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Basic Electronics » LED Array - Wall power problems

December 13, 2011
by Pretzel
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I'm able to use the 9v battery to run my lED array with no problems at all. However when I hook up the wall power I get nothing. I soldered the wires onto the adapter and tested the leads going into my breadboard on my multi-meter and am showing 14-15 volts on the leads but I'm getting nothing. I can disconnect that and use the battery and everything runs fine. Any ideas? I'm lost on this one.

December 13, 2011
by dgikuljot
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Are you sure you have the positive and negative connected correctly? Also is the DMM still showing 14v under load when you have it plugged into the circuit

December 13, 2011
by Pretzel
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Yes, I've checked the voltages on the leads and they are correct showing 14 volts going into the 5v regulator and 5v throughout the board afterwards. It seems like a weird problem to have.

December 13, 2011
by BobaMosfet
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What are the specs on this transformer? Perhaps it's not providing sufficient current? Does your LM7805 get warm? Is this transformer provided DC voltage?

BM

January 15, 2012
by Pretzel
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Transformer specs are 9V DC 300mA

I've tested the current and it seems erratic jumping between 2volts and 14-15. Nothing on the circuit gets warm. I've tested this over and over and the 9v battery powers it fine and with the adapter nothing. I've tried re-soldering the power adpater, splicing the transformer cable and putting the wire into the breadboard itself all with the exact same result every time. Battery works perfectly, nothing with the adapter. Any help would be appreciated I'm very new at this! Thanks!

January 15, 2012
by BobaMosfet
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Pretzel,

I would try a different transformer. What you are describing, sounds like the transformer is passing AC, not DC (perhaps it has failed in some way). Switch your DMM to AC, and see what the output on the transformer is, without it connected to your setup.

If the 9V battery powers your kit fine, you know it is not the kit or any component. You are isolated to just the transformer itself as the problem.

BM

January 15, 2012
by Pretzel
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oh wow your sure right. switched to AC on the multimeter and am showing 33V AC...

January 16, 2012
by hevans
(NerdKits Staff)

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Hi Pretzel,

That is certainly strange error. Very odd that you get 33V AC out of a wall outlet, since there should be 12V coming out of the wall outlet. You mentioned you spliced the leads on the transformer, did you try your multimeter in DC mode across those leads? Earlier you mentioned you were trying to measure current and then mentioned a voltage number.

Humberto

January 16, 2012
by BobaMosfet
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Humberto,

Don't you mean 120VAC (RMS)?

Pretzel,

In the interest of confirmation, I'd recommend you have another person with another voltmeter check things for you.

BM

January 16, 2012
by hevans
(NerdKits Staff)

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Ooops. Yes, there was probably supposed to be a 0 in there somewhere.

January 19, 2012
by sask55
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Over the last few days there has been a couple of threads that appeared to me to be addressing the same topic. http://www.nerdkits.com/forum/thread/1963/ and this tread. When I decided to power a Nerdkit board with a small wall adapter as I have done many times in the past, I was experiencing some of the same issues. I suspected that I could remedy the trouble by placing a fair size capacitor across the power supply output. When I did that it completely eliminated the trouble.

I decided to take a closer look at what was actually going on with some of these wall adapters. I did this test with a wall wart style adaptor that is rated as 9V DC 500 ma. I was interested in getting an indication of the output from this unit with various loads and with different levels of capacitor smoothing of the output. I used a few larger (wattage rating) resistors to load the supply at a number of levels. I also used a few different capacitors in parallel to (across) the output of the supply. I used a VOM in parallel to the cap. and supply to measure voltage. A second VOM was placed in series with the load resistors to measure current. The oscilloscope was also placed in parallel to the supply to give an indication of supply voltage over time.

The first thing I noticed was that with no load and no capacitor on the output this adapter produces bad DC power. There is a very large component of AC imposed on a DC offset voltage. I suspect that there is little capacitance after the rectifier built into this adapter. The second thing I noticed is that the peak voltages measured on the scope are quite high at around 20 Volts. The voltage swings from the highest to the lowest are more then 16 volts. The 60 hz AC power frequency is very evident in the “DC” output. I am posting a table showing some of my measurements with various load resistances and capacitor as well as a few screen shots from the scope.

