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| David Hess:
--- Quote from: nadona on September 24, 2014, 10:25:10 pm ---@ David Hess Thank you for showing your favorite set up. Could you elaborate the interesting things(at least to me) in the pictures? 1) Why two wire connection instead of one from the input cap? Is it factory default or you modified it? --- End quote --- The original aluminum electrolytic capacitors, one of which is shown in that photo for comparison purposes, were wired the same way. One side of each wire goes to the rectifier and the other side of each wire goes to the input to the regulator so the resistance in series with the capacitor itself is minimized. When I replaced the big can style capacitors, I selected replacements based first on diameter so they would snap into the gray mounting brackets which held the original capacitors. Also seen in that photo is an internal LED I added to show that the power supply is active when the cover is off. --- Quote ---2) What those two resistors do between Lo and Hi terminal of DM502? Why two Rs instead of one? --- End quote --- 1.10 Mohms is just a more convenient test resistance for the high resistance range and I did not have any 1% resistors higher than 1.00 Mohm handy. 1.00 Mohms would leave the most significant digit potentially changing between 0 and 1. That DM502 was measuring temperature using a probe at the time for no particular reason. --- Quote ---3) What is the purpose of the caps on the second DM502? --- End quote --- I was replacing solid tantalum capacitors which had shorted in a Tektronix PG506 pulse generator. The NOS (new old stock) solid tantalum capacitors shown in the photograph were being burned in and tested for leakage before installation. The leakage shown is 3.1 microamps for 10 capacitors in parallel but 2.48 microamps of that is caused by the 10 Mohm shunt resistance of the voltmeter so the actual average capacitor leakage was 0.062 microamps which is considerably better than the maximum rated leakage of about 4 microamps which might be specified for a typical 25 volt 15 microfarad solid tantalum capacitor. |
| nadona:
Thank you very much again for the detailed explanation. :clap: Could you draw how all these are connected for leakage test? Why leakage test for tantalum? Are you doing it because they are NOS or you do all the time for tantalum for a certain applications? The picture of tantalum caps drew my attention because recently two of my Datron 1062 were stopped working. Same reason. A tantalum cap in the power supply section shorted while bringing down the analog board's input resistance to less than 1ohm. First time I smell the tantalum. I have two of 1082 and thinking to replace the same tantalum to other type of cap. Maybe I start a new thread on how to test leakage and why? Regards, |
| David Hess:
--- Quote from: nadona on September 25, 2014, 12:04:56 am ---Could you draw how all these are connected for leakage test? --- End quote --- There is not all that much to it. The power supply is configured as a current limited 0 to 40 volt source with one of the multimeters measuring low values of current in series. The other multimeter is measuring the voltage across the capacitors which were all tied in parallel. If the leakage current had been high, I would have disconnected them in sets to find the bad one or ones. I just hacked the test together in a few minutes and let it run overnight. --- Quote ---Why leakage test for tantalum? Are you doing it because they are NOS or you do all the time for tantalum for a certain applications? --- End quote --- These were NOS solid tantalum capacitors of unknown heritage; there is a local surplus electronics store here which has bins of them and they are inexpensive compared to new ones. There *are* sometimes reasons to grade capacitors for low leakage but in this case it was just to weed out bad or marginal ones before installing them into the circuit. Why replace a shorted tantalum capacitor with another bad or marginal capacitor? I had already tested them for capacitance and dissipation factor. It would be nice if someone made a portable capacitor tester which did capacitance, dissipation, ESR, and leakage but I have never seen such a thing. When I was picking the capacitors out of the bins, I had my multimeter with me to check each one for capacitance and resistance which is not as good as a real leakage test at the rated voltage but better than nothing. I did not find any bad ones. --- Quote ---The picture of tantalum caps drew my attention because recently two of my Datron 1062 were stopped working. Same reason. A tantalum cap in the power supply section shorted while bringing down the analog board's input resistance to less than 1ohm. First time I smell the tantalum. I have two of 1082 and thinking to replace the same tantalum to other type of cap. --- End quote --- When solid tantalum capacitors first became available, they were optimistically advertised as *not* requiring voltage derating for reliable operation. In this specific case, 20 volt capacitors were used on the plus and minus 16.5 volt supplies although I have run across 16 volt solid tantalum capacitors used on 15 volt supplies and 6.3 volt solid tantalum capacitors used on 5 volt supplies before. One was shorted and after replacing it, the *other* one shorted a few hours later while I was watching so I stopped by the surplus electronics store and picked up some suitable replacements for all of the similar tantalum capacitors in the circuit; this was cheaper than ordering new ones online. The replacements were rated at 25 volts which I figured was derating enough. In general you can replace solid tantalum capacitors with good aluminum electrolytic capacitors of 2 to 4 times the value to get an equivalent ESR but if you have an inexpensive source of solid tantalum capacitors, I would use the same type as a replacement because they have better high frequency characteristics. No matter what kind of capacitor is used, reliability will be improved if they are voltage derated. |
| nadona:
@David Hess; As I expected, more than perfect answer :clap: :clap: :clap: Regards, |
| BravoV:
Shameless bump .... more please ... ^-^ |
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