Hi All,
I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.
As the output of these is (in theory) already smooth,
Could I convert them to 5V by the simple addition of a 7805 on the output?
TIA
Chris
It happens that snipped-for-privacy@o2.co.uk formulated :
Yes, but it would be more efficient if you were to find the resistor network or trimmer which sets the output voltage of the SMPSU and adjust it to directly output 5v.
They all use some sort of network, by which the output voltage is used as feedback to control the output.
I converted several ex-BT 50v (Farnell) SMPSU's so as to deliver 13.8v, by simply tracing the correct resistors to change.
Depends on how much current you're planning to draw I think.
A 7805 is a linear regulator, so you could be disspating a lot of power in it. At 12v in and 5v out, delta V is 7v. So at a current of
1A, you'd be dissipating 7w. That'll need a heat sink.Obviously efficiency will be low. The 7805 will have an efficiency of
5/12 = 41%. And that's after the losses in the 12v PSU.Yes but the disipation will be laess as you are in effect using the device to dump the volts. I'm not sure how these devices are made, but surely there has to be some kind of chip in there that is multi standard to give out the range of voltages one might need. Brian
You could use a 5V LDO (low drop out) regulator instead of a 7805, e.g. if you need 1A, try this, or search for higher current ones ...
As others have mentioned a linear voltage regulator wouldn't be very efficient and get quite warm.
So, whilst not very pretty ... and depending on the power required, a 'buck converter' (or converters) might be a cheap and easy way of making use of some 'nice' SMPSU's?
That way you could also use them for other voltages at the same time (and higher if you use a 'bang converter'). ;-)
Cheers, T i m
Adjustable Buck converters on eBay are about a quid a piece
10 PCS DC-DC Buck Step Down Converter Module LM2596 Voltage Regulator TE097Then again 5V 1A power supplies in Poundland are the same.
Yes, and according to this video are very good.
Another Dave
Thanks for the replies everyone. I think I will invest in some buck converters.
If you only need a 100mA 5v supply and you've already got a bunch of
7805 regulators to hand, then ok. Otherwise, if you need a 1A 5v smpsu, you might as well buy a bunch of Poundland USB wallwarts and have done with it.As I've mentioned elsewhere, the no-load standby consumption of these Poundland USB wallwarts is less than 100mW, quite possibly less than
50mW. Certainly more than low enough to make switching them off at the socket when unused a total waste of calorific effort just to shave 10 or 15 pence a year off the electricity bill.I couldn't detect the slightest hint of power draw using my Metrawatt analogue watt meter (on the 100W scale -2 watts per division) with a jeweler's loupe to closely observe the needle pointer's expected movement from a gnat's dick's worth of displacement from its steady state reading back down to the zero mark when switching the mains off[1]. IOW, the power consumption was immeasurably small on a device not crippled by the limitations of a digital readout limited to a resolution, at best, of a tenth of a watt.
[1] Switching on results in the usual 3 or 4 watt 'kick' due to the HT smoothing cap's inrush current so it has to be inferred from the switch off transient.====snip====
Oh yeah, I nearly forgot to mention that replacing just one of those 12v wallwarts however modified (add in buck converter or change of resistor in the volt sensing circuit) with a Poundland 5v1A USB wallwart is likely to pay for itself after little more than 6 to 11 months use[1].
Most of those older 12v wallwarts typically drew anywhere from 1 to 2 watts from the mains just by being plugged in before being connected to a load.
[1] If you're going to buy them a fiver or a tenner's worth at a time, just make damn sure you get (and keep) the receipt. :-)
You could but is it worth the hassle etc?
If you are an experimenter and want something for 'bench' use, rather than general domestic use, better to get some variable supplies off Ebay.
I bought 5 or 6 for about £9. Wide range input, up to something like
30V, variable (lower) output. Smaller than a match box. Ideal for all kinds of things. 'Clean' output, switchmode. Claimed >95% eff.En el artículo , Adrian Caspersz escribió:
Meant to grab a couple, but didn't spot them last time I was in. Clive certainly seemed happy with them.
En el artículo , Johnny B Good escribió:
:)
I thought the accepted term of measurement was a gnat's chuff.
Maybe we have to wait until article 50 goes a head :)
Sounds just like the 5 that arrived this morning, £5.99 free delivery, ordered Monday midday. I'll be checking how much RF muck they radiate and how clean the output is. The idea is buy meaty 12 V PSU and hang these DC-DC converter of it for the devices that normally live from wall warts at 3.3 V, 5 V, 7.5 V, 9V etc.
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