Surprise in wall wart.

I needed a "plug" to stick on a box and found an old big plug DC adaptor with switched variable output. To my surprise it had a multi tapped transformer, bridge rectifier and smoothing cap., no regulation..

[turretslug]

I recall that Maplin offered this kind of thing till quite late in the day, I'm sure there's one in the wall-wart box somewhere. Presumably, a lowest-bid item from a Chinese supplier until the day that x amount of iron and copper finally cost more than a bit of iron dust core, less copper and a few semiconductors!

[Herald1360]

Presumably it cost a few pence less to do it that way than use a LM317 plus a few resistors.

[broadgage]

The type of wall wart described was the norm until switched mode technology became affordable.
The output voltage was not normally regulated and care had to be taken not to over volt the items powered thus.

The nominal "6 volt" setting was about 8 or 9 volts at low load and ideal for use in place of a small 9 volt battery.

[G6Tanuki]

I've got - somewhere - an "Eagle" brand wall-wart from the late-60s that has a little switch on the front offering 4.5/6/7.5/9/12V outputs.

Never investigated it deeply. I guess the switch changed secondary-taps. It worked just fine to power a Philips "monoknob' cassette-recorder [which would otherwise have needed five C-cells] when recording Top-of-the-Pops.

[TonyDuell]

Some of the wall-wart and similar power supplies with voltage regulation had a 2-pole voltage selector switch. One side switched resistors to the voltage regulator chip (often an LM317), the other side switched taps on the transformer secondary. I guess that reduced the heat disipated in the regulator.

[usradcoll1]

Originally Posted by broadgage

The type of wall wart described was the norm until switched mode technology became affordable.
The output voltage was not normally regulated and care had to be taken not to over volt the items powered thus.

The nominal "6 volt" setting was about 8 or 9 volts at low load and ideal for use in place of a small 9 volt battery.

I normally dislike modern switch-mode power supplies, but the regulation is spot-on! You don't have to worry about over-voltage damage in a device powered by one.
I usually look though the AC adaptors at the charity shops and it's amazing what is available. Many intended for cell phone chargers can be used for other applications.
Dave, USradcoll1

[ortek_service]

These were quite common in the 1980's, with many made by Altai (even if re-branded as another make).
There probably wasn't much room inside the case (and were usually fully-sealed so difficult to cool the insides) to have a fully-heatsinked regulator at a few hundred mA's so could be a fair power-dissipation as Low-dropout ones were also much-rarer then
Plus the switch would have to be more complicated to switch regulator's feedback / output voltage and also the transformer's secondary to minimise output voltage / drop across regulator to keep efficiency a bit higher.
Although the cheap transformer in many of these ran hot with no load, and often the > +105degC thermal fuse in these failed.

Even most 'fixed-voltage' equipment PSU's were unregulated - like Sinclair computer '9V' ones, that were much-higher at less than 1A etc. load.
So you could get a 'pre-regulator to put inline with the computer to regulate the voltage down much lower to minimise heat inside the computer.

Power-efficiency regulations and desire to minimise weight & size has probably eliminated linear-PsU's (even regulated ones), with Switch-modes now the norm. Although you could always add a small DC-DC step-down converter module on the output of an unregulated transformer DC-rectified / smoothed adaptor.

[Jolly 7]

The version of the Maplin multivolt adaptor I had failed because of an open transformer primary winding. It did have an LM317 regulator inside it and was free from any RF spoiling my radio reception. I have the newer Maplin multivolt version too, but it is a switched mode supply and no good for AM radio circuits.

[MeerKat]

Evening All,

This reminds me I have one which I have just dug out, as you can see it's badged Dixons. It weighs 9oz, a transformer I suspect and multi-voltage. The brightness of the output indicator LED depends on the volts selected, it's smoothed but unregulated. Note the plug/socket in the cable to reverse the polarity!

[ortek_service]

Originally Posted by Jolly 7

The version of the Maplin multivolt adaptor I had failed because of an open transformer primary winding. It did have an LM317 regulator inside it and was free from any RF spoiling my radio reception. I have the newer Maplin multivolt version too, but it is a switched mode supply and no good for AM radio circuits.

You'll probably find that it was actually a thermal-fuse in series wit the primary that failed. If you can open the case (sometimes had to resort to carefully hacksawing around centre of the case then glueing it back together), to get to the transformer, you often see a slight bulge in the primary-winding outer insulation tape. Carefully cutting this in the centre, and folding away, often reveals the small rectangular thermal fuse and can often snip this out and replace with another (from CPC etc.) but have to be very careful when soldering not to blow it, doing it very-quickly / maybe adding a heat shunt clip near to fuse as extra precaution.

The LM317 has a nearly 3V dropout voltage - And could be more across it if they didn't switch input voltage.
So probably couldn't get anywhere near its max 1.5A current, without it overheating.
Although the transformer in these probably limited current to < 0.5A (especially with a regulator to get hot)

[ortek_service]

Originally Posted by Andrew64

Evening All,
>>
>>

Note the plug/socket in the cable to reverse the polarity!

Ah yes, then infamous gamble on what the actual polarity is, as often not clear without a multimeter-check.
Many, as well as o/p voltage (transformer tapping / possibly regulator feedback amount) switch, also had a polarity-switch.

Once I'd set these correctly, I often put tape over these, to stop these accidentally getting knocked and possibly damaging equipment being powered - It was quite common back then for these not to be reverse-polarity protected never mind any over-voltage shut-off.

[Jolly 7]

[QUOTE=ortek_service;1343295]

Originally Posted by Jolly 7

The version of the Maplin multivolt adaptor I had failed because of an open transformer primary winding. It did have an LM317 regulator inside it and was free from any RF spoiling my radio reception. I have the newer Maplin multivolt version too, but it is a switched mode supply and no good for AM radio circuits.

