A/C voltage regulator help

Thread Starter

Erick825

Joined Nov 23, 2021
2
New to this site and looking for some help. Trying to help my mom out. She has a Christmas decoration that has lights and motorized movement. It requires 6V a/c. I went on eBay and purchased a 110v ac to 6v/12v ac (dual tap) transformer. The problem I have is the actual output of the 6v tap on the transformer is actually 7.17v. I hooked it up and everything works but the lights seemed too bright, so I disconnected it so I wouldn't burn them out. Any ideas how I can regulate the output of the transformer so it is a true 6v.
 

AlbertHall

Joined Jun 4, 2014
11,704
AC is difficult to regulate. If the lights are on continuously - they don't flash - then the current drawn should be constant and a series resistor should do the job. The value of the resistor will depend on the current drawn. Do you know, or can you measure that current?

Second thought: Two diodes in series would drop about 1.2V.
 

Papabravo

Joined Feb 24, 2006
17,614
Transformer output voltages are normally given as RMS (Root Mean Square). 6VAC is actually about 8.48 Volt Peak. I don't know what kind of meter you might have used to measure the voltage, but 7.17 volts does not compute. If it was a True RMS meter then it should have read very close 6V.
 

djsfantasi

Joined Apr 11, 2010
8,078
Second thought: Two diodes in series would drop about 1.2V.
And may also the operation of the rest of the decoration. Rather than receive the expected 6VAC, it will receive half-wave AC.

Lights will appear much dimmer. A diode in series is used to provide two levels of light in lamps that don’t have a three-way base, for example. Motors may not operate on the effective 3-4 volts. Electronics may not operate at all.

Edit: missing word
 

crutschow

Joined Mar 14, 2008
28,551
Second thought: Two diodes in series would drop about 1.2V.
And may also the operation of the rest of the decoration. Rather than receive the expected 6VAC, it will receive half-wave AC.
You can use two sets of two diodes in series, with the pairs connected with inverse polarity (below) so they will conduct the AC in both directions and give full-wave AC.
That will drop about 1.2V in both directions of the AC waveform.

To size the diodes we will need to know the approximate current drawn by the lights.

1637730563163.png
 

Papabravo

Joined Feb 24, 2006
17,614
The open circuit voltage of small (and likely cheap) transformers is often significantly higher than the rated voltage to account for the transformer winding resistances.
At full-load it will likely be near its rated value.
Good point, but I'm still curious about the meter used and what configuration options it had.
 

BobTPH

Joined Jun 5, 2013
4,297
The open circuit voltage of small (and likely cheap) transformers is often significantly higher than the rated voltage to account for the transformer winding resistances.
At full-load it will likely be near its rated value.
Which is what I was getting at in post #3.

Bob
 

Thread Starter

Erick825

Joined Nov 23, 2021
2
Yes it is a cheap transformer, I was testing it with no load on it. The meter used to test it was an inexpensive digital volt ohm meter set to ac voltage. The transformer is rated at 3A. Not sure of current used by decoration but I would guess it to be in the range of 500-1000mah. So no where close to the capability of the transformer.
Thank you to everybody who gave me some ideas to try. It will be a couple weeks before I can get back to this project but I will defiantly try some of these suggestions.
 

old_beggar

Joined Jan 29, 2021
38
Erick,
If the transformer is rated at 3A, then get diodes rated at least that (i.e. 3 or more amps) and make the circuit posted by
crutschow , connect that circuit in series (ask if you're not sure what this means) with the lights, and then measure the a.c voltage across the lights. Let us know this value, and if the lights now look ok. Best of luck.
 

crutschow

Joined Mar 14, 2008
28,551
If the transformer is rated at 3A, then get diodes rated at least that
If the transformer is only used for the lights, then the diodes need to be no larger than the current the lights require (and that's conservative since each set of diodes only carries 1/2 the current).
 

RPLaJeunesse

Joined Jul 29, 2018
196
Consider also this circuit:

English: https://www.electroschematics.com/voltage-stabilizer-with-ldr-photoresistor/

You can change the lamp into an LED and the LDR into a photosensitive transistor.
View attachment 253622
This is a simple AC voltage regulator but it has serious issues. The output is very lossy and probably not good for more than a few mA of load current, the regulation is highly dependent on lamp to LDR coupling, compensated for (somewhat) by adjusting the variable resistance, and dissipation in the variable resistance is highly unpredictable. Stray light on the LDR cannot be allowed or proper operation would be disrupted. Also, replacing the LDR with a photosensitive transistor would only be possible if the photosensitive transistor worked for both polarities of the AC wave (they don't). It would be possible if the photosensitive transistor was placed "inside" a full wave bridge rectifier, but now all the other concerns still apply.
 

MisterBill2

Joined Jan 23, 2018
10,580
If the devices operate on AC, then they probably do not have any DC motors, Or they have internal rectifiers. Putting two devices in series across the transformer may be satisfactory and would be the easiest of all things to try.
Aseries resistor could work if the current was not very high, so that not a lot of heat would be produced. It would not be very efficient, but that may not be an issue.
 

dl324

Joined Mar 30, 2015
13,846
I hooked it up and everything works but the lights seemed too bright, so I disconnected it so I wouldn't burn them out.
You should have considered that before connecting power. Have you opened the device to see what its tolerance to higher voltage might be?

If you have access to a Variac (autotransformer), you can adjust the input to the transfer to get 6VAC out of the transformer and compare how the device operates at 6VAC and the nominal output from the new transformer..
 

MisterBill2

Joined Jan 23, 2018
10,580
Actually, 4 diodes, 2 in each direction, will provide the voltage drop you need to get it down to just about six volts. It will drop about 1.5 volts in each direction, rather than cutting the power in half
 
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