So why is it indicated in the circuit above that I uploaded?

I did some testing of adding energy storage (bipolar capacitor wired right across the motor) inside locomotives as a consulting gig about 10 years ago. It greatly smooths out the travel across switching points and sections of dirty track.

Paisley's circuit is a bipolar regulated power supply. The output voltage can go from +V to zero to -V. The capacitor is likely there to help make the power supply stable (not oscillate) for all settings of output voltage at various loads. Because the capacitor shown can get the full output voltage across it fully one polarity to fully the opposite polarity, it MUST be a special bipolar capacitor; it cannot be a simple electrolytic.

 

I have several new electrolytic capacitors and quite a few removed from old boards. All of them are several years old. What is the shelf life of electrolytic capacitors? What would happen if I were to use old capacitors? Would they cause a short or just not conduct? Is there any simple way of testing them. I only have a multimeter and no access to an oscilloscope.

Electrolytic capacitors are "formed" during manufacture - that is one of the aluminium foils is anodised to form an oxide layer, that layer is the actual dielectric whose thickness is proportional to voltage rating and inversely proportional to capacitance.

In normal use, aluminium electrolytics always have some leakage current, that maintains the forming anodisation process. The caustic electrolyte very slowly etches the oxide layer away, so in long periods of storage the oxide gets thinner - lower breakdown voltage, but higher capacitance.

Capacitors that have been stored a long time should be re-formed before you try applying their marked rated voltage. You need a current limited power supply so that you can let the voltage find its own level without causing hot spots or destructive breakdown.

The current limit depends mostly on how big the capacitance, but I'd suggest a few tens of mA as safe in most cases, you should allow up to 24h for forming.

 

Electrolytic capacitors are "formed" during manufacture - that is one of the aluminium foils is anodised to form an oxide layer, that layer is the actual dielectric whose thickness is proportional to voltage rating and inversely proportional to capacitance.

In normal use, aluminium electrolytics always have some leakage current, that maintains the forming anodisation process. The caustic electrolyte very slowly etches the oxide layer away, so in long periods of storage the oxide gets thinner - lower breakdown voltage, but higher capacitance.

Capacitors that have been stored a long time should be re-formed before you try applying their marked rated voltage. You need a current limited power supply so that you can let the voltage find its own level without causing hot spots or destructive breakdown.

The current limit depends mostly on how big the capacitance, but I'd suggest a few tens of mA as safe in most cases, you should allow up to 24h for forming.

 

Thank you for your reply . Can you please post a simple circuit I can build to restore these capacitors?

 

Thank you for your reply . Can you please post a simple circuit I can build to restore these capacitors?

There's plenty of constant current circuits you can find online.

 

There's plenty of constant current circuits you can find online.

Thank you for the reply. The trouble is when I looked up constant current circuits on Google I saw numerous circuit diagrams and I don't know which one would be best suited for me. Can you help?

 

Thank you for the reply. The trouble is when I looked up constant current circuits on Google I saw numerous circuit diagrams and I don't know which one would be best suited for me. Can you help?

Do you want me to do everything for you?!

 

Sorry, didn't mean it that way. I was just overwhelmed by the amount of information available and didn't know what the most suitable circuit would be. I'll work it out somehow. Thanks anyway.

 

Sorry, didn't mean it that way. I was just overwhelmed by the amount of information available and didn't know what the most suitable circuit would be. I'll work it out somehow. Thanks anyway.

An emitter follower modified with an emitter current sense resistor who's volt drop turns on a second transistor that shunts the EF bias so regulating the current.

A simpler single transistor current limiter resembles the basic auto-bias emitter follower stage, the value of the emitter voltage and the base bias voltage set the collector current. You can use the basic resistor divider to feed the base, but some people prefer using a green LED as a 2.0V reference for better stability.

 

There are plenty of bipolars on digikey.

http://www.digikey.com/product-detail/en/UVP2A100MPD1TD/493-12727-1-ND/4328344