Hi all,
I have already done this before, but would like to know if there are any drawbacks of reducing the secondary voltages of a toroidal transformer by taking off a few turns off the secondary windings? I need to drop the secondary voltages by 2-3 volts.
I have already done this before, but would like to know if there are any drawbacks of reducing the secondary voltages of a toroidal transformer by taking off a few turns off the secondary windings? I need to drop the secondary voltages by 2-3 volts.
Shouldn't be an issue, but it's best not to expect the output voltages from a power transformer to be particularly accurate.
No electrical problems at all; you might find "physical" problems as in having one secondary end you want to unwind deeply buried under many layers of varnished copper wire.
Now 2-3 Volts is a very low value, you sure you can´t use it as is?
Now 2-3 Volts is a very low value, you sure you can´t use it as is?
H713, JMFahey,
Thank you for your responses.
The last toroidal transformer secondary I modified, I was lucky - the secondary windings were just under the insulation wrapping and I got access quite easily.
Typically the voltage of a secondary rated at 35V AC comes to 36.5 - 37V AC without any load. My AC mains is also on the high side nowadays - 240-245 V AC instead of 230V AC. So I am getting 38-39V AC without load on the transformer secondary.
After rectification, this will come to around 53V DC.
Would like to keep the voltages under 50V DC as my caps are rated for 50V DC, and there are thermal considerations also.
The other option is to use a transformer with 30V AC secondaries. That way I will get a DC voltage of around 43-44V DC after rectification even with higher mains voltages.
Thank you for your responses.
The last toroidal transformer secondary I modified, I was lucky - the secondary windings were just under the insulation wrapping and I got access quite easily.
Typically the voltage of a secondary rated at 35V AC comes to 36.5 - 37V AC without any load. My AC mains is also on the high side nowadays - 240-245 V AC instead of 230V AC. So I am getting 38-39V AC without load on the transformer secondary.
After rectification, this will come to around 53V DC.
Would like to keep the voltages under 50V DC as my caps are rated for 50V DC, and there are thermal considerations also.
The other option is to use a transformer with 30V AC secondaries. That way I will get a DC voltage of around 43-44V DC after rectification even with higher mains voltages.
I would NOT modify the Transformers. Leave them as they are..
USE a Variable DIAC OR TRIAC Circuit to control the input voltage.
These are cheat and stable. Check this out here..
Use them and these will be able to handle 2.8KW Easy to use and adjustable..
Hope this could help you. The only negative thing is that there is a switch on Noise, but only if these are used in Audio Circuits, what I expect that you want to use it for.
BTW a 50Cap will take easy 55volts. the 50 Volts these are Rated is not the maximum.. I have some 68'000UF Caps for 40Volts and running them at 44 without any problem.. If the caps can't manage the voltage, the will show it real fast.. You will hear the Transformers telling you by vibration, the next thing is the Bridge will blow. but the cap wont explode..
But todays Caps are made to HOLD the rated Voltage + will be able to hold Voltages with 10% increase.. because of that check your caps for Absolute MAX VOLTAGE, how much these can take.
The only thing you need to look at that these are not running hot.
Enjoy Sunday
Regards
Chris
USE a Variable DIAC OR TRIAC Circuit to control the input voltage.
These are cheat and stable. Check this out here..
Use them and these will be able to handle 2.8KW Easy to use and adjustable..
Hope this could help you. The only negative thing is that there is a switch on Noise, but only if these are used in Audio Circuits, what I expect that you want to use it for.
BTW a 50Cap will take easy 55volts. the 50 Volts these are Rated is not the maximum.. I have some 68'000UF Caps for 40Volts and running them at 44 without any problem.. If the caps can't manage the voltage, the will show it real fast.. You will hear the Transformers telling you by vibration, the next thing is the Bridge will blow. but the cap wont explode..
But todays Caps are made to HOLD the rated Voltage + will be able to hold Voltages with 10% increase.. because of that check your caps for Absolute MAX VOLTAGE, how much these can take.
The only thing you need to look at that these are not running hot.
Enjoy Sunday
Regards
Chris
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Changing the voltage by 2-3V is not something I would do.
Transformers are generally rated for voltage on full load, and a voltage drop of 2-3V is easily possible (probably more) on load.
Have you measured the voltage on load? I assume you are discussing the power supply for a Class AB amplifier which actually won't draw much current when idling? That of course will give a higher voltage, but have you checked or are you measuring the voltage with no load?
