Linear power supply ideas

[Vintageaudioholic]

Hi all, I've recently acquired an 80VA, 25v with ct output transformer. I'd like to hear your opinions and ideas to make the most out of its rather rustic casing by converting it into a linear power supply. For your information, I have a limited degree of understanding with regards to lps designs with monolithic regulators so I intend to keep the design relatively simple.
My main objective with this transformer is to literally squeeze every bit of power that I can get out of it, this is also important because I know that any excess power/voltage is transferred into heat which is what I want to avoid given that its got limited space and I intend to just bolt the regulator to its chassis wall. My question here is whether it's wise to use an 18v or 24v regulator with this transformer? The primary can be wired from 200-250v and heat is my main concern here.
Beyond that, I'd like to hear some practical experience from you when it comes to picking regulators, rectifiers and a circuit, as far as I can tell, 78xx would be a wonderful monolithic regulator to use, but I'd like something with more current capabilities seeing that 80VA would be quite a waste just to output 1A/1.5A. I know nothing about rectifiers so I probably need information as to why people specify fast recovery (?) diodes for that purpose. As far as my reading goes, voltage regulation starts to be less effective with high current devices so my question is what circuit should I integrate with that in mind?
Finally, a lot of audiophile LPS use tons of capacitor which I think add the total capacitance to the tens of thousands of microfarads, far more than what manufacturers specify, some also use inductors to regulate noise whilst manufacturers also don't necessarily specify, I just wanna ask about your opinions on this too!
Hopefully this will be a reasonably easy project to kickstart my DIY side of designing electronic circuits!

 

[ruffrecords]

The first question is what voltages do you want to get from this power supply? A single 24V like Neve gear or split supplies for op amps or something else?

Cheers

Ian

 

[Vintageaudioholic]

The first question is what voltages do you want to get from this power supply? A single 24V like Neve gear or split supplies for op amps or something else?

Cheers

Ian

Dear Ian,
I did consider using it as an external transformer for split supplying amplifiers but most headphone amps I found use higher voltages so I dropped the idea. It's a waste of its casing too since it could still hide some components within.
Honestly, I don't mind whether it outputs 18v or 24v, I just want to make the best out of this transformer provided that it doesn't overheat.

 

[ruffrecords]

OK, if you just use it as a 25VAC transformer an add a bridge rectifier and smoothing cap then the peak raw dc voltage will be around 35V. As you add a load some ripple will occur depending on the load and the size of the smoothing capacitor. A typical approach would be to follow this simple circuit with a regulator. For example you could use a standard 1 amp fixed voltage 24V regulator. This will give you a 24V supply at up to 1 amp. Worst case this would consume 35VA from the transformer so it will not even get warm.

Cheers

Ian

 

[Vintageaudioholic]

OK, if you just use it as a 25VAC transformer an add a bridge rectifier and smoothing cap then the peak raw dc voltage will be around 35V. As you add a load some ripple will occur depending on the load and the size of the smoothing capacitor. A typical approach would be to follow this simple circuit with a regulator. For example you could use a standard 1 amp fixed voltage 24V regulator. This will give you a 24V supply at up to 1 amp. Worst case this would consume 35VA from the transformer so it will not even get warm.

Cheers

Ian

Hi Ian,
It's me again, I think I found a way out of this
Basically, I was trying to make a 24v 2a dc power supply with a 7824 to fully utilise the transformer, the problem was that the math simply didn't add up. I'm basing this on a worst case scenario where the equipment is expected to run 24/7 on high mains voltage, here's how I did my math:
25vac÷250v x 253v (UK upper limit) x 1.414 =35.77vrms approx.
35.77vrms - 1.2 (bridge rectifier drop, being a bit harsh here) -24vdc = 10.57
10.57 x 2a output = approx. 20w
Now, my assumption was to use a 7824 with a to-220 package, the typical bolted thermal resistance is 5°C, I would also assume worst case with mounting resistance at 1.6°C, I found a heat sink at 1.2°C...
20 x (5+1.6+1.2) = 156°C... this isn't healthy at all given that the To-220 package maxes out at 125°C
However, once I realised that To-3 Packages are rated at 1.5°C max, the math is very attractive:
20 x (1.5 +1.6 + 1.2) = 86°C
In fact, I could easily raise the bar to 30W (3A) and it will just be 4°C over theoretically, of course I'm aware that the math is based on 25°C ambient temperature which means the unit is definitely not going to sit well during the summer so I'm scrapping that and dropping it to 2A max. Besides, I wanted to make a power supply that would match the audiophile dc power supplies available on the market and I believe that 2A is already pretty much ahead of the match so I'll be happy with that power.
Now I'm trying to find any To-3 packaged 7824 that supply 2A or change the circuit slightly for a To-3 package voltage regulator, I'd also like to work on the aesthetics of retrofitting a heatsink to the transformer, this is going to get interesting
p.s. still working on the Pi filter, I just need more understanding on it.

