Constructing trouble

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Postby Andrew Davie » Mon May 14, 2007 1:19 pm

DrZarkov wrote:I still have a resistor-problem I think. I've testeds my 18 V transformator, it has 18 V. I've tested the LED-array, it is working. Connected to the circuit, I get a faint glowing, by far not enough. At the LEDs something between 10.4 V and 11 V arrive. Unfortunally I haven't such a good equipment like an oscilloscope to test. So now I have to start debugging in a more conventional way...


Did you get this working? Assuming (say) strings of 3 LEDs each, at about 3.5V each, that would be 10.5V (in your range). Is the 18V you measured the DC voltage after passing through your rectifier? If so, then the voltage across the resistor for a single string would be (18-10.5) = 7.5V and for a current of (say) 25mA that would be 7.5/.025 --> 300 ohms. So each string should have about a 300 ohm resistor, and if there were 48 LEDs in total that would be 400mA -- too big for the diodes, I think. Also, I wonder if your power supply can deliver 18V @400mA ?

How about cutting the LEDs down to 12, or 24, to see how it behaves?
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Postby DrZarkov » Mon May 14, 2007 4:08 pm

No, no, I sized it down to 18 LEDs after Klaas asured me that it won't work with 48 LEDs. (I use now an "Andrew circuit TM") Unfortunally I had not much time this weekend, so no progress yet...
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Postby Klaas Robers » Tue May 15, 2007 2:26 am

Gents, it is not that difficult with the resistors.
1. If you have series of 3 LEDs of 3,6 volt each,
2. the voltage accross the LEDs is about 11 volt.

3. Suppose that you have a DC voltage of 18V,
4. then you have to get rid of an extra 7 volt.
5. if you want to run e.g. 30 mA through a series,
6. then you need a series resistor of 7 / 0,03 = 233 ohm
7. so you take 220 ohm in series with each string.
8. Then you can run them immediately on the 18 volt.

9. However, if you use the LED-driver then the transistor takes over the function of the resistor.
10. Then in principle there is no resistor needed,
11. regardless the value of the voltage, as long it is more than 11 volts + say 5 = 16 volts.
12. The voltage may even vary, so needs no stabilisation.
13. The transistor takes up the remaining voltage.
14. But to match the light output of each string of LEDs it is advisable to place a low ohmic resistor, e.g. 33 ohm, in series with each string and then connect the strings in parallel.
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Postby DrZarkov » Tue May 15, 2007 2:47 am

Point 1. to 13.: O.k.

But 14.: "Of course" I've taken in my array resistors of about 220 Ohm (or let it be 270 Ohm, but I've got the calculated value). There is some very slight flickering, so I will try this evening just to change the resistors. I have a bunch of 27 Ohm (I think) resistors which I've got together with my LEDs.
I hope that is the only mistake in the two circuits...
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Postby DrZarkov » Wed May 30, 2007 4:00 am

The last thing I did 2 weeks ago was replacing the resistors on my LED-array with 27 Ohm types, the array is working. Connected with the driver it was still not working. The last 2 weeks I was on holiday and I had not much time, and I concentrated on the mechanical work (which is working very fine, the disc is spinning, the magnifying glas is installed, the LED-array with the diffusor is on the right place, and I started with the housing), and the 5 W audio amplifier is working fine, too.

Together I came back to the video-driver. I found a mistake: A completely other resistor had three "0" to many, I took the wrong one. A good reason that it hasn't enough power. I controlled with my multimeter the in- and outputs, and I had ridiculous numbers on the display. I had to change the power input capacitor and the IC HEF4053, and now the power is normal. 18 Volt input, and right from the transistor the 12 Volt output. Short test: Still not working. :-(

I had to stop working for today, we are invited this evening to friends. I hope it is something very silly and easy before I despare... Unfortunally my testing-equipment is very bad. I have a digital multimeter and an old one from the 50th, which seems to work better than the new one. Not perfect...
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Postby DrZarkov » Sat Jun 02, 2007 12:41 am

