Selection Guidelines - Convert DC Wall Wart to AC Wall Wart

[Vons Garage]

It seems I have no shortage of DC power adaptors - at all! Occasionally I need an AC version; for example, clocks requiring 60Hz for time base.

Given a needed AC voltage, which DC power adaptor voltage rating would you experts select to break open, pull out all the DC conversion components, retain just the transformer, and then glue the whole thing back together again?

Should you try to match the desired final AC voltage exactly? Or select one that has a higher or lower voltage than the target?

And finally, your opinions: what do you think these things should be officially called: external power supplies, power adapters, AC adapters, or power bricks? Has anyone come up with a more elegant name than “wall wart” that clearly identifies these type that actually have the guts at the point-of-connection to the mains?

 

[Diver300]

You can also measure the open circuit voltage. If the open-circuit voltage is maybe 40% higher than the rated voltage, it's almost certain to be a linear, unregulated supply.

Switch-mode supplies are always regulated so the no-load output voltage is very close to the rating. There are regulated linear supplies which will also have a no-load voltage near the rated voltage.

 

[Nigel Goodwin]

You can also measure the open circuit voltage. If the open-circuit voltage is maybe 40% higher than the rated voltage, it's almost certain to be a linear, unregulated supply.

Switch-mode supplies are always regulated so the no-load output voltage is very close to the rating. There are regulated linear supplies which will also have a no-load voltage near the rated voltage.

In either case, the weight is a dead giveaway

 

[Les Jones]

Measuring the resistance between the mains input pins will also tell you if it is switch mode or one with a normal mains transformer. Switch mode ones will read a few megohms. Ones with a normal mains transformer will measure a few hundred ohms.
Les.

 

[ronsimpson]

clocks requiring 60Hz for time base.

In 1978 I had a clock used in factory automation that uses 60hz from the power line. The clock did not keep time well. I tracked it down to every time the elevator motors turned on there was an extra pulse added to the time.
60hz See link to a good stable 60hz without the power line. I made the circuit in the link and disconnected from the power line. The IC is not made now days. I found then on ebay.com just now.

 

[For The Popcorn]

Working at an engineering services company, I received a strange request.

An oceanographic research vessel thyristor-controlled variable speed propulsion had a problem. The digital clock in the chief engineer's stateroom sometimes ran at twice normal speed.

Obviously, the control system was generating a lot of 2× crap into ship's power system. Sadly, they didn't care for my proposal to run a bunch of tests under varying operating conditions to determine the source and work out a solution.

The clock operation was just a symptom. Many instruments could have been affected as well.

 

[Vons Garage]

Consolidated all this excellent advise. Thanks!

 

[Diver300]

The only think that I would say is that regulated, linear supplies exist, and they do not meet the point about the unloaded voltage.

Switch mode supplies never have such large output voltages, linear ones often do.

 

[For The Popcorn]

One more piece of advice – slightly off topic for this discussion:

For a DC supply, always check polarity. Red may or may not be positive, black may or may not be negative.

This is particularly true if you cut the connector off a center-negative supply (the devil's work!) – center positive is the common polarity, so manufactured cables are red wire for center contact, black wire for shell (where the cable is two conductors inside a jacket). If a power supply manufacturer gets an order for center-negative supplies, they just reverse the connections to the pre-built cables they already have.

Some people here may dispute this claim, but I have let the magic smoke out more than once when re-using a former center-negative supply. Just check to be safe.

 

[Nigel Goodwin]

One more piece of advice – slightly off topic for this discussion:

For a DC supply, always check polarity. Red may or may not be positive, black may or may not be negative.

This is particularly true if you cut the connector off a center-negative supply (the devil's work!) – center positive is the common polarity, so manufactured cables are red wire for center contact, black wire for shell (where the cable is two conductors inside a jacket). If a power supply manufacturer gets an order for center-negative supplies, they just reverse the connections to the pre-built cables they already have.

Some people here may dispute this claim, but I have let the magic smoke out more than once when re-using a former center-negative supply. Just check to be safe.

No, you're quite right - sometimes things are just 'wrong'.

Long ago I upgraded the radio in a car I had, and found out that the manufacturers wire colours for the radio were red for -ve and black for +ve

Luckily I had a spare fuse! - and a multimeter to check the polarity (better late than never).