CTT May 2013

[webenda]

Yikes! I forgot. Thank you Bob, I am sure glad you were watching. Will edit schematic ASAP.

I have blown up enough LEDS from too high reverse voltage; you would think I would remember the reverse voltage protection circuit.

To those following this discussion, you can use another LED for D1 (as Bob says he likes) - if you wire it in anti-parallel (like parallel but each component in opposite direction) as shown. If you use something like a 1N4001 diode, you can wire it in series or anti-parallel. For series operation, turn it around so both the LED and 1N4001 are aimed in the same direction and put it in line with the LED. The advantage of anti-parallel operation is the LED lights about one volt lower than when the diode is in series with it.

[lionelsoni]

I haven't seen that issue yet; but I suggest that that LED should have some reverse-voltage protection.  Practically all LEDs are rated for 5 volts; and this one will have 35 volts across it.  There are various ways to fix this, for example, a diode with a suitable reverse-voltage rating in series with the LED or a diode in anti-parallel with it.  My favorite is a second LED in anti-parallel.

[KentJJ]

Outstanding, Webenda! Your work is very much appreciated -- in fact, Jay says "You're hired!" 

Kent

[webenda]

Thank you Kent. May I offer a correction?

Parts List:

DIODE D1

1N4001 1A Diode : Model: 1N4001 : Catalog #: 276-1101 : $1.49 /pair

RadioShack

LED

5mm Red LED : Model: 276-041 : Catalog # 276-041 : $1.99

RadioShack

RESISTOR

3.9K ohm 1/2W 5% Resistor pk/5 : Model: 271-1123 : Catalog # 271-1123 : $1.49 /5 pack

RadioShack

SWITCH, ON/OFF, SPST

SPST Toggle SwitchModel: 275-651 | Catalog #: 275-651 $3.19

RadioShack

SWITCH, SPDT

SPDT Toggle SwitchModel: 275-654 | Catalog #: 275-654 : $3.99

RadioShack

VOLTMETER

CAT# PMA-25V : 0-25V AC : 3.15" x 2.5" x 2" depth : 2.5% accuracy : $12.00 each

All Electronics

AMMETER

CAT# PMA-10A : 0 - 10A AC : 3.15" x 2.5" x 2" depth : 2.5% accuracy: $12.00 each

All Electronics

[KentJJ]

Glad to hear you liked the train box tester tip! Sorry Webenda, but this time the contributor didn't have the benefit of a computer to provide a more precise technical rendering. Although this can sometime thwart the publication of some tips, I thought the very clever application of a repro box was still quite worthy of sharing with CTT readers -- even all we had to work from was a hand-drawn diagram. At any rate, I'll be sure to let Jay know that you appreciated the work he did to spruce up the original sketch!

Kent

[webenda]

In the May 2013 issue of Classic Toy Trains there is a "Train Box Tester." (page 82) The picture labeled, "MULTI-TESTER WIRING SCHEMATIC", has a zener diode and 4 K resistor connected to a switch contact. The schematic switches are drawn so poorly I have no idea which switch should switch in the zener or what the zener's function is. :-( Tell Jay Smith, other than that, nice schematic. :-)