Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Maybe if you posted the actual connection of the cap, this debate wouldn´t be so academic. I expect to see three parts in this scenario, the charging resistor/supply/whatever, the cap, and the input of the timing IC.
Now you replace the original resistor or charging pin from the trace that goes to the cap, and replace it with a voltage source and a current meter in series and measure with circuit powered on obviously. Measure on both bad and good boards, then disconnect the input of the IC and measure again to see where the problem lies.
The Charging Pin is either
1.) The output of an Op-amp from the stage BEFORE
2.) Or the Charging pin is a Pull Up resistor connected to the Op amp from the stage Before
1.) Well some of the timing caps are in series with a charging resistor that goes to ground , this connects to the input of an Op Amp IC
2.) Others timing caps go to ground before the input pin of the IC op-amp
a.) The charging resistor is in series with a timing cap going to ground
b.) Or the charging resistor is a Pull Up resistor tied to the power supply , with the charging capacitor that goes to ground
RC timing networks , 3 types they use
1.) Resistor in series, capacitor is grounded
2.) Capacitor in series, Resistor is grounded
3.) Resistor is tied to VCC, capacitor is grounded, ( Not sure what this RC network is called tho) do you? The capacitor gets charged through the VCC through the resistor
There is an Op amp before the RC timing network and an Op amp After the RC timing network
To test a cap for leakage you apply a voltage and measure the current. But the current could be low so you need a low current meter to be able to measure it. You can apply different voltages by starting at a low voltage like 5v, then measure the current. Then turn the voltage up a little at a time and see that the current does not increase too much. You'll have to decide for yourself what is too much leakage current in the given application. It could also be that the leakage increases with temperature so keep that in mind.
The leakage current acts to change the charge and discharge time as Eric pointed out. The difference is that it acts like an equivalent resistor that is put in parallel with the charging or discharging resistor. So the timing itself changes because the equivalent resistance is acting in parallel with the normal charge resistor. The approximate equivalent resistor is equal to the voltage divided by the leakage current, where this voltage would be the average voltage over the charge period as an approximation.
Timing circuits with RC elements are always subject to variations in timing due to temperature anyway though. So for best results you have to keep the circuit at a relatively constant temperature for good results.
As others have pointed out, a single stage RC is not a good idea for very long time delays, although coupled with the 555 timer IC it works to some extent. But better is the faster oscillator and digital divider circuit made using counter IC chips like the 74LS93 for example. With this one chip you can make the oscillator 16 times faster and still get the longer time delay. With two chips you can make the oscillator 256 times faster, etc. So you can get some VERY long delays like this.
You can also get chips that are made just for this purpose as they contain many divider stages inside. So you can see very large division ratios like 1000 or something.
This is also a technique used for some of the older TTL frequency counters. They divide down the output frequency of a crystal oscillator and end up with a 1 second gate. So that's a division of over 32000. That would clearly be a good way to develop a time delay that needs to be accurate.
Of course if you can use a microcontroller you can do this in code. You then have the option of using an external crystal or using the internal RC oscillator, but this RC oscillator is compensated to some degree so it not your typical RC oscillator made by the hobbyist. It is also calibrated at room temperature, and you can even use software to calibrate it yourself. The typical variation is about 1 percent over a fairly wide temperature range which is much better than a stand alone RC oscillator.
Are you incapable of rational thought? You want to measure the current passing through the cap, with voltage difference between the cap´s terminals. Look at your circuit and apply what you know on what you have, figure out how you can measure the current inside your own circuit. Adapt your circuit if you have to to fit the requirement for steady DC voltage across the cap. Measure the current through the cap and decide where is your problem.
The thing about not desoldering the cap was just that soldering could possibly alter it´s leakage, and spoil your results, so you´d prefer not to do it unless you really have to.
Another thing, are you absolutely sure the problem is leakage? Could it be that the cap and the resistor don´t have spot on values? Caps vary usually +/-20% in value. Resistors not that much, but still they could change things. Make sure the supply voltage for charging the cap is the same as well.
To test a cap for leakage you apply a voltage and measure the current. But the current could be low so you need a low current meter to be able to measure it.
True, these circuit boards are from the 60's and 70's so the resistors and caps are aged , which has caused the timing to drift UP close to the Upper Limit range
Wow, components that old should really be discarded.
Yes you have to be able to measure microamps, and also you have to be careful not to blow out the meter. It is also possible to measure the voltage across the charge resistor once the cap is charged and determine the current that way, although you need a volt meter that has high input impedance.
Leakage is the current passing where it should not, so more curent provided by the supply equals to more leakage Ideally the current should be zero meaning no leakage at all.
The voltage across the cap should be made the same so that you get comparable results. Vastly different voltage will produce vastly different leakage current.
Leakage is the current passing where it should not, so more curent provided by the supply equals to more leakage Ideally the current should be zero meaning no leakage at all.
The current meter should measure HIGH current than if the capacitor is leaky right? cause it's drawing more current from the power supply?
The voltage across the cap should be made the same so that you get comparable results. Vastly different voltage will produce vastly different leakage current.
True, So do I really have to use a current meter to measure the current going through the charging cap? when I can just make voltage measurement across the cap
The current meter should measure higher current when the capacitor is more leaky.
you have to use a current meter because you want to measure current. Setting the voltage the same is just a way that allows you to accurately compare that current betweenthe different boards.
Or just change the bloody caps and be done with it.
If I take voltage measurements across the charging cap or take voltage measurements referenced to ground and compare it to different boards , will this tell me if I have a leaky cap?
Ok, so let me say this once again. Every cap is leaky. Every pcb trace is leaky. Even the air surrounding the board lets some current pass.
Now you have two boards that behave differently, and you want to know if the capacitor is the problem. Because current depends on voltage, you need the voltage to be the same on both boards. Different voltage across the cap will produce different current fllowing through the cap, even if the two suspect caps are exactly the same.
Now you take a voltage measurement AND a current measurement. Make sure that voltage is the same on each board, for example connect the cap to an external supply. Then compare the currents measured.
The cap will charge to whatever voltage the supply has, regardless of leakage. You need to make sure that the supply voltage is the same in all measurements.
The cap will charge to whatever voltage the supply has, regardless of leakage. You need to make sure that the supply voltage is the same in all measurements.
I don´t know how many times i have already said this. You want to measure the current passing through the capacitor when the voltage across it is X volts.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
