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power supply

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sam2

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I built this power supply **broken link removed** for a 9v am radio and it has a noticable hiss over the 9v battery I was using. Can I do anything to clean it up or should I try another power supply.
Thank you, Sam
 
sam2 said:
I built this power supply **broken link removed** for a 9v am radio and it has a noticable hiss over the 9v battery I was using. Can I do anything to clean it up or should I try another power supply.
The LM317 is an excellent voltage regulator.
Maybe yours has its wires too long so it oscillates.
Did you use all the capacitors it shows?
 
audioguru said:
The LM317 is an excellent voltage regulator.
Maybe yours has its wires too long so it oscillates.
Did you use all the capacitors it shows?

So that is unusual? If I built it correctly, I should not be having problems with it.
I was using test leads from the power supply to the radio about 15 inches long,but the circuit is built on perfboard using component leads and one small jumper, nothing longer than an inch.
I did not use the 0.1 tantalum capacitor, because the regulator and filter are close, but everything else is the same.
I will keep trying .
Thanks, sam
 
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You said the power supply causes hiss from your AM radio. Hiss is treble audio frequencies. AM radios are narrow band and don't produce treble frequencies. AM reproduces amplitude interference from light dimmers and sparking motors.

The LM317 needs an input voltage at least 2V higher than its output voltage. Is your unregulated input voltage higher than 11V?
 
I say hiss, because thats how it sounds to me. That may not be the proper term. Its not rough sounding with lots of popping and cracking. The input to the regulator is 17.5 v . The power supply seems to work well enough except for the static. I have tried different dummyloads and it is always right on voltage wise 8.8v.( I used the recomended resistors for 9v, but am getting 8.8v but its consistant)
 
I think the power supply is coupling interference from the electrical mains to the AM radio. AM is usually full of static sounds.

Some pc printers have a ferrite filter clamp on their power cord to keep from spreading digital interference. Try one on the power cord of your power supply.
 
Are you using a 22000 uF capacitor after the bridge rectifier?? (with proper voltage rating?). 1000 uF might be a more practical value. The 0.1 uF capacitor is useful even if the LM317 is close to the filter, you can use a ceramic or polyester capacitor.
An optional 10 uF capcaitor across R2 helps improving the ripple rejection (if you add it, put a diode across R1, as suggested in the datasheet of the LM317).
Another 10 uF or 100 uF capacitor should be mounted as close as possible to the radio (if it hasn't any).
 
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hi sam,
Google for 'LM317 instability', you will find a number of causes/cures.

Regards
EricG
Let us know if you fix it, it appears to be a common problem.
 
Thanks you guys, I used a 2200 uf 35v cap. Sorry I forgot to mention that. The transformer is only 12.6 v 300 mah.
I assumed that 22000uf value was a typo.
I have a couple of 23000uf caps and they are 2" in dia and 2'' high!
 
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I have tried three other power supplies- a lab supply ,a homemade variable supply(LM338) and a converted computer power supply with a LM 317 on its output. They all have more static than the 9v battery.

The computer power supply was the worst and the lab supply the best , but none as good as the battery.

eng1 ,I looked at the 317 data sheets and found the circuit you mentioned, and will build it on a breadboard first.
Sam
 
Try powering the LM317 regulator from a 12V battery to see if it makes hiss in the AM radio.
 
sam2 said:
The transformer is only 12.6 v 300 mah.
That makes no sense, you can't have a 300mAh transformer, do you mean a 300mA transformer?

With a 300mA transformer the maximum current you can safely draw from the regulator is 212mA; how much current does your AM radio draw?
I assumed that 22000uf value was a typo.
I have a couple of 23000uf caps and they are 2" in dia and 2'' high!
I doubt they are 23,000:mu:F capacitrtors; it isn't a standard value.

Anyway 22,000:mu:F is pretty excessive for only 212mA, you only really need 2,200:mu:F I would 3,300:mu:F to be on the safe side. Even so using larger capacitors is unlikely to do any harm so I don't see the problem.
 
Larger the capacitor, higher the current spikes and the surge current in the diodes... the high inductance can cause noise amplification too.
 
Hero999 said:
That makes no sense, you can't have a 300mAh transformer, do you mean a 300mA transformer?

With a 300mA transformer the maximum current you can safely draw from the regulator is 212mA; how much current does your AM radio draw?


I doubt they are 23,000:mu:F capacitrtors; it isn't a standard value.

Anyway 22,000:mu:F is pretty excessive for only 212mA, you only really need 2,200:mu:F I would 3,300:mu:F to be on the safe side. Even so using larger capacitors is unlikely to do any harm so I don't see the problem.

sorry,I meant 300mA
150mA
They are sprague powerlytics 23,000uf 35v
I have built a few power supplies( I still dont know sh!!# about electronics)and have looked at many schematics and have a couple of books on building and designing them, so when I saw 22,000uf for the filter, I thought it was a typo. Its way out of line with any of the formulas in the books and any "rules of thumb" I have read.

The problem I am having is finding a power that wont make noticable noise on an AM radio, all the ones I have tried are noisier than a battery.I am going to try one more.
I think audioguru might be right and its noise from the mains.
Thanks, sam
 
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I took eng1's advice and used the "Adjustable Regulator with improved Ripple Rejection" from the 317 data sheet. Next time, the data sheet is the first place I will look when I use a component.

It worked great, its as smooth and quiet as the battery.

I also adjusted the voltage down from 8.8 volts to 8.5, and that also removed some backround noise and made the voices clearer.
Can anyone tell me why lowering the voltage made it clearer? It did not just lower the volume, there is noticeably less static and the voices are clearer.

This was a project for aviation tech school. Our school is at an airport, and the instructor wanted a radio to listen in on local traffic from the class room.
I got the pcb artwork and parts list from: **broken link removed**
I was told this is a crude RX, but it works very well.

Thanks everyone for the help. sam
 
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