How to share a transformer

[dennnic]

Hi,

I'm looking for the best way on how to get multiple outputs from a single secondary coil transformer.

I've got a 9 volts 50 VA secondary and would like to build several independent outputs for raspberry and it's hats.

Would there be any downsides on sharing a single secondary?
As far I as see it, I can just use regulated output and perhaps connect multiple things in parallel; use a DC output from diodes and regulate each output independently; or use AC and independent rectifiers, caps, regulation for each one... How about a second approach?

Thanks in advance,

Regards,

Stefan

 

[Doodski]

If me I would use make a basic linear power supply and then use a linear voltage regulator(s) for the final regulation.

 

[dennnic]

If me I would use make a basic linear power supply and then use a linear voltage regulator(s) for the final regulation.

So, multiple regulators after rectifier and capacitors?

 

[Doodski]

So, multiple regulators after rectifier and capacitors?

Yes, that works. Very tight voltage regulation providing the transformer can deliver.

 

[Doodski]

Introduction to Linear Voltage Regulators

Linear regulators are simple voltage regulator circuits commonly used in electronics. This paper briefly discusses how linear regulators work, their advantages and disadvantages, variations on the linear regulator, and important datasheet parameters.

www.digikey.ca

 

[Doodski]

You might need to use a small heatsink on the linear voltage regulator(s). Also some can be configured for variable output voltage so you can dial in the voltage you require.

 

[dennnic]

Yes, that works. Very tight voltage regulation providing the transformer can deliver.

Thanks, I've been using lm 317 and 337 before, so I'm familiar with it. Just looking if it would be a good idea to use the spare transformer, or get another one with multiple secondaries.

Now that you've mentioned it, would a 50 VA secondary be adequate for up to 2 amps (9 volts secondary).

 

[Doodski]

It's been a long longgg time since I've performed power supply calculations. I don't want to make a mistake. But for the sake of curiosity if the secondary is rated at a apparent power of 50VA and it's a 9V secondary then it should be good for ~5.5A. Do you have a datasheet on the transformer you want to use?

 

[antcollinet]

You also need to think about the capacitor bank. The capacitors must support the voltage in between the AC peaks from the transformer. So you need sufficient capacitance to control the ripple voltage at two amps.

What is the output voltage (post regulation)? Oviously you need the lowest voltage of the ripple to exceed output voltage, PLUS the regulator voltage drop.

EDIT - and what is your maximum current for one regulator. You need to worry about the power dissipation. Unloaded, the DC input voltage will be about 12.5 V (more if the transformer is 9V at full load). If you have a 5V regulator, and 1 amp, then the regulator is dropping 7volts or so at 1amp so 7W

The thermal resistance junction to air is about 50C per watt, so without a heatsink you'll be putting the junction temp up to about 350C and fry it. Or it self protects and stops delivering volts.

So at 1amp you'll need a significant heatsink.

 

[dennnic]

It's been a long longgg time since I've performed power supply calculations. I don't want to make a mistake. But for the sake of curiosity if the secondary is rated at a apparent power of 50VA and it's a 9V secondary then it should be good for ~5.5A. Do you have a datasheet on the transformer you want to use?

Well, yes, but I suspect it's going to have overheating problems way before. I don't know what's the recommended procentual usage of a transformer, in order to get decent performance.

You also need to think about the capacitor bank. The capacitors must support the voltage in between the AC peaks from the transformer. So you need sufficient capacitance to control the ripple voltage at two amps.

What is the output voltage (post regulation)? Oviously you need the lowest voltage of the ripple to exceed output voltage, PLUS the regulator voltage drop.

EDIT - and what is your maximum current for one regulator. You need to worry about the power dissipation. Unloaded, the DC input voltage will be about 12.5 V (more if the transformer is 9V at full load). If you have a 5V regulator, and 1 amp, then the regulator is dropping 7volts or so at 1amp so 7W

The thermal resistance junction to air is about 50C per watt, so without a heatsink you'll be putting the junction temp up to about 350C and fry it. Or it self protects and stops delivering volts.

So at 1amp you'll need a significant heatsink.

So far I've tried just Lm317 with 2-3 volts of input-output difference. I usually put another resistor in-between capacitors to aid in heat dissipation and with the ripple size.

I assume that's a power dissipation calculation at full power. That 1 amp output was meant for a raspberry. It's average consumption should be around 700 mA, but with spikes at over an amp as some process is being run. Would a large capacitor bank after the rectifier (15-20 000 uF) make those 1 amp+ spikes invisibile to a transformer? So, theoretical, a smaller transformer could be used.

Also, would a large capacitor bank after a regulator alleviate the load on the regulator itself? So the spike could be drawn from capacitor and not induce additional heat on it's heatsink.

 

[Doodski]

Well, yes, but I suspect it's going to have overheating problems way before. I don't know what's the recommended procentual usage of a transformer, in order to get decent performance.

Can you post the transformer datasheet here? If the ASR website won't take a <.pdf> then zip it and that should work.

 

[dennnic]

Can you post the transformer datasheet here? If the ASR website won't take a <.pdf> then zip it and that should work.

No, it's custom ordered... I did get a better look at it now:
100 VA toroidal with electrostatic shield, out of which one secondary is rated at 1 amp at +/-15V (center tapped). Other two secondaries are rated at 9v at 3.8 amps.