Work continues: Now the LED-array starts flickering! That could be good news, if the CD-Player was connected. I wonder what causes the flickering (not a constant glowing) of the LEDs. Hm. The LEDs are not fully bright, so there must be a disturbing signal somewhere in the circuit.
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Postby gary » Sat Jun 02, 2007 1:10 am

DrZarkov wrote:Work continues: Now the LED-array starts flickering! That could be good news, if the CD-Player was connected. I wonder what causes the flickering (not a constant glowing) of the LEDs. Hm. The LEDs are not fully bright, so there must be a disturbing signal somewhere in the circuit.


Mains hum perhaps? Check all grounds.
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Postby DrZarkov » Sat Jun 02, 2007 2:34 am

Found it, fixed. But now the LEDs stay dark, still no picture. :(
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Postby Viewmaster » Sat Jun 02, 2007 2:53 am

DrZarkov wrote:Found it, fixed. But now the LEDs stay dark, still no picture. :(


I daresay this all sounds very obvious, especially if you have done a great deal of vero/pcb board soldering, but it is surprising how tiny slithers of solder can short out tracks, or sometimes joints not be made, or overlooked!
Over the years, I view all my soldering carefully with a watch makers eyeglass before ever considering powering them up.
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Postby gary » Sat Jun 02, 2007 3:02 am

I would hazard a guess that if you were getting mains hum to turn your LEDs on before that the output stages are working and that now the problem is that you input signal isn't arriving so check that part of the circuitry first.
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Postby DrZarkov » Sat Jun 02, 2007 7:38 pm

Very strange: By measuring the input I've pressed by accident my thumb on the backside of the PCB where the two capacitors of 1 uF and 0.82 uF are soldered in, and it worked! The contacts are rock.solid, so it must be a short circuit caused by my thumb which let it work. So i guess that there is something wrong with the capacitor. Maybe it had been grilled, too, together with that IC. That means waiting until monday, because i can't get any spare parts by now... Or I have to watch with a pressed thumb on the circuit :)
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Postby gary » Sat Jun 02, 2007 8:15 pm

DrZarkov wrote:Very strange: By measuring the input I've pressed by accident my thumb on the backside of the PCB where the two capacitors of 1 uF and 0.82 uF are soldered in, and it worked! The contacts are rock.solid, so it must be a short circuit caused by my thumb which let it work. So i guess that there is something wrong with the capacitor. Maybe it had been grilled, too, together with that IC. That means waiting until monday, because i can't get any spare parts by now... Or I have to watch with a pressed thumb on the circuit :)


Just as a sanity check... I noticed that on the PC Board that there are multiple holes for different sized capacitors, some holes are for vertically laid caps and some for horizontally, you couldn't have used the wrong ones could you?
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Postby Klaas Robers » Sat Jun 02, 2007 8:51 pm

Volker, have you connected the Clamp select input to ground? Otherwis the HEF4053 doesn't know what to do. What you describe is that the 4053 switches are in the centre (off) position. Then the DC-restorer doesn't work, nor the clamp. The conductivity of a finger can make the difference.
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Postby DrZarkov » Sun Jun 03, 2007 7:17 am

Yes, the clamp-select wire is grounded. I even tried it to ground at several places, but no difference. The only thing is, if I switch on the monitor, the LEDs will glow up very shortly. But no reaction when the signal from the CD-player comes. (Until I use my magic thumb, then I even have a picture, but very dark. Strangely it is a normal positive picture, on my other device with the simple driver I get a negativ picture with the same CD-player.)
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Postby DrZarkov » Sun Jun 03, 2007 8:55 pm

Update: I've used the daylight to controll all contacts on my PCB. I found no short circuits and no missing contacts. It is still not working. :(

Update: Can it do any harm, when I replace my thumb with a high-Ohm resistor? This is of course not very scientific, but if it works...
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