Fluctuating output voltage from wall wart adapters.

[Sunny55]

Hi,

I noticed that the output voltage from wall wart adapters are not constant(keeps fluctuating). If i apply that fluctuating voltage to a step down converter circuit, the output voltage from the converter circuit also fluctuates.

Fluctuating voltage is measured in no load condition.

How can i solve this problem? The step down circuit is an LM317 variable voltage regulator circuit.



Thanks,
Sunny55.

 

[Genovator]

Fluctuations are highly noticed in transformer-type adapters. Is your adapter based on transformer or SMPS? Is it AC or DC type? What is the rated voltage output? Try adding a large value electrolytic capacitor (if it is DC), and if possible use a zener diode of desirable output voltage.

 

[betwixt]

Most transformer based wall warts are not regulated. Most SMPS ones are, but often not very well. However, if you are saying the output of your LM317 regulator is fluctuating there is either insufficient input voltage to it from the wall wart (you need around 2V more in than you want out) or it is faulty.

Brian.

 

[KlausST]

Hi,

Yes, the output of the regulator must be stable.

What output voltage do you expect and what do you see instead?
What measuring method do you use? Is it reliable?

Klaus

 

[bigdogguru]

Please post a schematic of your LM317 voltage regulator circuit, so that it may be examined.

What is the stated output voltage and current rating of the wall wart adapter?

What is the intended output voltage and current rating of your LM317 voltage regulator circuit?


BigDog

 

[Sunny55]

Hi guys,

I am giving a 1 time response here to all.

@Genovator : Should be transformer type. It AC to DC adapter. Rated input voltage 240VAC.

@betwixt : My Wall wart supplies 16.6VDC. I connect it to LM317 and step down to 13.5V.

@KlausST : 13.5V. I saw voltages fluctuating. Measured using voltmeter.

@bigdogguru : Circuit as here :**broken link removed**
Wall wart output rating: 12V, 800mA., LM317 output voltage needed: 13.5V, 100mA.

 

[KlausST]

Hi,

Please provide more detailedinformations.

@KlausST : 13.5V. I saw voltages fluctuating. Measured using voltmeter.

If it is always 13.5V, why do you speak of fluctuation?

You don't say what output voltage you expect. You don't say what fluctuations you see.

We can not know this informations..

Klaus

 

[Sunny55]

Hi,

Please provide more detailedinformations.


If it is always 13.5V, why do you speak of fluctuation?

You don't say what output voltage you expect. You don't say what fluctuations you see.

We can not know this informations..

Klaus

Expected output Voltage: 13.5V. i saw only voltmeter.... 13.5V, 13.3V.. numbers kept changing.

 

[D.A.(Tony)Stewart]

Step-down converter with no feedback to regulate output will follow the peak voltage of an unloaded AC input. Loaded may drop up to 40% to Average after rectification depending on effective RC filter and loading.

 

[Sunny55]

Step-down converter with no feedback to regulate output will follow the peak voltage of an unloaded AC input. Loaded may drop up to 40% to Average after rectification depending on effective RC filter and loading.

Hi, i am using the wall wart as a trickle charger for a SLA battery. Can i use it to charge the battery with small voltage fluctuations?

Pls guide.

 

[KlausST]

Hi,

My Wall wart supplies 16.6VDC.

Is this the rated voltage or the measured voltage at no load?

Please measure the LM317 input voltage. What range (min ... max) do you see?

Fluctuating voltage is measured in no load condition.

Is this true for all your given measurement values? Or is there sometimes the battery connected?

How reliable are your DVM readings? Are they stable when you measure the voltage of an unloaded battery?

Wall wart output rating: 12V, 800mA., LM317 output voltage needed: 13.5V, 100mA.

I hope you are aware that the LM317 needs about 1.5V voltage drop. So if the input voltage is 12V you maximally (reliably) can expect an output voltage of 10.5V.

Klaus

 

[chuckey]

Sounds to me as though the DVM cannot cope with a DC with 100% ripple. Or its internal sample frequency/LPF is not compatible with the ripple frequency.
Frank

 

[betwixt]

@betwixt : My Wall wart supplies 16.6VDC. I connect it to LM317 and step down to 13.5V.

16.6V is an unusual voltage for a wall wart, is that the voltage marked on it or is it what you are measuring with your meter. If it's your meter reading it could well be a nominal 12V adapter but measured under no load, in other words it would be expected to drop to 12V at or near to it's rated current, it may not be proving enough voltage to the regulator while under load. Can you measure the voltages on all three pins of the LM317 and tell us what they are please. Take the measurement with no load and also with the SLA connected.