Four output regulators needed - two for +/-15v crossover, one for raspberry pi 4 (up to 1.2 amps), one for both it's screen (up to 500 mA) and allo hat (around 100 mA). At least I think to connect a screen and a hat to the same regulator, to save up on space and costs.

 

[Doodski]

No, it's custom ordered... I did get a better look at it now:
100 VA toroidal with electrostatic shield, out of which one secondary is rated at 1 amp at +/-15V (center tapped). Other two secondaries are rated at 9v at 3.8 amps.

Four output regulators needed - two for +/-15v crossover, one for raspberry pi 4 (up to 1.2 amps), one for both it's screen (up to 500 mA) and allo hat (around 100 mA). At least I think to connect a screen and a hat to the same regulator, to save up on space and costs.

So let me confirm details:

100VA primary transformer
2 regulators @ +/- 15VDC, 1A max +/-
1 regulator @ 9v DC, 1.2A max
1 regulator @ 9v DC, 100mA +500mA = 600mA max

 

[dennnic]

So let me confirm details:

100VA primary transformer
2 regulators @ +/- 15VDC, 1A max +/-
1 regulator @ 9v DC, 1.2A max
1 regulator @ 9v DC, 100mA +500mA = 600mA max

That's would be it, yes.

 

[Doodski]

That's would be it, yes.

From what I can see the +/- 15 VDC secondary is a bit low on output current capacity. By the time you allow for circuitry power inefficiencies and losses. If the transformer does overheat there may be a thermal fuse in the primary. There are thermal fuses in Canada according to safety code. Perhaps in your country there are too.

 

[dennnic]

From what I can see the +/- 15 VDC secondary is a bit low on output current capacity. By the time you allow for circuitry power inefficiencies and losses. If the transformer does overheat there may be a thermal fuse in the primary. There are thermal fuses in Canada according to safety code. Perhaps in your country there are too.

My Bad, +/-15v secondaries are rated at 1 amp, the load on them would be just 7 op amps.
There's both a thermal fuse inside (forgot it's rating) and a thermal switch glued on the side of the transformer.

Reason for all of this writing really is - I've read on several sources that secondary should supply at least twice the required current, or, preferably, three times more.

If 9v secondary would allow for 3.8 amps, then 1.2 amps load is slightly above the 1/3rd rule.
I'm wondering if to add a large capacitor storage before and after the regulator (average usage should be around 700-900 mA), or to connect both 9v secondaries together and connect two regulators to that. (Or maybe three. That 100 mA load, that's the only one that actually needs linear power supply)

 

[Doodski]

I'm wondering if to add a large capacitor storage before and after the regulator

Before the regulator will smooth things out a bit better and provide some additional storage although the regulator will smooth out things very well if the there is just adequate capacitance without going overboard. Upon power ON there might be a surge that exceeds the diode(s) and transformer parameters if you over do the smoothing capacitance on the secondary side. If after the regulator then the regulator has to deal with that capacitance and the regulator will have limitations too. The regular output capacitance (If used) should be as close to the load as possible.

or to connect both 9v secondaries together and connect two regulators to that. (Or maybe three.

That would save on secondary side linear supply components and make things simpler to build. You could source a linear regulator IC that has more current capacity and use just one instead of three. But then you lose the dedicated linear regulator for the digital circuitry voltage supply. I think one regulator for the digital circuitry voltage supply and one for the analogue supply would suffice if you can get linear regulators that have the current capacity for your needs. If I remember correctly you are operating analogue and digital components from these regulators.

If I may ask. Why don't you buy pre-made power supply(s) and use them?

 

[dennnic]

Before the regulator will smooth things out a bit better and provide some additional storage although the regulator will smooth out things very well if the there is just adequate capacitance without going overboard. Upon power ON there might be a surge that exceeds the diode(s) and transformer parameters if you over do the smoothing capacitance on the secondary side. If after the regulator then the regulator has to deal with that capacitance and the regulator will have limitations too. The regular output capacitance (If used) should be as close to the load as possible.


That would save on secondary side linear supply components and make things simpler to build. You could source a linear regulator IC that has more current capacity and use just one instead of three. But then you lose the dedicated linear regulator for the digital circuitry voltage supply. I think one regulator for the digital circuitry voltage supply and one for the analogue supply would suffice if you can get linear regulators that have the current capacity for your needs. If I remember correctly you are operating analogue and digital components from these regulators.

If I may ask. Why don't you buy pre-made power supply(s) and use them?

Dual rail 15 volt is for analogue, an electronic crossover. Everything else is digital, the hat is just a separately powered spidif out.

I've ordered PCBs for regulators, but they are rated to 1.5-2amps. The problem is getting devices shipped to this part of the world ...
It would be best to put a raspberry and it's screen on one regulator (don't really think ripple and noise would matter a lot there) and keep the rest separated. However, that would mean perhaps even 1.7 amps on a single regulator. I could get perhaps a case with a heatsink integrated... How about adding a large capacitor (6-8000 uF) after the regulator? Could that buffer those relatively short high current spikes?

 

[dennnic]

If I may ask. Why don't you buy pre-made power supply(s) and use them?

Plus it makes for a fun project

 

[Doodski]

Plus it makes for a fun project

It's a learning experience for sure.