I would probably swap the caps out for 63V units instead of 50V. I never like to run components (like capacitors and resistors) beyond spec. (and when I have run transistors beyond the spec sheet BVceo it is only after testing as mentioned in another thread!)
(You may be able to run a cap a little over its rated voltage but it may shorten its life. Electrolytics are known to have a limited life anyway).
Transformers are generally rated for voltage on full load, and a voltage drop of 2-3V is easily possible (probably more) on load.
Have you measured the voltage on load? I assume you are discussing the power supply for a Class AB amplifier which actually won't draw much current when idling? That of course will give a higher voltage, but have you checked or are you measuring the voltage with no load?
I would probably swap the caps out for 63V units instead of 50V. I never like to run components (like capacitors and resistors) beyond spec. (and when I have run transistors beyond the spec sheet BVceo it is only after testing as mentioned in another thread!)
(You may be able to run a cap a little over its rated voltage but it may shorten its life. Electrolytics are known to have a limited life anyway).
DO NOT DO THIS, especially with a toroid transformer. Phase control circuits contribute a lot of high frequency hash, and if not *very accurately controlled*, as in with a microprocessor, you can get a DC component on the mains. This was *the biggest issue* with the Carver Magnetic Field amplifiers. Those transformers really HATED being used that way.
Either use it as is, with 63 volt caps, or change the turns ratio. Most toroids are easy to un tape and pull a few turns off. If you don’t trust yourself there, there are other safe options. You can add a bucking winding in reverse phase to the secondary, or add turns in phase to the primary. Either will bring it down. Use #14 or #12 THHN solid building wire, which can be wrapped over the existing winding (and tape). And will have sufficient insulation and mechanical robustness that you don’t have to worry about it shorting. It’s not as space-efficient as magnet wire, but you probably only need a dozen turns. If you DO use magnet wire, you MUST tape it properly, and do NOT use the primary modification option (only the secondary side). Way too much trouble, and you can’t buy magnet wire at Home Depot.
Unwinding is probably easy as long as it’s on the outer winding. For me, I wouldn’t worry about 3 or 4v over 50v on the caps as long as they are a quality brand and test them first for 10min standing far away. Obviously, if you have 63v caps then use them but I can see that being in Dhaka, such things can take months to order sometimes.
I wonder if 6x 10A10 diodes (or similar 10A basic Si diodes) in series on each rail after the main bridge can be used to easily drop 3.6v before the main cap bank?
US $1.10 | 20pcs/lot 6A10 10A10 20A10 Rectifier Diode R-6 DIP 1000V
20pcs/lot 6A10 10A10 20A10 Rectifier Diode R 6 DIP 1000V|diode 10a|diode 10a 1000vdiode 10a10 - AliExpress
It’s a cheap and quick solution to test.
I wonder if 6x 10A10 diodes (or similar 10A basic Si diodes) in series on each rail after the main bridge can be used to easily drop 3.6v before the main cap bank?
US $1.10 | 20pcs/lot 6A10 10A10 20A10 Rectifier Diode R-6 DIP 1000V
20pcs/lot 6A10 10A10 20A10 Rectifier Diode R 6 DIP 1000V|diode 10a|diode 10a 1000vdiode 10a10 - AliExpress
It’s a cheap and quick solution to test.
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wg_ski, X,
I have already untaped a couple of transformers in the past and reduced the voltages. I had help from another kind member of diyAudio, and he had explained the process to me in detail via email before I tried doing it. Of course the windings were taken off the secondaries only, I did not touch the primary. I was told that the procedure would only work if the transformer secondaries were wound on top of the primary, and if they were accessible after taking the tape off.
If there are no electrical disadvantages, I would be open to modifying transformers with 35V AC secondaries and bringing it down to 32V AC. Of course 30V AC secondaries will also work in my application.
The amplifier I have in mind is a Class AB amplifier. If it was a Class A project, I would have expected some voltage drop due to the load. Also I could drop a few more volts by using a cap multiplier after the power supply or use a SLB type power supply (cap mx built in).
Since it is Class AB, I expect the voltage drop from the load to be low vs Class A.
I have already untaped a couple of transformers in the past and reduced the voltages. I had help from another kind member of diyAudio, and he had explained the process to me in detail via email before I tried doing it. Of course the windings were taken off the secondaries only, I did not touch the primary. I was told that the procedure would only work if the transformer secondaries were wound on top of the primary, and if they were accessible after taking the tape off.
