# Reducing AC supply voltage - Slightly

#### Jak74

Joined Aug 17, 2019
4
What I have is a 250VA power transformer with an input of 117v and output of 24v or 48v. So when I apply the 120vac (121vac - 122vac actual) from the wall the outputs are a few volts higher 28v or 54v. This will be used to momentarily supply power to coils and motors up to 5 amps. A few volts high shouldn't hurt anything i wouldn't think however, if i wanted to reduce the voltages so that they are closer to 24v or 48v what would be the best way?

I considered a resistor voltage divider with 140w film power resisters with heatsink 33ohm and 1ohm at the VAC input to reduce the voltage coming in to around 117VAC.

#### AlbertHall

Joined Jun 4, 2014
10,576
The transformer output voltage will be lower when loaded. The reduction may be sufficient for your purposes.

#### Jak74

Joined Aug 17, 2019
4
The transformer output voltage will be lower when loaded. The reduction may be sufficient for your purposes.
Okay, yes that is true. I guess I shouldn't worry about a few volts.

#### Papabravo

Joined Feb 24, 2006
14,701
A voltage divider is probably the WORST possible solution where power is concerned, especially if the load is not a constant impedance. With a variable impedance load the whole voltage divider fails to work as intended rendering it a COMPLETE waste of time and energy. About the only viable application I have seen in a career spanning half a century is to create a voltage reference for a high impedance load, like an opamp input.

The actual solution to the TS problem is called a Variac. It is an adjustable transformer that takes AC in and produces a variable AC out.

https://variac.com/staco_Variable_T...MItYq8su6J5AIVhZ-fCh06_ghsEAAYASAAEgL7FfD_BwE

I agree that in many cases it is a less than cost effective solution, but sometimes there is no substitute.

#### ScottWang

Joined Aug 23, 2012
6,963
1. Is that an adapter or a real transformer?
2. How much current of the output?
3. To use a suitable resistor load to in parallel on the output and to measures the output voltage.
4. If you need to use the resistor(small values and higher Watts) to reduce the output voltage then you can in series with the resistor on the DC output side.

#### Jak74

Joined Aug 17, 2019
4
It is a Toroid Power Transformers. I have came across a toroid transformer that outputs 22v and 44v with a 117v input, but I think the few Volts high from my current transformer will not be an issue in my case. Plus I might find that 120vac outlets at other locations other than here in the shop might be lower. What I have tested it on has worked fine. Seems like the best fix would be a precisely wound transformer so if there isn't a simple circuit solution I wont worry with it. Thank you all for your input!

#### AlbertHall

Joined Jun 4, 2014
10,576
As it is a toroidal transformer you could possibly wind some extra turns and connect this extra winding in series the primary to effectively increase the number of primary turns and reduce the output voltage.

You would need to experiment with the number of turns and which way round you connect it (the wrong way round would increase the output voltages).

#### MrChips

Joined Oct 2, 2009
22,113
You need to determine if your equipment can handle the over-voltage.

Note that 24VAC is actually a signal with amplitude 24 x 1.414 = 34V peak.
Similarly 48VAC is 68V amplitude.

#### Jak74

Joined Aug 17, 2019
4
Experimenting with a winding in series does sounds interesting.. I may try the other transformer I found with the 22v and 44v outputs. That might also do the trick.

Joined Jul 18, 2013
21,639
I think you are concerned unnecessarily, normally those type of loads can handle over-voltage with no problem.
However, If what you have is a toriodal transformer, they are usually very easy to modify by preferably removing turns, or in some instances, adding extra buck wound turns.
Also these loads do not require a smoothing capacitor, and can be used with the 120hz ripple resulting in the DC voltage the same as the AC level.
Max.

#### Bhante

Joined Dec 15, 2009
42
... by preferably removing turns, or in some instances, adding extra buck wound turns ...
Ah, the preference for removing turns over adding forward wound turns would be to prevent possible saturation, I take it? That would mean changes to the secondary as opposed to the primary as suggested above, since removing turns from the primary would increase voltage further.

If the OP is worried about variation in input voltage, perhaps an interesting advantage of adding buck wound turns could be to add buck wound turns with several alternative outputs! Then if the voltage error is critical you can choose a suitable compensationfrom among several - converting a fixed transformer into a custom variac, effectively (add some forward wound turns also if increasing voltage might be a useful additional option).

My interest in this topic is that I have an application using about 750 watts at theoretical 230V (the load is a heater). The input voltage unfortunately varies enormously from around 210V to 240V (and less often from 200V up to 250V), which causes regulation problems. Using a custom wound roughly 1:1 toroid variac as in the previous paragraph with several outputs with different windings in series, what sort of size and weight of transformer would we be talking about? Actually it is mainly boosing the excessively low voltages that worries me, and if following this route I would definitely want to boost power to 240V, i.e. about 820W.

Perhaps a suitable way of regulating the voltage would be to switch in or out additional turns with several SSDs, using a signal from an arduino and a voltage monitor. I am guessing there is probably a small chip available that will monitor ac voltage and directly give a digital output for the arduino.

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Joined Jul 18, 2013
21,639
Ah, the preference for removing turns over adding forward wound turns would be to prevent possible saturation, I take it? That would mean changes to the secondary as opposed to the primary as suggested above, since removing turns from the primary would increase voltage further.
.
The preference is always the secondary, also easier to access, as the primary is wound first.
Using a buck winding just reduces the efficiency a little.
I have used toroids extensively in power supplies and have often added a custom winding, as long as all is within the rated VA.
Max.

#### n1ist

Joined Mar 8, 2009
188
The power line is usually spec'ed at +/- 5% tolerance; most consumer equipment is designed to +/- 10% at the extremes (ie, most of my designs were done for 105-305V for a nominal 120 to 277V input).

/mike

#### KeithWalker

Joined Jul 10, 2017
1,307
When you measured the output voltage was there a load connected? The open circuit voltage will always be higher because the winding voltage is rated at full load, when there will be avoltage drop across the winding resistance.
Keith