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Changing capacitors on CDU - feasible or not?


Jinty3f

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I have built a diode matrix for point control, and it works pretty well, except that there are 2 routes which require 4 points to throw at the same time, and my CDU struggles with this. I have checked that the seep motors are all functional (the ones with the switch built in, so extra drag I suppose), I have used 3 core mains wire for the short distances involved, and the power is from a 16vac Gaugemaster transformer. The CDU has 2 x 2200 capacitors, charged via a pretty substantial looking 2n3055 transistor

so I am wondering if it is feasible / safe to unsolder the caps and replace them with 2 x 4400 to give some extra muscle, or am I being a bit simple? I am OK with most layout wiring, but am totally ignorant about electronic stuff. Any advice gratefully received.

JD

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No such thing as a 4400uF, would be a 4700uF. Needs to have the same voltage rating (or higher) than the originals too.

You might also need to consider beefing up the return wire, common to all motors, as it will be carrying the combined current and liable to voltage drop, which if excessive can be just as bad as not having enough capacitance.

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If the circuit can stand it, also consider increasing the supply voltage. The energy stored in a capacitor increases with the square of the voltage, but only linearly with capacitance.

 

Andrew

Agreed, the OP needs to check the voltage rating of the capacitors, first. Typically they are 25V and are therefore inadequate & need to be replaced to at least 35V.

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You need to balance the load when using a Diode Matrix.  My matrix throws two three or four points at a time so  I have two dead loads which are simply two point motor coils so when just two points are to be thrown the two dead loads are also energised,   If you don't balance the load you risk points not throwing or worse the point blades being knocked out of the tie bars when two points only are thrown

 

Watch the voltage rating when changing Capacitors, they charge to the very peak of the applied voltage not the average.  I found out the hard way that they explode when over charged.  However I use big bulky 50 ish volt capacitors connected across my 16 volt nominal power unit for point motor operation through a diode matrix and electric pencil or rotary selector, matrix and push button, without a CDU and it works really well.  It was even better before the 24 volt transformer failed.

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  • 2 weeks later...

Thank you all for your advice. Just to update you, I successfully replaced the CDU 2 x 2200 caps with 2 x4700 - replacement was easy, and now the points, a mixture of mainly Seep and a few old Peco, all click over in groups without problem via the diode matrix, and using the same 16vac as before. Problem solved, thank you everyone.

JD

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