You'll probably find that it was actually a thermal-fuse in series wit the primary that failed. If you can open the case (sometimes had to resort to carefully hacksawing around centre of the case then glueing it back together), to get to the transformer, you often see a slight bulge in the primary-winding outer insulation tape. Carefully cutting this in the centre, and folding away, often reveals the small rectangular thermal fuse and can often snip this out and replace with another (from CPC etc.) but have to be very careful when soldering not to blow it, doing it very-quickly / maybe adding a heat shunt clip near to fuse as extra precaution.

Thank you. This solution was also suggested by other forum members when I posted in relation to my failed wall wart in a previous post. However, I was unable to locate the fuse. Please see photos of the wall wart innards. I have thrown away the transformer, but have kept the PCB sans the LM317T, which I have used to build another regulated power supply. My new PSU goes from 0 to approximately 9V continuous and can be smoothly adjusted via a 5K pot.

[Maarten]

Originally Posted by ortek_service

These were quite common in the 1980's, with many made by Altai (even if re-branded as another make).

I'm pretty sure Altai wasn't the manufacturer either. Various European brand registrations for Altai point to UK based importers.

[ortek_service]

Originally Posted by Maarten

I'm pretty sure Altai wasn't the manufacturer either. Various European brand registrations for Altai point to UK based importers.

Altai did appear to be the original Chinese / Hong Kong manufacturer for a large number of these, also putting their name on many of these sold in the UK.
They also did many 'CB' power-supplies, that were identical apart from the front label. And quite a few were Altai-branded, as well as other names.


Altai certainly seemed to be the original manufacturer behind a lot of Multimeters, where there were a lot of identical looking different-brands (with actual Altai brand less common, but may have also used Eagle etc.), so were most-likely made in the same factory.

[ortek_service]

[QUOTE=Jolly 7;1343500]

Originally Posted by ortek_service

You'll probably find that it was actually a thermal-fuse in series wit the primary that failed. If you can open the case (sometimes had to resort to carefully hacksawing around centre of the case then glueing it back together), to get to the transformer, you often see a slight bulge in the primary-winding outer insulation tape. Carefully cutting this in the centre, and folding away, often reveals the small rectangular thermal fuse and can often snip this out and replace with another (from CPC etc.) but have to be very careful when soldering not to blow it, doing it very-quickly / maybe adding a heat shunt clip near to fuse as extra precaution.

Thank you. This solution was also suggested by other forum members when I posted in relation to my failed wall wart in a previous post. However, I was unable to locate the fuse. Please see photos of the wall wart innards. I have thrown away the transformer, but have kept the PCB sans the LM317T, which I have used to build another regulated power supply. My new PSU goes from 0 to approximately 9V continuous and can be smoothly adjusted via a 5K pot.

Yes, it's normally under the layer of red cardboard electrical-insulating vulcanized-fibre 'fish paper', just over the yellow polyester transformer-windings insulating tape / sometimes a bit of masking tape.

I presume it wasn't found on the other side either (May have to cut plastic insulating sheet away) - As sometimes they brought the long lead of fuse around to the other side, to give more strain relief / thermal soldering protection.
It might be they omitted it, for cheapness / early ones didn't have this, and was a later addition for added-safety. There was often a symbol on the PSU's label, with the temperature rating of the thermal fuse - which despite being usually over 100degC, would rather worryingly often fail so wondering if the transformer really got that hot!

[Herald1360]

100C isn't necessarily that hot for a transformer, depending on the insulation materials used. The 100C trip probably relates to the other components in the unit and the touch temperature of the plastic case.

If the (cheap) unit is only designed for normal room temperature operation- up 25C, say, there could easily be problems running it hard in a confined space or direct sunlight.

[jez_145]

Something I've always done, and highly recommend, is checking the output(s) of all wall warts before using them for the first time; whether supplied on their own or with a piece of equipment.

In the early 90's I built a NICAM TV tuner/decoder unit purchased in kit form, from Maplin and quite expensive at the time. I spent a weeks worth of evenings populating the (five ?) circuit boards and doing all the wiring. Fortunately, I checked the 12V PSU Maplin had supplied with the kit, only to find it was giving out twenty plus volts. Internal investigation revealed a 7812 regulator and heatsink, literally broken in half! It must have been rammed in the case with some force and later snapped. Externally, the PSU looked perfect. I let Maplins know my concerns about this in no uncertain terms! Happily, the decoder worked fine but it was a good job I checked.

Jerry

[ortek_service]

Originally Posted by Herald1360

100C isn't necessarily that hot for a transformer, depending on the insulation materials used. The 100C trip probably relates to the other components in the unit and the touch temperature of the plastic case.

If the (cheap) unit is only designed for normal room temperature operation- up 25C, say, there could easily be problems running it hard in a confined space or direct sunlight.

Well as transformers should be close to 100% efficiency, then you'd hope they wouldn't get anywhere near that sort of temperature which could be a fire-risk, depending on rating of the plastic (and which may eventually get as hot, if in fairly-close contact with it)
Especially as I think some of the thermal fuses used were >120degC, when they did start using these.

I have noticed that the cheap ones often got quite hot, even with no load. And had read that some may have been designed for 60Hz, and running them at 50Hz meant the cores weren't large enough for the power (Maybe similar with some ones in certain (FKI etc) CB PSU's used to buzz quite loudly).
Whereas, if you bought a replacement transformer from a UK distributor like Maplin back in the 1980's, these would run completely cool.

[Herald1360]

Large power transformers may approach 100% but the cheap wall wart variety are more likely to be around 85%. Depending on where the cheese paring has gone in the design, they may well run almost as hot on no load as on full load.

Insulation temperature ratings range from Class A at 105C to Class H at 220C.