Brian.

 

[Sunny55]

Hi guys,

I have another idea in mind.

Planning to connect the wall wart to a 16V zener diode which will give constant 16V and then transport to the LM317 circuit and step down to 13.5V.

Wall Wart(16.6V) >>> 16V zener >>> LM317 >> down to 13.5V

What do you guys think about it? Going to work smoothly?

Thanks,
Sunny55.

- - - Updated - - -

Hi,


Is this the rated voltage or the measured voltage at no load? (Ans: measured voltage at no load using voltmeter)

Please measure the LM317 input voltage. What range (min ... max) do you see? (Ans: 16.6V, 16.5V )


Is this true for all your given measurement values? Or is there sometimes the battery connected? (ans: no battery is connected)

How reliable are your DVM readings? Are they stable when you measure the voltage of an unloaded battery? (ans: without battery connected, output voltage keeps changing from 13.5V, 13.4V)


I hope you are aware that the LM317 needs about 1.5V voltage drop. So if the input voltage is 12V you maximally (reliably) can expect an output voltage of 10.5V. ( Ans: Will need more time to find out)

Klaus

Hi, all the replies in ().

- - - Updated - - -

16.6V is an unusual voltage for a wall wart, is that the voltage marked on it or is it what you are measuring with your meter. If it's your meter reading it could well be a nominal 12V adapter but measured under no load, in other words it would be expected to drop to 12V at or near to it's rated current, it may not be proving enough voltage to the regulator while under load. Can you measure the voltages on all three pins of the LM317 and tell us what they are please. Take the measurement with no load and also with the SLA connected.

Brian.

Wall wart rated output V @ 12V. No load output V measured @ 16.6V,16.5V. I am not connecting the SLA battery since the voltage is not stable .

 

[D.A.(Tony)Stewart]

it can work but a better way is to open a MOSFET switch when Vbat reaches 13.8V using any accurate reference.

 

[KlausST]

Hi,

Planning to connect the wall wart to a 16V zener diode which will give constant 16V and then transport to the LM317 circuit and step down to 13.5V.

Wall Wart(16.6V) >>> 16V zener >>> LM317 >> down to 13.5V

I can't see why this should give an improvement. It will increase power dissipation..In the zener.
The LM317 has no problem when input voltage is above 16V. But it has a problem when it is below 15V.

The input voltage must be more than 15 V all the time. If it is only a part of a millisecond below 15V, it can not safely output 13.5V.
Maybe a electrolytic capacitor at the input can help.

***

How reliable are your DVM readings? Are they stable when you measure the voltage of an unloaded battery? (ans: without battery connected, output voltage keeps changing from 13.5V, 13.4V)

Here I ask to measure the battery voltage. But your answer sas without battery....
....But now you tell 13.4V....13.5V --> This means only one LSB of your display jumps. If this is your problem, then it can't be avoided.
Every analog to digital conversion has this problem: There is more than 1 LSB uncertainty.
--> use a DMM with more digits, to see if the analog voltage changes in the range of 100mV.

Klaus

 

[betwixt]

Adding a Zener and associated series resistor is pointless and will probably make matters worse.

The underlying problem is that wall wart. You say it is rated at 12V but actually produces 16.6V, this is suspiciously like (12 * sqrt(2)), in other words the output of an unregulated supply that has reached crest voltage. The voltage will drop rapidly as current is drawn from it and under the load of charging the SLA, particularly if it is deeply discharged, it will be insufficient ahead of the LM317. This is why I requested the voltages on all three pins with no load and with the SLA connected, it would give a clue about the wall wart being capable or not.

To be honest, it may be easier to just use a series resistor to limit the charge current and forget about regulating the voltage. Under charging conditions you may not need to drop the voltage at all and with such a small overhead available it would just put the SLA in trickle charge as it topped up. It isn't the ideal method but it would probably be most practical and cetainly cheapest!

Brian.

 

[Genovator]

@Sunny55

16.6V in output is indicating it is a 12v RMS transformer based adapter. You need something higher than that. Or you have to use a large capacitor bank trying to make the peak voltage as the RMS (quite inefficient). A 16v zener is not of much help.


How about using a voltage doubler at the output of the transformer? (If high current is not an issue...)