If there are no electrical disadvantages, I would be open to modifying transformers with 35V AC secondaries and bringing it down to 32V AC. Of course 30V AC secondaries will also work in my application.
The amplifier I have in mind is a Class AB amplifier. If it was a Class A project, I would have expected some voltage drop due to the load. Also I could drop a few more volts by using a cap multiplier after the power supply or use a SLB type power supply (cap mx built in).
Since it is Class AB, I expect the voltage drop from the load to be low vs Class A.
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"Quote By wg_sky
This was *the biggest issue* with the Carver Magnetic Field amplifiers. Those transformers really HATED being used that way. End Quote"
I use that on Secondary 25Ampere Toroidal Transformers, never any problems. Not on high frequency thou.
Don't forget as much as I remember, Carver uses Hi frequency Switching's on their Amplifiers as well.
Also, Carver was in the 70'ies.. Today many Components are much better and I doubt that a Microprocessor would do any good..
As also I recall they used Triac's inside the amp already and this was the disaster..
For any DIY Build which runs with 50/60 hertz power then there is nothing to fear.
But I agree with wg_sky to some points about using Triacs..
This was *the biggest issue* with the Carver Magnetic Field amplifiers. Those transformers really HATED being used that way. End Quote"
I use that on Secondary 25Ampere Toroidal Transformers, never any problems. Not on high frequency thou.
Don't forget as much as I remember, Carver uses Hi frequency Switching's on their Amplifiers as well.
Also, Carver was in the 70'ies.. Today many Components are much better and I doubt that a Microprocessor would do any good..
As also I recall they used Triac's inside the amp already and this was the disaster..
For any DIY Build which runs with 50/60 hertz power then there is nothing to fear.
But I agree with wg_sky to some points about using Triacs..
If the primary is on top, you can just add some turns on the primary. Makes your transformer quieter (mechanically) too.
Jan
Jan
The Carver circuit used a triac in the *primary* in a vain attempt to regulate the output using a transformer that was *far too small* for the job. They had rubber shock mounts for the trafos to keep the amp from vibrating off the shelf in use. And when the phase control circuit effed up there was fire and smoke.
The efficiency improvement they offered was totally independent of that whacky power supply, and would have been better off with a regular multi winding transformer.
If you just want to drop a couple volts you don’t even really need phase control, primary or secondary. A stack of diodes works.
The efficiency improvement they offered was totally independent of that whacky power supply, and would have been better off with a regular multi winding transformer.
If you just want to drop a couple volts you don’t even really need phase control, primary or secondary. A stack of diodes works.
The extra turns can go on the outside, even if the rest of the primary is at the core. Just put it in the proper phase and all is good. The only thing you have to worry about is the insulation. THHN is good for 600 volts, tough as nails, and can handle fairly high temps. As close to fool proof as you’re going to get.
The lower flux density is a plus.
And that will solve the problem of having to adjust two secondary windings when one might be buried under the other.
You don't even need to check if the primary is on the outside - if turns are added over the current insulating layers.
What you will need to do though is to be very careful with the phase (connecting it in series to add to the primary rather than subtract) and turns per volt.
The concern I have is that the unit as it stands is properly insulated and (presumably) passed insulation tests.
Adding turns to the outside needs to be electrically and mechanically sound. Using insulated wire is OK for a single insulation, but often current rules require double insulation so an additional insulating layer over whatever wire is used is I would say essential. Protecting that mechanically is the next problem, perhaps using two of the neoprene disks instead of one (top and bottom) might be useful to provided added protection as the outer wire will suffer physical compression when bolted down.
In summary 63V capacitors look like a better option. Newer caps generally fit in a smaller space than older ones (or the higher voltage rating in the same size).
You don't even need to check if the primary is on the outside - if turns are added over the current insulating layers.
What you will need to do though is to be very careful with the phase (connecting it in series to add to the primary rather than subtract) and turns per volt.
The concern I have is that the unit as it stands is properly insulated and (presumably) passed insulation tests.
Adding turns to the outside needs to be electrically and mechanically sound. Using insulated wire is OK for a single insulation, but often current rules require double insulation so an additional insulating layer over whatever wire is used is I would say essential. Protecting that mechanically is the next problem, perhaps using two of the neoprene disks instead of one (top and bottom) might be useful to provided added protection as the outer wire will suffer physical compression when bolted down.
In summary 63V capacitors look like a better option. Newer caps generally fit in a smaller space than older ones (or the higher voltage rating in the same size).
