Small 5.5v+3v power supply

[Theo Markettos]

I have an electronic appliance (Logik IR100 internet radio) with what I suspect is a dodgy internal power supply. It runs off both 5v and 3.3v, don't know the current split but total power consumption about 10W. I'd like to buy a new power supply for it, either a wallwart/brick that produces both voltages, or a regulator board that takes (say) 12V from a brick I already have.

I could hack up something with some 78L05s and similar, but I don't really want to have to worry about the thermal envelope, and a linear power supply isn't too efficient (it runs 24/7). Being lazy, I don't desperately want to design my own switching PSU. Off-the-shelf power supplies (like for PCs) are for hundreds of watts and have terrible efficiency as such low current draws if they even start. Even mini-ITX are 80W+.

So can anyone suggest a source of a small 10-20W power supply? For example, is there some common consumer gadget where I can steal the PSU? USB chargers might give me 5V at 1-2A, but is there an easy/cheap way to get 3.3V?

Best so far I have is to buy two of these and feed them with 12V...

other suggestions?

Cheers, Theo

[Harry Bloomfield]

Theo Markettos expressed precisely :

Probably the simplest solution is to repair the failing PSU that you have.

[Tabby]

Best is to stop suspecting and find out, otherwise you're likely to be wasting time.

If it is the psu, and for some reason the existing one were unrepairable, I might go with a pair of simple asynchronous switched mode regulators. These arent issue free though, you need capacitance on the output end to filter the crud, thus you need either current limiting or on time limiting.

NT

[The Natural Philosopher]

Oddly enough a good source of very low RF interference switched mode regulators can be sourced from the model radio control industry, where regulating an uncertain battery down to a very clean 5, is a common problem.

These guys are the dogs bollocks

And pretty much will allow you to generate what you want from any source that is capable of at least 6v and adequate current

for low current consider straight linear voltage regulators instead.

[tony sayer]

In article , Theo Markettos scribeth thus

I've got Two of those thing's here broken. I think they suffer from firmware problems among others. Be interesting to see if the power supply is now an addition to what they fall over from;(...

Also we've just had our REVO "Blick" wi-fi radio loose its memory so each time you switch it on the settings for the wi-fi connection have to be re entered..

Never a problem with fM reception;)..

[Theo Markettos]

Unfortunately the PSU design is terrible. From what I've read, it's underspecced, provides poor noise isolation, is prone to overheating, and is prone to dropping one voltage rail keeping half the radio module parasitically powered through the other. The result being a pile of intermittent faults. What I think is happening is that it's either noisy on boot, is susceptible to glitches, or isn't able to provide the inrush current to boot the radio module. Without putting it on a storage scope for days on end with the hope of catching glitches it's hard to verify this.

Which is exactly why I want to replace it with something else, to see if that makes a difference. But I don't have a suitable dual-voltage bench supply. Which is why I'm asking about off-the-shelf modules. I have a power brick that does 5V and 12V (for a USB CD writer), so I wondered if there's anything similar out there for 5V and 3.3V?

I can, if necessary, design a new switching power supply. But when these things come out of China at $5 a pop (if you know where to buy them, which I don't) is it really worth the hassle?

Theo

[Theo Markettos]

I have an IR100 and an Oxx Alto. Both suffer from the same fault: the radios often won't boot. The IR100 has always done this or crashed after a few mins of boot, the Oxx has only recently started. On the Oxx the lights come on, but nothing else happens. The IR100 just sits there dead. If you pull the plug enough times you can sometimes get them to boot, especially if they haven't been powered for hours/days.

That looks to me like thermal, power supply, noise, reset or clock problems, rather than software. They're both on the latest firmware (which is different for each, both from 2008-ish). I have yet to open up the Oxx to see what the power supply is like (unlike the IR100, it has an external wallwart and takes 7.5V DC)

Theo

[Terry Casey]

If 5V rather than the 5.5V in the thread title is ok, why not use the 5V supply from the brick and a 3.3V regulator.

If the extra half volt is important, use a suitable regulator off the 12V supply. If you,ve got a 5V regulator handy, but a silcon diode in the earthy leg to raise the output to 5.7V

[Terry Casey]

+0000 (GMT)

supply from the

the output

or...

