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Old 6th Jul 2007, 4:48 pm   #1
Pete_kaye
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Default Interference from a battery charger

I was testing an oldish 12V car radio yesterday and it worked well on FM and MW from a short aerial but on LW :drowned out by the battery charger I was using as a supply.Is this what you would expect?I will try to connect it to a car battery soon but intersted in what was said.
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Old 6th Jul 2007, 4:54 pm   #2
mikelect
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Default Re: Interference from a battery charger

Yep charger voltage isn't smoothed so is the most wonderful source of crud try it with a battery across the terminals that should streighten it out a bit

Mike
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Old 6th Jul 2007, 4:55 pm   #3
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Default Re: Interference from a battery charger

Hi Pete,

First of all, be aware that the voltage you can get from some "12V" battery chargers can be MUCH higher off load, and so you can damage equipment if you try to use one as a power supply. I would check with a meter to make sure the output voltage from yours isn't too high.

As for the interference, does your charger use a modern switch-mode power supply? In other words, does it have a small PCB with a tiny lightweight transformer on it, rather than using a conventional, heavy "lump of iron" transformer? These SMPS are notorious for interfering with am radio transmissions.

Nick.
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Old 6th Jul 2007, 5:00 pm   #4
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Default Re: Interference from a battery charger

Not smoothed and most of them were half wave rectified. Usually, the transformers buzz and rattle as well. Definitely built on the cheap but good for what they were intended for.

Pete.
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Old 6th Jul 2007, 8:41 pm   #5
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Default Re: Interference from a battery charger

LW is more prone to this sort of interference.

A car battery charger is definitely not a good bench PSU for this sort of work. A PSU from a scrap computer should work much better though you still might get a bit of hash. If you get a PSU from an old AT computer (one with 2 motherboard power connectors not one) it will 'just work'. If you use an ATX supply you will need to strap one of the connector pins to turn it on (specs here: http://www.hardwarebook.info/ATX_Power_Supply)

Paul
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Old 6th Jul 2007, 8:55 pm   #6
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Default Re: Interference from a battery charger

I've found those old 12v mains units intended for use with C.B. radio work well in this application, They still turn up cheap at car boots .

Regards, Mick.
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Old 6th Jul 2007, 9:16 pm   #7
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Default Re: Interference from a battery charger

A long time ago, after much collecting of heatsinks and meters to build a bench PSU, I took the plunge and bought a £100 bench PSU new. 2.5 amps 0-30 volts, current limited. It also has fixed 5 and 12 volt outputs up to .5amps.

I've used it a lot and never regretted the purchase. They're a lot cheaper now, about £50. If you can find one second hand, they should be cheaper still.

Battery chargers whether linear or switch mode just aren't good for testing radios, or playing with transistor circuits. I haven't tried switchers from PCs for anything like this, but they tend to chuck out a lot of RF noise and I'd avoid using them.

Pete.
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Old 6th Jul 2007, 9:33 pm   #8
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Default Re: Interference from a battery charger

Quote:
Originally Posted by XTC View Post
I haven't tried switchers from PCs for anything like this, but they tend to chuck out a lot of RF noise and I'd avoid using them.
I've actually run a car radio from a PC PSU. It worked well with very clean DC. I've no idea how typical this is though.

Paul
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Old 6th Jul 2007, 10:27 pm   #9
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Default Re: Interference from a battery charger

Quote:
Originally Posted by paulsherwin View Post
I've actually run a car radio from a PC PSU. It worked well with very clean DC. I've no idea how typical this is though.

Paul
In that case, I bow to your experience, but I'd still rather use switchers to power computer equipment or something else designed to use them, or where the noise doesn't matter.

Pete.
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Old 6th Jul 2007, 11:48 pm   #10
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Default Re: Interference from a battery charger

A car battery charger is also a good way of exploding any supply decoupling capacitors in there as they try to smooth the supply out!!!

Regarding SMPSU's for radios, the cheaper wall wart adapters these days can have switching regulators (but a mains transformer) and these will often interfere / cause whilstling/motorboating with a tranny, however I would imagine a car radio is more robust as its meant to withstand alternator/dynamo/ign noise to some degree! I would say a PC PSU is filtered beautifully and meets modern CE EMC standards for noise...but will deliver a whole load of current if there's a fault! Use a bench power supply!
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Old 7th Jul 2007, 12:08 am   #11
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Default Re: Interference from a battery charger

Quote:
Originally Posted by plumbweiss View Post
AI would say a PC PSU is filtered beautifully and meets modern CE EMC standards for noise...but will deliver a whole load of current if there's a fault! Use a bench power supply!
Or an appropriate fuse
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Old 7th Jul 2007, 8:28 am   #12
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Default Re: Interference from a battery charger

