The first project I am working on is a preamp.
I was about to hook my transformer to the power supply but have higher than expected voltage. The transformer was designed for 115V mains, where I had 123.x around the house. I've tested 125 in the past.
Instead of the 230V I was expecting from the transformer, I think it was 246.3 - so by the time it is converted to DC...
246.3 * 1.414 gives me 348 which I need to knock down to 280.
68 volts to drop sounds much further than 42 and I was already worried about dropping that much with a resistor. (does it increase ripple? I'm a noob, but just downloaded the first chapter of the Morgan Jones book from my library - probably available to me thanks to the engineering school
)
Is there a simple way to condition the power coming in... the equivalent of a resistor on the AC without starting one of those discussions for millionaires or ... I'll not say any words that will hurt anyone's feelings.
Don't know how to search for transformers designed to work with mains voltages between 120 and 125.
I was about to hook my transformer to the power supply but have higher than expected voltage. The transformer was designed for 115V mains, where I had 123.x around the house. I've tested 125 in the past.
Instead of the 230V I was expecting from the transformer, I think it was 246.3 - so by the time it is converted to DC...
246.3 * 1.414 gives me 348 which I need to knock down to 280.
68 volts to drop sounds much further than 42 and I was already worried about dropping that much with a resistor. (does it increase ripple? I'm a noob, but just downloaded the first chapter of the Morgan Jones book from my library - probably available to me thanks to the engineering school
)Is there a simple way to condition the power coming in... the equivalent of a resistor on the AC without starting one of those discussions for millionaires or ... I'll not say any words that will hurt anyone's feelings.
Don't know how to search for transformers designed to work with mains voltages between 120 and 125.
There is also another trick called bucking, where you use the secondary low voltage output of a small transformer to reduce the output of your transformer.
The main advantage of this trick is that you don't have to dissipate the excess voltage with resistors.
You can find the trick here :
Winding configurations : Transformers - Electronics Textbook
Iso the primary winding, as shown in the link, you can do the same with the secondary of your transformer.
The main advantage of this trick is that you don't have to dissipate the excess voltage with resistors.
You can find the trick here :
Winding configurations : Transformers - Electronics Textbook
Iso the primary winding, as shown in the link, you can do the same with the secondary of your transformer.
If it's a preamp, extra voltage is a bonus, not a disadvantage.
Rather than knocking AC down, knock DC with an extra resistor.
Add an extra filter cap and you've just gained an extra ripple killing RC filter.
What's not to like about that?
If anything, check you don't have way too much filament voltage.
Which again is an advantage, because it can be brectified and filtered to supply DC to filaments.
Rather than knocking AC down, knock DC with an extra resistor.
Add an extra filter cap and you've just gained an extra ripple killing RC filter.
What's not to like about that?
If anything, check you don't have way too much filament voltage.
Which again is an advantage, because it can be brectified and filtered to supply DC to filaments.
Very helpful perspectives, and like the link but haven't finished reading it.
My notes indicate a 1.6K ohm resistor to drop the voltage on the PS for the original calculated DC voltage drop, but it appears I used a 2K for some reason but don't have notes as why.
Another option which has been difficult to calculate is using a hash choke in place of that resistor, but it might be a hash choke in series with a resistor or filter cap. From what I can tell hash chokes are used for small drops but again, new to all of this.
Ripple killing sounds good, from my limited understanding it is the biggest concern I have at this point in the build.
Suggestions or some reading on using a filter cap sought.
Thanks again.
My notes indicate a 1.6K ohm resistor to drop the voltage on the PS for the original calculated DC voltage drop, but it appears I used a 2K for some reason but don't have notes as why.
Another option which has been difficult to calculate is using a hash choke in place of that resistor, but it might be a hash choke in series with a resistor or filter cap. From what I can tell hash chokes are used for small drops but again, new to all of this.
Ripple killing sounds good, from my limited understanding it is the biggest concern I have at this point in the build.
Suggestions or some reading on using a filter cap sought.
Thanks again.
Thanks Andrew,
I can't define, but here is a link proving I didn't make up the term:
Hammond Mfg. - R.F. & Hash Chokes - (1531 - 1537 Series)
or quoting a Broski manual "...The resistor can be replaced by a choke, either a small hash choke on the PCB, or a large off-board choke."
So far I haven't been able to drop the voltage with a resistor alone... but I didn't have the tube circuit hooked up to the power supply so I was measuring the power supply without a load. Yes, a mistake.
While not exactly on the bright side, my house voltage fluctuated to just under 121 yesterday. I had a clothes dryer running, but I didn't have a drop in voltage when I had the air conditioning running last week. Even with the lower mains voltage, I was still measuring 348VDC (I didn't measure them at the same time, but it would have only been a few hours difference).
Welcome to the joys of designing for, say, +10%, -30% mains variation. Consider a Maida regulator.
The voltages in most pre-amp circuits are not that critical, and until you hook up the pre-amp to the power supply you will not know whether or not you have a problem unless you know all of the transformer losses that come into play under load and have taken those into account.
Start with what you've got.
Hash chokes are not really that useful in high voltage applications unless using mercury rectifiers as an example where they will remove most of the HF hash from the rectifier. What you really want is a standard choke of 5H - 10H rated for sufficient current.
Start with what you've got.
Hash chokes are not really that useful in high voltage applications unless using mercury rectifiers as an example where they will remove most of the HF hash from the rectifier. What you really want is a standard choke of 5H - 10H rated for sufficient current.
Really your 246 x 1.41 will not give you 348v There are always losses, voltage drop across the rectifier ( if you use a tube rectifier this can be a whole lot ) and voltage drop in the transformer as soon as any current is drawn from it. Depending on the curent rating of your transformer, your load current and choice of rectifier you could end up with much less than 348v once it's under load. I would build it as is and measure the voltage under load you may be trying to solve a problem that will not be there.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.