The voltages and currents shown are as measured with the digital VOM’s. The resistances and capacitances are the nominal values marked on the components I did not measure them. The measured voltage did increase by just adding a capacitor on the output. This makes sense as with little or no load the cap will charge to near the peak voltage.

I think that if a person is intending on using a wal wart type power supply it is probably a good idea to incorporate a capacitor across the ouput to help smooth any Ac influence in the output. From the scope screen shots we see the effect that the capacitors have on smoothing and raising the measured output voltage. By adding load to the supply the output voltage will pull down with little effect on the quality of the DC. All this is to be expected but the extent of the variation with this adapter is interesting (at least to me).

table of results

Table of results of test

A few screen shots of the voltage output from the adapter

no load no capacitor about 20 V peak voltage

0 load no cap. 20 V peak voltage

165ma of load no cap. about 16 V peak

165ma load no cap. 16 V peak voltage

68ma of load large 6800uf cap the output is nearly flat.

68ma load 6800uf cap

there is just a small ripple left in the output.

68ma load 150uf cap still considerable 60hz ripple in ouput.

68ma load 150uf cap.

so; use a capacitor with your wall wart adapter It will likely help, a lot, and I dont think it could have any negative effects.

Darryl

January 19, 2012
by sask55
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Since my last post I have noticed that my results do not satisfy Ohms law, I=V/R. In some cases my results are not even close to what should be measured. I am trying to reason out why that may be the case. After measuring some of the resistors, the measured values do vary considerably from the nominal value. Then there would be the resistances in the alligator clip style test leads I was using, and the possible resistance in the connections. I was using two separate VOMs to measure the current and the voltage perhaps there is an accuracy issue there. However I think a good part of the discrepancy can be attributed to the voltage levels displayed on the VOM are not really 100% meaningful. The scope shows that without any capacitors the output voltage cycles between about 3.8V and 20 V 60 times per second. so; really what is the meaning of a DC voltage reading from that source. When measuring an AC source and the meter is indicating the RMS value of the changing voltage over a number of cycles. Since the voltage fluctuation seen on the scope does not appear to be a true sine wave and the VOM is set to measure DC (which is normally near static), I am not certain what that voltage reading is actually indicating. It is some approximation of the mean or average voltage based to some extent on properties of the meter itself.

Darryl

January 19, 2012
by Rick_S
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That sure doesn't look like DC to me. Even DC through a cheap wall wart that just has a diode bridge would be flatter than that. I don't know what you have going on, but when I look at an AC - DC wall wart that is supposed to put out 9VDC with my scope, I see a flat line at about 14VDC with no load.

Rick

January 19, 2012
by sask55
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Rick

I took a quick look at the output of another 8 adapters of various voltages and power ratings. These units where salvaged over the years from just about anything and are sitting in a drawer. One of the 8 has an uneven output, not as bad as the one I was testing earlier but not good either. The others are relatively flat and reasonably close to their rated voltage. As I said, I have made use of a number of these adapters in the past and never had problems like what I was experiencing with this one. I recognized that if other people have units that produce an output anything like this one that may explain the problems they have described. I was able to use this adapter on a Nerkit board with no apparent problems, after adding a 150uf capacitor. The measured output voltage is higher then required or desired but the 7805 seams to be handling it for now.

It clearly states DC9V 500mA on the adapter. I don’t know if it ever did produce what I would consider DC power but it is not doing it now. The output of this unit looks to me to be close to a sine wave with the negative portion of the cycle inverted, and then the entire output offset from 0V by about + 3.8 volts. I suppose it is DC in the sense that the current is all going the same way.

You may have noticed that in this cold weather I have way too much time on my hands and spend it doing relatively useless things that I find interesting.

Darryl

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