Noise Filters on Radio Shack Power Supplies

[Anonymous]

Must be my day for questions.... 

Radio Shack Power Adapters (Wall Warts) come with Noise Filters on the low voltage end of the DC accessory cord.  Do these noise filters have any real purpose or need in our modeling situation?

I don't want to cut them off if they are worthwhile but I would like to get rid of the excess cable if they are not needed.  I will have four of them (15vdc, 2 x 9vdc, 12vdc) and that is a lot of excess cable to manage.

Thanks,
-John

 

 

[gandydancer19]

Well, I can't speak directly to the filters on the RS wall-warts, but all a Wall-wart is, is a transformer that reduces the AC line voltage, then a diode rectifier of some type on the low voltage side of the transformer to change the AC to DC. Either a full wave bridge rectifier or maybe a simpler/cheaper half wave rectifier using one diode. The waveform out of these devices can be quite "lumpy" (but it is still DC now), so a capacitor (cap) may be used to smooth this out. This may be what RS is calling a "filter". So, you could cut them off and add your own capacitor across the output wires if necessary. *** You will need to observe the polarity when hooking up the capacitor ***. 100 mfd (value) and above should work, and the WV (working volt) rating should be higher than the output voltage of the Wall-wart. If the output is 15VDC, the cap can be 100 to 400 mfd at 25WV or higher. The higher the mfd value, the smoother the output voltage will be. (less "lumpy") If you are just going to use them for lights, LED's, or small motors, you wouldn't have to add the caps. If you were going to power an electronic block detector circuit or some other electronic device with them, you should add the caps.

[cacole]

Those noise filters are just ferrite iron beads that surround the wire and create a small inductance.

Cutting them off should present no problems except, as already mentioned, in a sensitive electronics circuit, particularly something with integrated circuitry.

I use computer power supplies for all of the lights, crossing signals and gates, tortoise switch motors, etc. on my home layout and our large HO scale club layout.  These give you very smooth, regulated + and - 5V, 12V, and sometimes 3.3 V, depending on the model chosen.  A 100 Watt computer power supply can run an entire layout except track power, and they are far more reliable than a wall wart.

[Anonymous]

Thanks to both of you.

I will just leave the filter blobs in place and coil up the excess cord.  I might as well keep my power clean whenever possible.

Thanks again,
-John

 

[NeO6874]

the idea of the "filter" is to combat EMI (Electro-Magenetic Interference), and really doesn't do anything to the electricity flowing through the wires....

Pretty much the magnet on the wire cancells out some of the EMI that is caused by coiling the wires between the two endpoints.

[60YOKID]

You don't really need the iron bead filter for lighting, but it won't hurt. In fact, it is there to smooth out high frequency transients, which can burn out out fragile electronic devices, and possibly reduce the life of small incandescent lamps. 

Many of these beads can simply be unsnapped and moved toward the transformer for more convienience.  I would still leave a couple of small 4 inch loops of wire between the bead and the transformer so the bead can work best. 

[jackn2mpu]

You don't really need the iron bead filter for lighting, but it won't hurt. In fact, it is there to smooth out high frequency transients, which can burn out out fragile electronic devices, and possibly reduce the life of small incandescent lamps. 

Many of these beads can simply be unsnapped and moved toward the transformer for more convienience.  I would still leave a couple of small 4 inch loops of wire between the bead and the transformer so the bead can work best. 

You don't need it for lighting, but I'd leave it in anyway. Most of these wall warts can generate a lot of rf hash and the ferrite core on the cord is to quash that. Any filtering of the dc is done inside the wart's case, if they even do anything more than just rudimentary filtering to lower the ripple output of the rectifier in the power supply.