If you've got a 5V regulator handy, PUT a silcon diode in the earthy leg to raise the output to 5.7V ...

[Theo Markettos]

D'oh. That was a brain holiday, I really want 5V+3.3V.

That does sound a sensible approach, if I can get appropriate current out of the brick (which is likely). Even for 3.3V@2A, 3.4W lost in a linear regulator down from 5V is probably OK. I'll give that a go...

Thanks Theo

[tony sayer]

Every wondered what their like at 5 quid a pop;?...

Just changed Three recently on Moxa Ethernet to serial units and the price they are .. you'd have thought they'd be Rhodium plated;!..

[robgraham]

Just as a thought, how critical will the 3v3 be, and how stable will it require to be? An option is just to drop the voltage with a couple of diodes - if a bit of bleed current is added, which would help the stability, that would give 3v6. Add a Ge diode and you'll get your

3v3. OK, that nearly falls into the category of a bodge but it works and qualifies on the KISS basis! Rob

[Man at B&Q]

You'll need a low drop out 3.3V regulator.

MBQ

[Theo Markettos]

Useful idea. Since the 3v3 powers a processor, it needs to be fairly stable. Also the diode forward voltage varies (a little) under load, so when the processor starts taking more current the power rails will dip a bit. Given that I have a system that's slightly flaky to start with, I don't think this is such a great plan for debugging purposes!

Theo

[Terry Casey]

This WON'T sag under load ...!

LM1084 5A Low Dropout Positive Regulators

Line Regulation 0.015% (typical)

Load Regulation 0.1% (typical)

The LM1084 is a series of low dropout voltage positive regulators with a maximum dropout of 1.5V at 5A of load current ...

The LM1084 is ... also available in three fixed voltages: 3.3V, 5.0V and 12.0V. ...

you only need 2A, there is a 3A verion, the LM1085

[robgraham]

My thinking Theo, was to do a potential divider network with the diodes doing the dropping but running with something like 25mA through them with a resistor to ground in parallel with your load. That will control the load variation and also make sure they are dropping a near- enough 0.7v. etc. Rob

[John Williamson]

Is there any reason not to start with, say, a 7.5 or 9 volt supply, and use a couple of LM317s to supply the rails? One chip, one resistor, one preset (Or calculate the second fixed resistor value) and a couple of small capacitors to cut down instability per rail. Or just use a fixed 5 volt, 3 leg regulator for the 5V and a 317 for the 3.3.

[Theo Markettos]

Power consumption. The unit takes about 10W. For the sake of argument, let's assume that that's 5V @ 1A and 3.3V @ 2A (so 11.6W total, an overestimate for various reasons). Let's say that the input is 9V. With a linear regulator we need to dissipate 4Vx1A = 4W and 5.7Vx2A = 11.4W. Total wasted 15.4W. That's 135KWh per year, or a cost of £13.50 at 10p/KWh.

Of course, that isn't a reason not to do that for testing purposes. But you still need to arrange to dissipate that much heat, which is a bit messy to arrange on a breadboard.

Switching regulators are definitely the way forward IMO, but designing them yourself for a one-off project really isn't worth it unless you have particularly special requirements.

Theo

[John Williamson]

Fair enough. You'd get those losses with any linear solution, though. The three leg regulators bolt nicely onto a heat sink, and the added components can easily be wired between the pins, even for long term use in a vibration free environment. The input voltages only need to be smooth(ish) and 2 volts above the supply voltage, though.

RS components list a couple of 0.6 - 3.63 volt output adjustable DC-DC converters at 3 amps, and a whole range of 5V output DC-DC converters, all for under a tenner a unit plus P&P at single unit pricing. If you don't want to set up an account, your local TV repair place will have one, and may be willing to order for you, given the order codes and a bit of money for their time.

[robgraham]

I'm bailing out of this thread - a radio that consumes 10W and has a processor taking 2A. What nonsense is that when you look at the power that the average battery driven radio is taking ? How long does my trannie last on a pair of AA cells ?

Rob