
Stuey
Hi,
have you measured the output voltage yet?
What is the transformer regulation?
What will be the output voltage when mains input is running low?
What is your tolerance on supply voltage from your supplier?
Mine can vary from 216Vac to 254Vac. That range is rarely if ever used, but minimum Vdrop of a regulator almost disappears to insignificance when design input voltage varies this much.
Then you need to consider Pd of the first regulator stage when running at high input voltage. Unless it is a tracking first stage reg, then Pd of the second stage becomes the issue.
Finally, all these voltages must be measured to the bottom of the ripple.
Reduce the ripple and you ease the regulated voltage problem.
Double or quadruple the planned smoothing to get back a lot of volts on the higher loads.
have you measured the output voltage yet?
What is the transformer regulation?
What will be the output voltage when mains input is running low?
What is your tolerance on supply voltage from your supplier?
Mine can vary from 216Vac to 254Vac. That range is rarely if ever used, but minimum Vdrop of a regulator almost disappears to insignificance when design input voltage varies this much.
Then you need to consider Pd of the first regulator stage when running at high input voltage. Unless it is a tracking first stage reg, then Pd of the second stage becomes the issue.
Finally, all these voltages must be measured to the bottom of the ripple.
Reduce the ripple and you ease the regulated voltage problem.
Double or quadruple the planned smoothing to get back a lot of volts on the higher loads.
Hi Andrew,
If you mean a one time measurement of transformer output irrespective of supply voltage, that's as above ~16.8VACx2. The transformer regulation is unknown as it has been removed from a 20 year old surplus amp and I can't locate any data about it - I'd already tried, to find out its VA rating.
Our mains voltage regulation unfortunately lowered (in quality) to IEC standards 6 years ago so is the same as yours; 216 to 253 volts. That is, it used to be more closely regulated...
I guess all of the issues you mention would affect the standard design using the recommended transformer. I'm not looking to enhance the design.
So if I live dangerously and assume that the design of the PSU is pretty close to OK except for my slightly lower voltage transformer that leaves the issue you state - that Pd of the second stage becomes the issue...but that is the essence of my question.
Thanks and regards
Stuey
If you mean a one time measurement of transformer output irrespective of supply voltage, that's as above ~16.8VACx2. The transformer regulation is unknown as it has been removed from a 20 year old surplus amp and I can't locate any data about it - I'd already tried, to find out its VA rating.
Our mains voltage regulation unfortunately lowered (in quality) to IEC standards 6 years ago so is the same as yours; 216 to 253 volts. That is, it used to be more closely regulated...
I guess all of the issues you mention would affect the standard design using the recommended transformer. I'm not looking to enhance the design.
So if I live dangerously and assume that the design of the PSU is pretty close to OK except for my slightly lower voltage transformer that leaves the issue you state - that Pd of the second stage becomes the issue...but that is the essence of my question.
Thanks and regards
Stuey
Hi Stuey,
is the first regulator a tracking reg?
or is it arranged to give a fixed output voltage?
Don't just assume that the original is correct for a normal range of mains input voltages.
You are now confirming that the 16.8V is the open circuit voltage.
That means that you probably have a transformer between 13Vac and 15Vac not 16.8Vac.
Your problem is worse than you thought.
For a single regulator PSU, it is normal to pick a transformer that has a Vac >=regulated Vout.
If you want 15Vdc then use a 15Vac transformer.
If you want to add a second pre-regulator I would suggest adding at least 3Vac or preferably 5Vac to give adequate headroom. So we are talking 18Vac to 20Vac to give 15Vdc after twin cascaded regulators.
is the first regulator a tracking reg?
or is it arranged to give a fixed output voltage?
Don't just assume that the original is correct for a normal range of mains input voltages.
You are now confirming that the 16.8V is the open circuit voltage.
That means that you probably have a transformer between 13Vac and 15Vac not 16.8Vac.
Your problem is worse than you thought.
For a single regulator PSU, it is normal to pick a transformer that has a Vac >=regulated Vout.
If you want 15Vdc then use a 15Vac transformer.
If you want to add a second pre-regulator I would suggest adding at least 3Vac or preferably 5Vac to give adequate headroom. So we are talking 18Vac to 20Vac to give 15Vdc after twin cascaded regulators.
Hi Andrew,
No the first stage isn't a tracking regulator.
As for your comments about the transformer - of course! I should have known that myself. I'm so used to buying new transformers with their appropriate mfr's rating I never even thought.
The page I linked to in my first post suggests an 18V transformer for a single stage (down to 15V) and to set the first stage to regulate down to 21V if you are employing two stage regulation. It doesn't mention using a different transformer for the higher voltage first stage, but obviously from your comments this would be needed - 21V or above.
I'll just buy a new toroid...would 25V be OK? The off-the-shelf ones here are usually 18V then 25V.
Thanks for the input Andrew, again.
Stuey
No the first stage isn't a tracking regulator.
As for your comments about the transformer - of course! I should have known that myself. I'm so used to buying new transformers with their appropriate mfr's rating I never even thought.
The page I linked to in my first post suggests an 18V transformer for a single stage (down to 15V) and to set the first stage to regulate down to 21V if you are employing two stage regulation. It doesn't mention using a different transformer for the higher voltage first stage, but obviously from your comments this would be needed - 21V or above.
I'll just buy a new toroid...would 25V be OK? The off-the-shelf ones here are usually 18V then 25V.
Thanks for the input Andrew, again.
Stuey
Hi,
you now need to calculate your requirements.
Start at the output and work back towards the mains voltage.
If you play safe by using a higher voltage to ensure you do not suffer drop out in the first stage regulator, then you must balance against this extra safety margin the higher dissipation in the regulator, particularly when mains voltage is high and output current is also high.
you now need to calculate your requirements.
Start at the output and work back towards the mains voltage.
If you play safe by using a higher voltage to ensure you do not suffer drop out in the first stage regulator, then you must balance against this extra safety margin the higher dissipation in the regulator, particularly when mains voltage is high and output current is also high.
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