Hi,
I've been playing with very low power valve/tube amplifiers and I've used boost converters to generate 180V-240V DC. As I cannot get rid of the switching noise, I'm just going to try transformers (encapsulated = cheap).
The idea is to use two identical transformers back to back (240V to 2x6V). The transformers are rated 10VA and the output current is, ideally, around 830mA whereas the input current is around 40 mA.
The question is: after applying 6V to the secondary of the second transformer to get 240V again( in an ideal world), how much current, taking into account the losses, can I expect to have to supply the circuit?
After the transformer, I will use a silicon full wave rectifier made with diodes for both voltages.
For the heaters I will take the 6V, rectify it to obtain 8.4 DC and use a 7806 to apply 6V DC to the heaters of the valves. I will do the same for the HV output and then lower the voltage with resistors to get the voltages I want.
https://www.switchelectronics.co.uk/products/2-x-6v-10va-230v-encapsulated-pcb-transformer
Another option would be to go to toroidals, since they can deliver much higher current for an ok price.
https://www.switchelectronics.co.uk...06-toroidal-transformer-30va-0-6v-vigortronix
Kind regards,
Pedro
I've been playing with very low power valve/tube amplifiers and I've used boost converters to generate 180V-240V DC. As I cannot get rid of the switching noise, I'm just going to try transformers (encapsulated = cheap).
The idea is to use two identical transformers back to back (240V to 2x6V). The transformers are rated 10VA and the output current is, ideally, around 830mA whereas the input current is around 40 mA.
The question is: after applying 6V to the secondary of the second transformer to get 240V again( in an ideal world), how much current, taking into account the losses, can I expect to have to supply the circuit?
After the transformer, I will use a silicon full wave rectifier made with diodes for both voltages.
For the heaters I will take the 6V, rectify it to obtain 8.4 DC and use a 7806 to apply 6V DC to the heaters of the valves. I will do the same for the HV output and then lower the voltage with resistors to get the voltages I want.
https://www.switchelectronics.co.uk/products/2-x-6v-10va-230v-encapsulated-pcb-transformer
Another option would be to go to toroidals, since they can deliver much higher current for an ok price.
https://www.switchelectronics.co.uk...06-toroidal-transformer-30va-0-6v-vigortronix
Kind regards,
Pedro
Last edited:
Hi
Sorry, I was not very clear. The input current of the transformer at 240V is 40mA and the output current at 6V is 0.8 amps.
Cheers
Pedro
Sorry, I was not very clear. The input current of the transformer at 240V is 40mA and the output current at 6V is 0.8 amps.
Cheers
Pedro
You haven't said how much current you are going to devote to the heaters. I'm gonna assume one 6V winding is for heaters, 830mA RMS, which will amount to about 400mA DC rectified.
That leaves 5VA for powering the second transformer. I'm gonna guess the second transformer needs perhaps 3VA of magnetising power, leaving you with 2VA for your HT supply.
In practice you'll probably get 200V AC at most, which gives you 2VA/200V = 10mA RMS for the HT, which amounts to perhaps 7mA DC rectified.
That leaves 5VA for powering the second transformer. I'm gonna guess the second transformer needs perhaps 3VA of magnetising power, leaving you with 2VA for your HT supply.
In practice you'll probably get 200V AC at most, which gives you 2VA/200V = 10mA RMS for the HT, which amounts to perhaps 7mA DC rectified.
Last edited:
Thanks for your reply.
You are right, I will need a separate transformer for the heaters.
And, according to your calculations, I will end up with 14mA available after rectification for the valves, which is ok for my sub 1W (0.5 or 0.6W) amplifier, powered with subminiature valves. The full circuit needs less than 10mA.
You are right, I will need a separate transformer for the heaters.
And, according to your calculations, I will end up with 14mA available after rectification for the valves, which is ok for my sub 1W (0.5 or 0.6W) amplifier, powered with subminiature valves. The full circuit needs less than 10mA.
You can do this, but with two small encapsulated transformers back-to back - expect absolutely -dreadful 'regulation'.
