I'm building a power supply for the tent clock XO following
the generic schematic shown on www.tentlabs.com
It looks like a pretty straight forward circuit, but I'm
getting 1.68V at the XO input. probing backwards, the
voltage divider is giving me
1.68V across the 56K, and 6.15V across the 22K. This
doesn't make sense. I was expecting 5V+diode drop across
the 56K when the transistor is ON. I checked my soldering,
all seemed okay. I'm using NTE123ap as the npn, and it's
supposedly to be equivalent to BC547.
Any ideas? Thanks.
the generic schematic shown on www.tentlabs.com
It looks like a pretty straight forward circuit, but I'm
getting 1.68V at the XO input. probing backwards, the
voltage divider is giving me
1.68V across the 56K, and 6.15V across the 22K. This
doesn't make sense. I was expecting 5V+diode drop across
the 56K when the transistor is ON. I checked my soldering,
all seemed okay. I'm using NTE123ap as the npn, and it's
supposedly to be equivalent to BC547.
Any ideas? Thanks.
related problem
Guido,
I just built a copy of your XO supply schematics and I'm getting only 4.49V measured after the BC547 and before the bead. It's exactly 8.00V after the MC7808 (supposedly equivalent to LM7808), and I have the exact resistor values in place. As caps I am using 0.1 uf Wima MPK10 and 100uF 16V Blackgate N's.
Any ideas what may be off? Did I overheat the tiny BC547 during install? Searched the archive and found this thread, but it doesn't answer my problem.
Your site lists the supply should be 5V +/- 5% and I'm around minus 10%, so I don't really want to hook this up. Just wondering how close to 5.00V I should be getting with that circuit.
thanks
Peter
Guido,
I just built a copy of your XO supply schematics and I'm getting only 4.49V measured after the BC547 and before the bead. It's exactly 8.00V after the MC7808 (supposedly equivalent to LM7808), and I have the exact resistor values in place. As caps I am using 0.1 uf Wima MPK10 and 100uF 16V Blackgate N's.
Any ideas what may be off? Did I overheat the tiny BC547 during install? Searched the archive and found this thread, but it doesn't answer my problem.
Your site lists the supply should be 5V +/- 5% and I'm around minus 10%, so I don't really want to hook this up. Just wondering how close to 5.00V I should be getting with that circuit.
thanks
Peter
Hi Peter,
I just did a quick analysis of the circuit and it looks like its pretty dependant on the current gain of the transistor. The spec sheet gives a fairly wide spread of current gain, if yours is on the low side the current through the base will be only a little less than the current through resistive divider, thus throwing off the voltage.
Guido's suggestion here sounds verry good in this case, it increases the current through the divider by 10x so the base current is a much lower percentage.
You can measure the voltage at the base, unloaded it should be 5.74 volts, the transistor has a Vbe of right around .7V so it should come out just about right if the transistor isn't pulling too much current.
John S.
I just did a quick analysis of the circuit and it looks like its pretty dependant on the current gain of the transistor. The spec sheet gives a fairly wide spread of current gain, if yours is on the low side the current through the base will be only a little less than the current through resistive divider, thus throwing off the voltage.
Guido's suggestion here sounds verry good in this case, it increases the current through the divider by 10x so the base current is a much lower percentage.
You can measure the voltage at the base, unloaded it should be 5.74 volts, the transistor has a Vbe of right around .7V so it should come out just about right if the transistor isn't pulling too much current.
John S.
Guido's suggesteion worked - things went right up to 5.01V stable.
Now if I could only figure out what I messed up in my DAC after installing this, because now the pin 6/8 voltages on the DACs are fluctuating up and down from 3.7 to 4.5V - none of that should have anything to do with the circuit I modified. John, you have the same DAC - I just cut the two jumpers going to the reclocking part of the DDDAC board and inserted the output from the new supply into the same holes, adding a bead right before the clock and bypassing the clock underneath with the suggested 0.1uF cap. Why now the DAC is all over the place is a mystery to me.
Oddly, when powering it up, the thing draws almost 2 amps for just a second, then goes down to the usual 1amp. Before this mod it never did that. Maybe I have something shorted out, but that usually would cause smoke to rise within short time, but I had it going for 30 mins last night without failure, other than the output sounding weird with crackles and other distortion at times. I doubt this has anything to do with the power I am sending to the clock, but perhaps with the grounding of the whole board. I'll have to spend some time thinking this through tonight.
Peter
Now if I could only figure out what I messed up in my DAC after installing this, because now the pin 6/8 voltages on the DACs are fluctuating up and down from 3.7 to 4.5V - none of that should have anything to do with the circuit I modified. John, you have the same DAC - I just cut the two jumpers going to the reclocking part of the DDDAC board and inserted the output from the new supply into the same holes, adding a bead right before the clock and bypassing the clock underneath with the suggested 0.1uF cap. Why now the DAC is all over the place is a mystery to me.
Oddly, when powering it up, the thing draws almost 2 amps for just a second, then goes down to the usual 1amp. Before this mod it never did that. Maybe I have something shorted out, but that usually would cause smoke to rise within short time, but I had it going for 30 mins last night without failure, other than the output sounding weird with crackles and other distortion at times. I doubt this has anything to do with the power I am sending to the clock, but perhaps with the grounding of the whole board. I'll have to spend some time thinking this through tonight.
Peter
Hi,
I also built Guido's XO psu and also encountered same problem. Simply changing the 56K to 5.6K and 22K to 2.2K solved it. I'm getting 5.02V at the XO supply (pin #8) but when I install the XO, the voltage drop to ~4.6V ... still within XO'sspecs.
I had better success in this circuit :
http://home.tiscali.nl/~t708955/schem/xoschema.jpg
The output voltage is stable at 5.00V with or with the XO .... really stable !!!! It is using the famous TL431 shunt regulator and BC550 low noise transistor.
You might want to try this circuit
cool,
jomar
I also built Guido's XO psu and also encountered same problem. Simply changing the 56K to 5.6K and 22K to 2.2K solved it. I'm getting 5.02V at the XO supply (pin #8) but when I install the XO, the voltage drop to ~4.6V ... still within XO'sspecs.
I had better success in this circuit :
http://home.tiscali.nl/~t708955/schem/xoschema.jpg
An externally hosted image should be here but it was not working when we last tested it.
The output voltage is stable at 5.00V with or with the XO .... really stable !!!! It is using the famous TL431 shunt regulator and BC550 low noise transistor.
You might want to try this circuit
cool,
jomar
Elso Kwak said:
Hi Peter,
Which circuit did you exactly build?
Guido's circuit, just swapping 22k and 56k for 2.2k and 5.2k, otherwise exactly as posted on his site for the XO clock.
Parts used were pretty much same grade as with the old supply - Wimas for the 0.1uFs and Blackgates for the large values.
Peter
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