Yes be my hope that they are done properly..

Agree! isolation is good yes

Sadly I think a lot of stuff getting into this country is very sloppily made and sometimes just out right dangerous

"Medical Power Supply", "Made in China" should I be worried?
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At the moment I need it, but as soon as it's replaced, I won't resist looking to see if it's just the ferrite making it a medical supply. Don't worry, it's not running medical equipment!

And no Australian approval marks...

To get back to the question of circuitry -

I'm interested in how much circuitry is needed to have the charger sense when battery is at an appropriate level charge, then turn itself off.

My undeducated guess is - the charging current would have to be pulsed, and there would need to be a zener diode or the equivalent linked to a transistor to turn off the charging current once the battery was feeding back a charge of the right voltage. Maybe a resistor one side of the diode?

The reason I am interested is: I am building a B+ voltage synthesiser battery replacement for a AWA /Hotpoint portable. I'm thinking of powering it from a 6v SLA rechargeable that I can also use as source for the heater voltage. The radio power circuit has a switch setting that used to send a DC current to recharge the original batteries while the radio was connected to AC. I'll want some protection so the charging current doesn't overcharge the SLA.

Maven

In smart batteries it's either done by detecting temperature of the battery or looking for voltage peak over time, or both.
Both of these could be done with a microcontroller with analogue inputs.

Actually, since posting the query I have done an internet search and find there are dozens of circuits online, mainly related to 12volt automotive battery chargers with their own 240vac transformers etc.

I'll have to check whether the charging current supplied by the AWA / Hotpoint is low enough to count as a "trickle" and therefore not need to be turned off. Obviously in its original usage it was just recharging dry cells, so must have been pretty low. I haven't noticed any special components.

For my application, it will be the heater power circuit charging 6v SLA battery, not the B+ charging a B battery. I'll need to know whether the charging characteristics of SLA battery are compatible with that, or might might draw too much current from the radio's transformer. If so I would have to just use an external charger.

Some measuring to do, but no room on the bench at the moment.

Maven

As for "carbon footprint", every time you exhale, you are dumping CO2 into the atmosphere...

Anyway, those switching power supplies have provided me with high voltage electrolytics, high voltage rectifier diodes, and low voltage high speed rectifier diodes. CFLs also have high voltage diodes and electrolytics.

Dear friends, I wish to add a few few comments regarding switch mode power supplies battery chargers.

I wish to make it known that I am not a designer, but a serviceman charged with the repair of such items, and therefore might not be 100% accurate in what I have to contribute.

First of all, I’m surprised to hear that one member was surprised to find that one of the power supplies examined was of the switched mode type! The weight difference gives away the type of supply. Iron-cored transformers are very heavy and large when compared to an equivalent ferrite (switched mode ) type.

Iron-cored (50Hz) transformer based power supplies are very reliable, have low component count, and are very much immune to noise/voltage spikes. After all, they are basicly just a big choke as far as the noise and spikes are concerned. They are therefore very effective in protecting the following equipment.

Producing a square wave switching waveform produces harmonics by the score. Zero point switching is one method of reducing interference.

The use of the switch mode of operation, is used for a number of reasons, one of which is to minimise the size of heatsink required to keep the switching device/s within their safe operating area. This is then countered by totally enclosing the the power supply in a non-vented case which is virtually an oven.

Years ago I built a power supply which was an Electronics Australia project. It was of a switched-mode series regulator design. What a disappointment. It had more birdies than an aviary. It could only be used on noise tolerant devices like car radios. Anything that was meant to operate from dry cells was swamped by noise.

The company that I work for makes a very tidy sum thank you, first of all installing switch mode power supplies for CCTV and Access control devices, and then replacing them as they fail. Clients always ask for the cheapest “solution” but fail to see that one single service call will cancel-out any cost saving that is made by being cheap.