AC Power Supply - Regulated

Thread Starter

Ed208.5

Joined Jan 21, 2022
3
Hi. New to building AC power supplies. I generally focus on digital circuits with DC.

However, I have a need for a 9V AC power supply, around 20VA. Obviously I can achieve this with a transformer, but do I need to regulate this to 9V, and if so, how? As I understand it, if the input voltage fluctuates, then so does the output.

TIA
 

Papabravo

Joined Feb 24, 2006
18,073
Regulating AC voltage is not normally a thing. Things like industrial control systems do not much care about the actual AC voltage as long as it is sufficient to pull in the relays used for the control logic. Regulation is something that is done more with DC voltage than AC voltage.
 

MrSalts

Joined Apr 2, 2020
1,517
Hi. New to building AC power supplies. I generally focus on digital circuits with DC.

However, I have a need for a 9V AC power supply, around 20VA. Obviously I can achieve this with a transformer, but do I need to regulate this to 9V, and if so, how? As I understand it, if the input voltage fluctuates, then so does the output.

TIA
Why are you building this? Most devices that require a 9vac power supply generally have a diode bridge, capacitor and voltage regulator inside. Often, you can simply use a 9 to 12VDC (the rectifier will get the direction right so no need to worry about polarity). If it is a motor, the capacitors and regulator are omitted. Also, if a motor, using DC will cause the low power switch not to work because that generally simply reduces power by disabling the negative half-wave from the rectifier.
Take a look inside and report back if help is needed.
 

Thread Starter

Ed208.5

Joined Jan 21, 2022
3
Why are you building this? Most devices that require a 9vac power supply generally have a diode bridge, capacitor and voltage regulator inside. Often, you can simply use a 9 to 12VDC (the rectifier will get the direction right so no need to worry about polarity). If it is a motor, the capacitors and regulator are omitted. Also, if a motor, using DC will cause the low power switch not to work because that generally simply reduces power by disabling the negative half-wave from the rectifier.
Take a look inside and report back if help is needed.
Thanks for your replies. It's a replacement for a Commodore 64 PSU. The C64 uses 9v AC as clocking if I remember correctly. i.e. it uses the 50Hz (UK). I've just been informed that it is rectified in the machine for -12V and +12V for RS232 as well. So I'm guessing that a small variation off a transformer won't be too much of an issue?
 

MrChips

Joined Oct 2, 2009
25,919
Welcome to AAC!

You asked:
1) How to make a 9VAC power supply?
2) How to build a replacement for a Commodore 64 PSU?

Can you see how the two are very different? You would have gotten better answers more appropriate to your needs had you asked question #2 in the first place.

Here is what the original C64 PSU looks like:
1642774907202.png

Now that we have some specs we can show you how to accomplish this.
 

Thread Starter

Ed208.5

Joined Jan 21, 2022
3
Welcome to AAC!

You asked:
1) How to make a 9VAC power supply?
2) How to build a replacement for a Commodore 64 PSU?

Can you see how the two are very different? You would have gotten better answers more appropriate to your needs had you asked question #2 in the first place.

Here is what the original C64 PSU looks like:
View attachment 258356

Now that we have some specs we can show you how to accomplish this.
Thank you for your time on this, but just to clarify, my question was actually "do I need to regulate an AC power supply?". I then went on to clarify what it was for, as I was asked to do.

The original C64 supply had issues with old units delivering way more voltage than they were meant to when they failed. I was hoping to mitigate these issues with a different design. I'd rather it fail altogether than with higher voltages.

If the generally accepted way of achieving 9vAC is simply via the transformer then that's fine. If there is a way to make it more robust/safe/foolproof (delete as required), then I'd like to explore the options.
 

MrChips

Joined Oct 2, 2009
25,919
In order to fix a problem you need to identify where the fault lies. Thus if the C64 fails because of over-voltage you need to find what part of the C64 is sensitive to over-voltage.

A study of the power distribution would be helpful.
Study the power supply circuit as shown on page 8 of the C64 Service Manual.

C64 PSU.jpg
C64 PSU description.jpg
 
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