uninterruptble 6 volt supply; make or buy

I've tried in some radio group which I've forgotten, but didn't get a good answer, so it's back to good old diy.
I replaced my old Sony icf7600d as it is very awkward to fumble with, with a Roberts badged Sangean, and found that it loses it's brains in a power cut, (a not unknown thing just outside the M25), so no alarm in the morning, and no stations tuned even if there were, and no clock anyhow.
That's progress.
My good old sony had a separate processor battery that lasts for years, and the radio section works off a wallwart or batteries as required.
I thought the ultramodern Roberts would be at least as good, but if the mains supply is plugged in it disconnects the batteries, so everything dies in a power cut.
The batteries last about 6 minutes.
I've looked at other sets, and asked my local Sony shop in Chelmsford, and it seems they're all like this.
I've finally arrived at the point, thanks for hanging in this long.
Can I buy a supplier that is in essence a battery on trickle charge, or I could make one, but that involves design, which I reckon I can do, and buying the right opamps, regulators, and especially low ohm resistors, a number of values for adjust on test, a right PITA as I'm retired so don't have access to a company store no more?
Any suggestions, please, I've got a sealed 6v lead acid, is there a trickle charger or conditioner, or can I get a unit- seems to me there's a crying need for one, as even Sony sem to have lost the plot after they made my old set. I'd put it back, but there's no dial light, the LCD's are microscopic, setting it up and doing daylight saving and alarm time is a pain - But - it woked right...
Bugrit, modern times, rant dies into mumble....any one help, please?
mike r
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mike ring wrote:

Your battery needs a constant voltage of about 6.75 volts to keep it charged. I suggest you use a wallwart followed by a LM317 adjustable regulator and a series diode to the battery (to avoid back through the regulator during mains failures). Diode will drop about 3/4 volt so the regulator should be set to 7.5 volts.
I've just thought, you may be able to buy a wallwart with a regulator in it and a 7.5 volt output eg http://custom1.farnell.com/cpc/product.asp?catalog%5Fname=CPC+Catalogue&category%5Fname=Electrical%2C+Security+and+Test+%2D+Power+Adaptors+and+Converters+%2D+AC%2DDC+General+Purpose+Plug%2Din&product%5Fid &3668
the 500mA one should do. Still put the diode (1n4001 or similar) in series with the battery as above and then wire the battery into the radio.
If you need more info, then come back to me.
HTH
Bob
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Maplins but got stuck on the low value resistors I need for the feedback loops, and the fact that I'm bound not to get the right values - I hadn't thought of the series diode, that's a good point.
But i am concerned about overcurrent, bothe for the battery ( a 4a/h, so I figure max charge 200mA), and safety as it will be on for ever ish.
So I need also to worry about that, a simple voltage control won't be enough, or will it?
PS I've collected loadsa wallwarts, wanna buy some?
mike r
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Use a preset pot instead.
--
*I'm not being rude. You're just insignificant

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Hi.
There's a much easier way. Battery mains radios have 2 supplies, battery and mains. The battery supply is normally switched by insertion of the mains lead bacause it avoids a diode drop in the batt supply.
The solution I've used is to add 2 diodes. One diode goes in series with the output of the maisn supply, and the 2nd diode goes across the switch on the mains lead so that in a power cut, with the lead in place, the battery runs thru its diode to power the radio.
Result: the radio has its own UPS built in, for the 8p cost of just 2 diodes.
Note that because of the additional diode drop, the batteries will cease to work a bit sooner when backing up mains failure. When you intend to use it on batteries specifically, unplug the mains lead and it will run with no diode drop. Using low Vdrop diodes minimises this minor detail.
Note that this must be done right. Leaving out a diode or getting one the wrong way round could result in batteries going bang. So its only for people who are competent to do it.
Regards, NT
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mike ring wrote:

Should not be a problem if you get the wallwart supply correct and it must be a regulated one. Lead acid batteries will normally manage the current draw by themselves. Another poster mentioned possible hum problems. This can be a problem but an electrolytic capacitor across the battery should help here. If you go the unregulated wallwart and 317 route then you do not need low resistance values. 1mA down the feedback chain will be ample. If you make the resistor from the adjust pin to -ve 90% fixed and 20% variable via a small pot then you should be able to trim the output. The 317 will give a current limit too.
Bob

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mike ring wrote:

What I did on a older Roberts, after the PP9 had died yet again, was to replace the 9v battery with a socket, into which I plugged a 8.4V NICAD pack that I had spare out of a model aircraft.
Since I have a fast charger for this anyway that racks the charge back in 20 minutes, and the radio now lasts about a month or more on one charge (1700mA/h cells), that's what we use. The charger also works om a car battery if we get cut off from mains supply for several days. As happened last year. Local radio was our lifeline in that case.
Trickle charging from a simple mains charger usually introduces a lot of hum if the radio is on. However it could be adapted.
I am not sure if this solution exactly matches your requirements, but it suits us perfectly. We now have a 'bathroom friendly' portable radio with batteries that cost nowt to recharge, and so far the battery cost (about 18) has more than repaid itself versus the cost of PP9's (about 5 quid each, and lasting no more than a month) over about 10 charges in the last year.
I believe you can even get PP9 format rechargeables, but I modified the case to fit what I had.

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Most wall warts provide higher voltage than nominal so the radio should be happy on a float charged lead acid battery. Which IIRC is a fixed voltage supply.
On Sun, 2 Nov 2003 20:49:39 +0000 (UTC), mike ring

Lawrence
usenet at lklyne dt co dt uk
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Many thanks for all the replies; I _will_ have to use pots to get the right answers, but I don't even have a low value pot - those few I have or can salvage are far too high, so it's schlep down to Maplins in Sarfessex, then again for the right Rs, and I always forget something quitical; I so miss the days of doing these things on the company time using the company's boxes of bits - sigh...ish.
I'll have a look at the 2 diode solution from bigcat, it sounds great, but I won't really understand it unless I draw it - if it's as good as it sounds it could be the next catseyes.
Ta again
mike r

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Hi, I've done it on several radios etc, just so you know it does work.
Regards, NT
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snipped-for-privacy@meeow.co.uk (N. Thornton) wrote in

Not this time, I'm afraid, got the case off and next to impossible to get at the 6volt connector, 100 to one I'd wreck it.
Still, if I ever see a properly built radio again..
mike r
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Hi. Theres no need to get to it. Think :)
Regards, NT
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You've got me, BC, the pin that is disconnected by plugging in is internal, isn't it? I've got one in front of me and can't see how I can get to it without getting at the pcb, whch is hidden under a plastic extrusion, and will need very careful surgery aroun surface mounted bits to dismantle enough to get an iron on,
So I'm dim - how is it done?
mike r
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OK, first draw the idea on paper so you get whats trying to be achieved. Now, every connection youre after is connected to wires, PCB tracks and either a mains socket switch, battery holder, on-off switch, etc. I've yet to find a set where not one single point is accessible.
To comment any more I think a picture or 2 would ne needed.
Regards, NT
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snipped-for-privacy@meeow.co.uk (N. Thornton) wrote in

Yes ISWYM, and the one to prevent the battery supply exiting through the psu would have to be external - might even put it in the psu.
Might find there's already a diode there - if it comes straight off the rectifier you shouldn't get a reverse current, and it doesn't have to hold up all that long, I'm only worried about those sneaky cuts where you wake up late to find all the clocks have stopped/are slow/restarted from 0000, the heating timer ditto, etc.
I'll haVE A GO
MIKE R
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