With some PSUs it is possible to fit a clamping diode - i.e. a zener diode rated at or close to the maximum safe voltage, accross the output rails, so for use with 12v. equipment, a 15 volt ZD could be fitted. Not sure what effect this would have on SMPS, though
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Old 11th Jul 2007, 4:34 pm   #13
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Default Re: Interference from a battery charger

Many thanks for all the replies.My Halford's charger must be ~15 years old but it is switchable from 6 to 12 volts and High/ Low.It was giving 13 V on Low with my Digital meter when in use.I am sure that the interference will disappear once connected to a battery as I have found it is possible to remove most of the interference on LW by touching the long wire loft aerial to the chassis while a length of coaxial is plugged in the aerail socket.Don't know why
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Old 11th Jul 2007, 4:56 pm   #14
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Default Re: Interference from a battery charger

Don't use a battery charger. It's filthy power that comes out of a charger. It may read 13v on a meter, but there will be brief peaks up to quite a high voltage.

Don't risk it. The set will fail eventually.

Cheers,

Steve P
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Old 11th Jul 2007, 11:39 pm   #15
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Lightbulb Re: Interference from a battery charger

Personally, I'd agree with the comments made above about the unsuitability of using a car battery charger for this application. Having said that 'though, if nothing else is available, perhaps the approach of fitting a high uF cap. (say, 10,000 uF) across the charger's O/P followed by a low R / high watt resistor feeding a 12v. zener (say 5-watt), ought to be better than using the charger "as it is".

The value and power rating of the R can be calculated, of course (knowing the rectification scheme, ac volts from the transformer and load current), but personally I'd try the traditional "suck it 'n' see" approach. As an off-the-cuff guess, choosing R = 2 ohms seems a good starting point.

Al / Skywave / 110707
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Old 15th Jul 2007, 4:33 pm   #16
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Default Re: Interference from a battery charger

Battery chargers generally provide a mean value voltage due to rectification,any attempt to smooth this will cause the voltage to rise to a destructive level.The interference may be due to the rectifier,it probably uses silicon diodes which can produce transients,this is common when these are used,get a proper regulated power supply.
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Old 19th Jul 2007, 8:33 am   #17
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Default Re: Interference from a battery charger

You will probably find LW poor when the set (or more importantly, the aerial) is out of the car anyway. The aerial matching circuits in car radios are odd to say the least (full band coverage with just a metre long rod of metal sticking up and no real earth?) and in part rely on the aerial being mounted on a vehicle with it's base well grounded to the body.

For bench testing I used a modified 4A car battery charger, it's fitted with a big silicon rectifier and an even bigger smoothing capacitor. I power it via a variac and monitor the output on a voltmeter, this arrangement works OK for occaisional use.

The forum members kindly helped me through similar problems with a Philips valve set a few weeks ago, lack of LW was disturbing on the bench but in the car out in the open Radio 4 booms out (shame it's all cricket!). I think I have the only ancient VW with a Bang & Olufsen loudspeaker in it though.
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Old 21st Jul 2007, 7:27 pm   #18
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Default Re: Interference from a battery charger

On a similar note, I was troubled by heavy "micro-noise" type interference a while ago on medium wave and couldn't find where it was coming from until I associated a flashing led on a portable drill battery charger with the pulsating noise. A lot of switched mode units are used in this type of application and seemingly aren't too well shielded.

Biggles
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Old 22nd Jul 2007, 7:49 pm   #19
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Default Re: Interference from a battery charger

I use a hefty Tandy 13.8v 3A PSU for fixing transistor car radios. This Made in England (!) unit has a traditional linear regulator and will just cope with powering a marine VHF radio on full transmit power (25W). For the big valve car radios I use one of those portable "power booster" thingies with heavy jump leads. Mine has a cigarette lighter socket and contains a 12v 17Ah jelly-acid battery. Power don't come much cleaner than that!

Regarding wall-warts. Yes, I've got one I bought intending to use it for running radios on soak test which would free up my variable voltage bench PSU. Only thing is, it wipes out MW/LW reception. It now runs my Evoke DAB workshop radio.

Beware of old CB PSUs. They often used very light heatsinks which caused the series pass transistor to overheat and fail, with predictable results when the unregulated 20+ volts appeared on the output. Best thing to do with one of these is to remove the old pass transistor and paint the heatsink both sides with matt-black paint, taking care to leave clear the transistor's seating area. Next, assuming the pass transistor is npn (as far as I remember they all were) fit a good quality 2N3055 using adequate h/sink compound and a new mica washer. Then check the driver transistor and zener and if you have any doubts, replace.

Finally, bear in mind that car radios in particular seem to be particularly sensitive to mains-borne interference from switchmode PSUs and compact fluorescent ballasts. It's so bad that sometimes I have to wait until after all the neighbours have gone to bed (hopefully after turning off all their pesky interfering gadgets) before I can test a repaired set for LW reception. I find that this type of interference blankets all of LW, but mostly just the centre of the MW band.

Pete
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