I am building a model railway using digital control and would like to add some ‘keep alive’ capacitors to a locomotive. An age old problem with these railways is that the trains will tend to hesitate or stop if they encounter dirty track and the power is momentarily cut off.

However each locomotive contains a digital decoder chip and if you add a large capacitor to the power input of the chip it will provide enough discharge power to help the loco over dead spots on the track. This is a tried and tested method and works well.

So I added 3 capacitors in parallel to a locomotive. These were all 4700uf 16v. The circuit also had a 150ohm resistor on the +ve lead to limit the charge rate of the capacitors, and a diode across the resistor to enable the current to bypass the resistor when discharging.

The works fine, if you lift a running loco from the track it keeps going without power for about a second.

My problem…

The caps are large and difficult to fit in smaller locos, so I thought I would switch to super capacitors instead, the type usually used for memory backup. However using the same circuit as above nothing happens, they do not work.

I have connected 4 backup capacitors in series, each is 0.33F and 5.5v. So connected in series they should give me a whopping 83250uf and 22v. They are also much smaller than electrolytic caps.

I have checked with a multi meter and they are charging up, and they slowly drain when you remove the train from the track, but the loco does not run.

Is there perhaps a difference between standard electrolytic and backup capacitors that I not accounting for?

 

Considering the difference and keeping balance between each super capacitor, you will need to adding a resistor for each capacitor, how big the values of resistor that it could count flows through the resistor from 1~10mA, it depends on you.

 

hi von,
What power does the loco motor draw from the rails, ie: voltage and current.?

The internal resistance of some super caps can be from 1R to 100R, by adding 3 caps in series you may have a high total cap resistance.

E

 

hi von,
What power does the loco motor draw from the rails, ie: voltage and current.?

The internal resistance of some super caps can be from 1R to 100R, by adding 3 caps in series you may have a high total cap resistance.

E

Do you have any info about this -- internal resistance about 1R to 100R ?

 

Thanks for you replies.

I've done some more tests using the 4 backup caps in series and a bare motor removed from a train. Even when fully charged the caps will not run the motor, but they do run low power devices like LEDs.

To check that connecting them in series was not causing the problem I also tested one backup cap on its own. When charged to 5.5v it would still not run the motor.

Although these caps have a high capacitance, is it possible they will only give up their power in a slow trickle. Since they are really designed to backup memory chips?

 

As Eric says, the ESR in supercaps is too high for your application.

 

hi,
Charge up one super cap to 5V, then connect a 4.7R 5Watt resistor across it and while its discharging thru the resistor, measure the voltage across the cap, see how long it takes to discharge.
Tell us the result.
As I said previously, your caps may have a high internal resistance which will limit the output current rate.

E

 

Do you have a part number for these super caps? If not, where did you get them?

I would like to see the data sheets.

 

I am curious as to why the OP is wiring the caps in series. Why not just get a single smaller cap at the proper voltage rating? Don't they make super caps in all kind of volatge ratings?

 

Supercaps tend to be low voltage.

Bob

 

I am curious as to why the OP is wiring the caps in series. Why not just get a single smaller cap at the proper voltage rating? Don't they make super caps in all kind of volatge ratings?

To get the big capacitance and small size.

 

To get the big capacitance and small size.

Actually Bob had the more correct answer if it is difficult to get higher voltage super caps. Placing caps in series reduces the capacitance not increase it. It is why I was curious as to why the OP would want to do that considering his space issues.

 

Actually Bob had the more correct answer if it is difficult to get higher voltage super caps. Placing caps in series reduces the capacitance not increase it. It is why I was curious as to why the OP would want to do that considering his space issues.

I just want to tell you that why the TS not to using the normal capacitor, because he want a big capacitance and small size, why he needs to in series because the most of super capacitors has low rated voltage.

 

Take a look at the 1200 Ufd here:
http://www.mouser.com/ds/2/420/al-kyb-e-140701-514759.pdf
I think it will be smaller than 3 in parallel.

18 X 40 mm