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At our train club we have several display layouts that run modern locomotives, i.e. with DC can motors. The locomotives get a good work out and are often run continuously for 4 hours. At times the boards have failed and we are left with a shelf queen. The most recent one was an inexpensive Lionel Thomas locomotive. The motor is fine, the board is not. I would like to convert this locomotive and others to one way forward with the addition of a simple full bridge rectifier. Perhaps some 2 motor locomotives would be converted in the future. Can someone suggest an amperage rating that would be suitable? Is 3 amp enough or should I go with something bigger? Thanks for your help.

Don

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If you actually have 10 amps flowing through the bridge, it'll be dissipating around 7 watts, that size of a part package will be hotter than a firecracker.



At any given time, two (2) diodes of a 4-diode bridge rectifier are "ON."  Hotter than a firecracker to say the least!   I think you can buy a 10 Watt soldering iron - as in hot enough to melt solder.

@Rich Melvin posted:

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At only 3 amps (30% of its max current rating) it will be quite cool.

3 Amps running thru 2 diodes (~0.7 Volts each) is just over 4 Watts.  One would have to be pretty thick-skinned to characterize 4 Watts "quite cool."  I've burnt my finger touching a component dissipating a measly 1 Watt.

@stan2004 posted:

At any given time, two (2) diodes of a 4-diode bridge rectifier are "ON."  Hotter than a firecracker to say the least!   I think you can buy a 10 Watt soldering iron - as in hot enough to melt solder.

Right, I guess I should double that, 14 watts!

@Rich Melvin posted:

GRJ, I figured they could mount it to the chassis somewhere with a #6 screw if they want to heat sink it.

Even if it’s not heat-sinked to the chassis, it’s not going to handle anywhere near 10 amps running a single can motor. At only 3 amps (30% of its max current rating) it will be quite cool.

Yep, a can motor probably isn't going to draw more than an amp or two at the most.  That's likely not a problem, I was just addressing the maximum load.

Since the rectifier is going to pull only as many amps as the motors require, is there any downside in having a rectifier that is oversized?

I recall looking at the rectifier in one of the early Williams motors and it appeared to be pretty "light duty".  Of course only an amp or two doesn't take much.  My recollection is that the rectifier wasn't attached to a heat sink., it was just held up by the wiring.  Of course that was a while ago so my recollections may be pretty foggy.

Thanks for your help.

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