1)no need for primary to be outside, just wound on the same core. Which it is.
Coupling is magnetic, not capacitive 😉 🙂
2) Triacs and such work fine ... on resistive loads.
On highly inductive transformers ... not that much, to say it politely.
And on capacitive input supplies, caps will charge to peak voltage anyway, even if duty cycle is reduced.
EDIT: as of the suggested Swiss/German "power controller" it is, as I suspected a plain vanilla "Triac speed/power" control, suitable only for heaters or incandescent lamps and "universal" wound motors, definitely NOT suitable to adjust Transformer voltage.
Coupling is magnetic, not capacitive 😉 🙂
2) Triacs and such work fine ... on resistive loads.
On highly inductive transformers ... not that much, to say it politely.
And on capacitive input supplies, caps will charge to peak voltage anyway, even if duty cycle is reduced.
EDIT: as of the suggested Swiss/German "power controller" it is, as I suspected a plain vanilla "Triac speed/power" control, suitable only for heaters or incandescent lamps and "universal" wound motors, definitely NOT suitable to adjust Transformer voltage.
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I'm a unlucky man.. even my toroidal transformers, have the primary outside, and I do not want to resize the voltage *35 - 0 - 35 * to nowhere.
The transformer is used in a Class A which I build in 2017,stacked Transformers meaning one top of each other...
One of these Transformers went DEATH, because of overheating., I have an End Heat Control on each heat sink which would cut off 230Volts when exceeding 100degrees Celsius..
But me, a stubborn old man, switched that safety off when I opened the case for maintenance, mainly for cleaning out the dust because that Amp uses 6 LOW Turning in sequence powered Fans,, and did not turn it back on when I finished with maintenance.
The other day some time ago, I forgot to switch off that amp before I went out to work in the morning 8 Am. when I came back 8 hours later, I got that Transformer Smell all though our home, and one of the Toroidal is death since that day..
But the transformers I build into that Case, are HEAT Fused in the Top layer winding, so I will need to replace that and then the Toroid should work proper again..
The same Scenario I had on the left Channel once before and then Temp meter read 149 Degrees on the Heat Sink.. but AMP, after repairing the Toroid, is still working same as ever.. hope fully this will be for the right Channel as well.. All this has nothing to do with that CLASS A SINGLE END I build last year, that one runs real smooth..
That's why I'm building that Variable Current Source Class A Amp which is on my Table in my working room.. Still waiting for some Parts to arrive, as soon these come then problems of overheating Class A Amps are PAST..
No FANS no more, no excessive heat when on idle anymore.. usually it stays @30 / 40 Degrees Celsius after 10 hours of running the amp.. consuming a 30 / 45 Watts per Channel.. PCB's are made already Double sided Through hole, PCB HOME MADE, bought that machine to make that through hole even better as with my home made machine..
BTW this is the first AMP which got broken from all the ones I have build.. and these are running real hot, draw a lot of current and have fantastic sound.
This latest Prototype of that Variable Current Source Class A can drive 2 ohms easily. Its on the test bench for almost 3 months.. all children ills are gone,
Hopefully I can start to assemble that one in the coming days..
@wg_sky yes using diodes would be another way to reduce Voltage, using high power and high speed diodes.
Guys enjoy weekend
Regards Chris
It CAN be done, not much recommended, and be certain to replace that thermal fuse the proper way, or next time more than a transformer will smell burnt when you return to home.
And I would not remove fans either, not only the transformer runs HOT in a Class A amp..
Some old HP power supplies used SCRs at the transformer output for a preregulator, so that the linear regulators didn’t have to dissipate as much when the output as set really low. They delayed the conduction angle MORE than 90 degrees, which reduces the peak voltage. Those old EI trafos could handle a little DC in the core, so it it wasn’t perfectly symmetric is wasn’t the end of the world. They could still handle the full 120 volts at the input without saturating (and weighed a ton).
*OR* wind 10-30 turns outside and add them to the Secondary, Series Opposition. 39VAC -minus- 4VAC is the 35VAC you seek.
It is very un-optimum but for a one-off it may be the best path in some cases. You don't care which existing winding is in/out-side. The added turns can readily be increased or decreased, even tapped, to taste. While you should use an ample gauge, being on the outside the heat may not be a big problem. (Not that heat should ever be a problem in speech/music AB.)
I used a triac on an attic fan motor. It ran slower. A half-hour later WONK and the attic and house was full of stinky smoke. Motor did not like the wacky waves.