You will not get 240vAC on the output side under load - I'd be surprised if you see even 190-200vAC under load. Run your numbers again on such a basis, and see if that kind of loss will still support your design ideal.
You will not get 240vAC on the output side under load - I'd be surprised if you see even 190-200vAC under load. Run your numbers again on such a basis, and see if that kind of loss will still support your design ideal.
Hi,
Thanks for the reply.
Let's think this way then.
The transformer 230V-6V means 6V under load, in order to calculate the number of turns (or the voltage ratio), we need the unloaded output value.
For this transformer, the no-load voltage is 7.6V which gives us 230V/7.6V ≈ 30 turns (the ratio of turns is the same as the ratio of voltages).
Now, if the first transformer's output is higher, let's say 230V-12V instead of 6V, the output of the second will be higher too since we are applying 12V to a 6V-230V transformer.
Taking 12V, we will have 12V x 30 = 360V. Let's now assume 20% loss under load, that would give us ≈ 290V ac and after rectification 290V * 1.41 ≈ 410V, which is too much for me. Maybe a 230V-9V would do the job. So, we can play with the voltage ratio of the transformers as we like.
However, my problem is the current and not the voltage, since I can increase the voltage the way described above.
Meanwhile I discovered the transformer below which is not expensive and will do the job, but it will cost more than the double while comparing to encapsulated transformers but it will be cheaper than using toroidal ones.
https://primarywindings.com/product/pwht03a/
Kind regards,
Pedro
Thanks for the reply.
Let's think this way then.
The transformer 230V-6V means 6V under load, in order to calculate the number of turns (or the voltage ratio), we need the unloaded output value.
For this transformer, the no-load voltage is 7.6V which gives us 230V/7.6V ≈ 30 turns (the ratio of turns is the same as the ratio of voltages).
Now, if the first transformer's output is higher, let's say 230V-12V instead of 6V, the output of the second will be higher too since we are applying 12V to a 6V-230V transformer.
Taking 12V, we will have 12V x 30 = 360V. Let's now assume 20% loss under load, that would give us ≈ 290V ac and after rectification 290V * 1.41 ≈ 410V, which is too much for me. Maybe a 230V-9V would do the job. So, we can play with the voltage ratio of the transformers as we like.
However, my problem is the current and not the voltage, since I can increase the voltage the way described above.
Meanwhile I discovered the transformer below which is not expensive and will do the job, but it will cost more than the double while comparing to encapsulated transformers but it will be cheaper than using toroidal ones.
https://primarywindings.com/product/pwht03a/
Kind regards,
Pedro
I've bought items from Primary Windings before - the quality and service are excellent!
Dig a little deeper into the website, and he makes a 240:240VAC isolation transformer, up to 43VA* or even 75VA - and that is what you actually want!
I have one - it's very useful for instrumentation uses, and since it it split-bobbin, I could barely measure the interwinding capacitance - it is single -tens of pF. This is a fine thing also for your use, also
* I think this is it:
https://primarywindings.com/product/pwgp43-120v-43va-2-x-120v-0-19a/
NB - don't be afraid to email them & ask for best way to achieve the outcome you are looking-for. Again - it is a company I can recommend from experience.
ATB!
M
Dig a little deeper into the website, and he makes a 240:240VAC isolation transformer, up to 43VA* or even 75VA - and that is what you actually want!
I have one - it's very useful for instrumentation uses, and since it it split-bobbin, I could barely measure the interwinding capacitance - it is single -tens of pF. This is a fine thing also for your use, also
* I think this is it:
https://primarywindings.com/product/pwgp43-120v-43va-2-x-120v-0-19a/
NB - don't be afraid to email them & ask for best way to achieve the outcome you are looking-for. Again - it is a company I can recommend from experience.
ATB!
M
I did exactly this and it works OK, but have since decided to go with 2x transformers in parallel.
Check out my post: Step down transformer in reverse - safety and performance
Check out my post: Step down transformer in reverse - safety and performance