AC Voltage Stepdown w/o transformer @ 250mA

[Infrared_Fred]

Does anyone know if there is a practical way to step down an AC voltage at a moderate current (say 250mA) without dissipating a lot of heat?  More specifically, I have a 24V transformer I am looking to use in a junkbox parts power supply, and from it I would like to generate a 5V supply for some LED voltmeters.  They draw something around 100mA each, but to power  them with a 7805 running of the rectified 24V would be >20W disspation.  Switching power supply is not an option really because of the noise and I don't want to have to order any new parts for this project.  Would a capacitive or inductive divider work?  I asked the Google and capacitive dividers disdn't come up with much other than theoreticcal type stuff, and there were some of the non-isolated usually driving LEDs type, but they seemed to be for low-current (<50mA).  Does anyone know if there is a way to adapt the circuit to electrolytics or otherwise use an non-resistive voltage drop?  JSYK Un fortunately there is not a center tap on the transformer.  If need be I could use a little 9V transformer from an old radio, but I would prefer it to all run of the 24V transformer for simplicity.  Thanks so much!

[bob91343]

If the load is constant, a capacitive divider will work.  But like any divider, it will have a fairly high sensitivity to load current.  You can put the divider either on the primary or secondary of the transformer.  You cannot use electrolytic capacitors of course,

Another approach might be to use a switching regulator.  National Semiconductor has made some good stuff; I have a few of their demo boards which put out, say, 5V at 1A with a wide dc input range.

[Terry Bites]

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiK6Yj_qbT1AhUzoVwKHYKuDmkQFnoECBQQAQ&url=https%3A%2F%2Fwww.designercircuits.com%2FDesignNote1c.pdf&usg=AOvVaw2wb_GvJLhyPXxiwDyQlydo

For 250mA you would need a massive class X cap an no fear of death.

Switcher input and ouput noise can be managed down to acceptable levels.
see https://www.maximintegrated.com/en/design/technical-documents/tutorials/9/986.html

Buck coverter modules can be bought for less than a decent bottle of beer.

You can always use an LDO and a 6V lead acid or LiPo battery- charge it once in a while. Super low noise. There is no, no noise solution. The DVMs will have thier own digital noise.

[Ian.M]

What sort of transformer?  If its toroidal, put on an overwind secondary to get exactly the voltage you want to feed your 5V regulator.  Its also got the advantage the 5V will be floating, so if you have any ammeters, you don't have to worry about the operating current of the meter displays showing up on the ammeters.

[NiHaoMike]

Assuming the 5V is to have the same ground as the rectified 24V, add two additional diodes from the 24V AC (no filter capacitor) going to a MOSFET then to a capacitor. Have a comparator turn the MOSFET on when the input side is 5V or less and off when it is greater. (Experiment with the gate resistor value to not get too much switching noise, try 100 ohms for a start.)

Here's a chip that implements the idea, in your case you're using low voltage AC which makes it a lot safer to work with.
https://www.microchip.com/en-us/product/SR087

[ogden]

More specifically, I have a 24V transformer I am looking to use in a junkbox parts power supply, and from it I would like to generate a 5V supply for some LED voltmeters.  They draw something around 100mA each, but to power  them with a 7805 running of the rectified 24V would be >20W disspation.

You need rectifier bridge and smoothing capacitor for 24V transformer and step-down (buck) DC-DC converter module capable to handle input voltages at least 35VDC or 40VDC and deliver 5V >=1A on output. Look for LM2596 Buck Converter module, they are all over the internet. This is cheapest, bottom of the barrel solution that can do the job.

[John B]

The trick is to use a 2nd capacitively coupled bridge rectifier off your 24V transformer alongside the regular bridge rectifier. It does require a handful of large electrolytic caps though, at least 4x of them - both AC terminals have to capacitively coupled to the transformer, and each has to use 2x back-to-back electrolytics to prevent reverse biasing the caps (aka DIY bipolar electrolytic).

One of the rectified terminals is then referenced to either ground or the positive potential of your other recitifier. This means you can generate a negative voltage, another positive rail or a boosted positive rail on top of your regular rail.

In this case you would reference it to ground. You can then modify the value of the coupling caps to increase or decrease the impedance to create a suitable voltage drop across them.

It works best with relatively constant loads so you can maximise the voltage drop while still ensuring you have sufficient headroom above your regulator.

This solution is best for low noise situations like power supplies where you would rather keep out switchmode solutions.

[John B]

Here's what I mean:

Falstad link

Due to the large voltage drop, the coupling caps can have significant impedance, so at least in simulations 4x 330u caps might be enough. Also when building the real circuit, check for any significant cap heating due to the current through them, you may need to parallel some smaller value electrolytics to reduce the ESR.

[BrokenYugo]

100mA sounds about an order of magnitude high for such a thing, have you measured the draw of these meters?

That said, John B's idea is probably the easiest to implement and lowest noise.

[mariush]

100mA sounds about an order of magnitude high for such a thing, have you measured the draw of these meters?

That said, John B's idea is probably the easiest to implement and lowest noise.

It's the leds in the 7 segment displays that consume the current ... they're probably driving them high at 20mA or so, even multiplexed they're still one digit on all the time, so between 2 and 8 segments at 20mA.

OP could modify the displays by changing the resistors for each segment so that the leds are limited to lower current... and could probably get them working with 3.3v or 3.6v, depends on the chip used and the internal voltage reference of that chip (if any).

[SeanB]

Got another smaller transformer with say a dual secondary, or with a 110VAC primary and a 48V secondary? You can drive one transformer off the other, and say if you had the 110VAC to 48VAC one, capable of providing 1A on the secondary, you will be getting 10VAC instead, or if it is 30VAC you will get 6VAC, enough to use with a LDO to drive the displays.

[Infrared_Fred]

The trick is to use a 2nd capacitively coupled bridge rectifier off your 24V transformer alongside the regular bridge rectifier. It does require a handful of large electrolytic caps though, at least 4x of them - both AC terminals have to capacitively coupled to the transformer, and each has to use 2x back-to-back electrolytics to prevent reverse biasing the caps (aka DIY bipolar electrolytic).

One of the rectified terminals is then referenced to either ground or the positive potential of your other recitifier. This means you can generate a negative voltage, another positive rail or a boosted positive rail on top of your regular rail.

In this case you would reference it to ground. You can then modify the value of the coupling caps to increase or decrease the impedance to create a suitable voltage drop across them.

It works best with relatively constant loads so you can maximise the voltage drop while still ensuring you have sufficient headroom above your regulator.

This solution is best for low noise situations like power supplies where you would rather keep out switchmode solutions.

  Thanks so much!  This sounds good and I will try it out soon! 
Thanks to everyone for the help!