Thanks a lot Ian!

 

[Vintageaudioholic]

Hello, I made a 24vdc 1A linear power supply with a 40VA vintage lighting transformer, I'm glad to say that my ethos of minimal invasion did apply here: I only drilled 1 hole which does not impede on the overall aesthetics. The voltmeter also fit perfectly as a retrofit. I think I'll put the 80VA transformer away for now cause the 1A supply seems to satisfy many applications. However, I would like to ask for more information on CLC (Pi) filters now...
My 40VA transformer has a very simple 7824 circuit that has no additional filtering circuit and I have no intention of retrofitting a filter unit inside the transformer given the limited space. Therefore, anything I implement will be beyond the output of the 7824. As far as my knowledge goes, The first C(apacitor) does the bulk filtering and the rest is effectively a low pass filter. I think what I need to know more about is the choice of inductor. I am aware that inductor choice will impact voltage drop, therefore I want to know more about this. For your information, I have my eyes on a 4H, 1.1A, 25 ohm choke. However, if Ohm's law (?) stands true, I can be expecting a terrible voltage drop if my transformer runs at maximum output? I'm just not too knowledgeable on this matter and yes I'm aware that a choke is a subset within the inductor family.
Many thanks!

 

[Vintageaudioholic]

FYI this is the 40VA transformer I was repurposing, I am quite satisfied with how it turned out

 

[ruffrecords]

An old GPO transformer! Cool. I have not seen one of those in ages.

Looking at the pics it seems you are using a 3600uf reservoir capacitor. Taking 25V at 1 amp as your output requirement we can calculate the raw 100Hz ripple across the capacitor using:

CV - iT which gives a peak to peak ripple of about 2.7V. You can expect 70dB of ripple reduction from a standard regulator which means your output ripple should be in the region of 1mV. Perfectly satisfactory.

CLC filters are not used much for low voltage supplies these days simply because large value capacitors are readily available.

Cheers

Ian

 

[Vintageaudioholic]

An old GPO transformer! Cool. I have not seen one of those in ages.

Looking at the pics it seems you are using a 3600uf reservoir capacitor. Taking 25V at 1 amp as your output requirement we can calculate the raw 100Hz ripple across the capacitor using:

CV - iT which gives a peak to peak ripple of about 2.7V. You can expect 70dB of ripple reduction from a standard regulator which means your output ripple should be in the region of 1mV. Perfectly satisfactory.

CLC filters are not used much for low voltage supplies these days simply because large value capacitors are readily available.

Cheers

Ian

Hi Ian, the 80va transformer is also a GPO transformer , I might install a voltmeter and ammeter into it should the opportunity present itself
The inspiration behind all this was how to make the best out of the transformer and it's aesthetics, surprisingly I did manage to make the best out of its electronics too! This was somewhat an experimental project as I learnt that distance matters for components regarding noise, I took my liberties and soldered SMD film caps as close to the regulator as possible and tried grouping everything into one neat package, glad that turned out well cause the chunky casing does make a wonderful Faraday shield . The Voltmeter was a surprise too as you can see that it fit perfectly for what was the socket outlet.

 

[Vintageaudioholic]

Reunited

 

[cleohot]

Have a look at this.
http://www.circuitdiagram.org/24v-5a-power-supply.html
ML