1.5 V power supply question

[ShadowNix]

Hey all,

 I have some 1.5V lamps to power, and besides miniatronics, I was wondering what you all use to power them?  There seems to be a paucity of them online at my usual train stores I use.  Have you all built them / modified your 5V output with resistors??? I am a bit rusty on my electronics... let me know.  Thanks for your help!

Brian 

 

[betamax]

How many milliamps of current does one of these bulbs draw?

For a series resistor, the formula is R=V/I, in this case, V equals 3.5. 

 

[Seamonster]

A cheap and dirty way to do it is to provide a load for your power supply with two diode in series with it and take the lamp power from across the diodes  For example, let's say you have a 12 volt power supply.  Connect a 12 volt lamp to it with two rectifier diodes in series with one lead.  Use 1N4001 diodes for currents up to 1 amp, 1N5400 diodes for currents up to 3 amps.  Attach the leads to your 1.5 volt lamps across both diodes.  The diodes drop 0.7 volts across them for a total voltage drop of 1.4 volts.  The 12 volt lamp is just there so the diodes don't short circuit the power supply.  The 1.4 volt drop across the diodes is what powers the 1.5 volt lamps.  If your power supply is somthing other than 12 volts, just change the lamp to the appropriate value.

 

[rrinker]

 The diode method is probably the best option, since even if the 12 volt supply isn't well-regulated, the drop across the two diodes will ALWAYS be 1.4 volts, keeping the lamps safe. If you do want a 1.5 volt power supply, make SURE it is well-regualted, as even a slight fluctuation under load will possible blow the bulbs. In fact operating the 1.5 volt bulbs at 1.2 volts would be far better, they will last a lot longer and still be plenty bright. The problem with 1.5 volt and under regulated power supplies is that this is at or below the minimum voltage for most adjustable regulator chips like the LM317. That doesn't mean it's not possible to obtain such a device, but the diode and bulb idea is a lot easier. You can also use a slightly higher voltage supply, say 5 volts, and put 5 diodes in series with the lamps. 5 diodes will drop 3.5 volts, leaving 1.5 for the lamps. An old computer power supply is good for this, although most need a minimum load on the 12 volt lines to regulate properly. The ones I have all meausre just under 5 volts when under load, so dropping 3.5 volts will put the lamp voltage just under 1.5 volts.

 

                                --Randy

[GAPPLEG]

I have used in the past and will do so again, Multi-tap transformers. The ones I have most of are Radio Shack, there are other sources too.  They have a switch on the front to select 1.5  3  6  9   12   volts. Great for lighting circuits etc.

[Gandy Dancer]

two diode in series with it and take the lamp power from across the diodes  ...  The diodes drop 0.7 volts across them for a total voltage drop of 1.4 volts.

This is what I do, except I use four (4) diodes.  One set in series in EACH polarity direction.  That way if for some reason the power comes through the opposite direction it protects the lamp.  Using a full wave bridge rectifier works the same way and often in a smaller package than four diodes.

[ShadowNix]

Brilliant... I like the diodes idea with a 12V lamp... THANKS a million...

Oh, would using a zener diode to clamp the voltage down to 1.5 work?  Sorry... been awhile since I have done serious electronics (would you believe I used to build prototype scientific instruments like 15 years ago... jeesh!!!)...

Brian

[cmarchan]

I have used in the past and will do so again, Multi-tap transformers. The ones I have most of are Radio Shack, there are other sources too.  They have a switch on the front to select 1.5  3  6  9   12   volts. Great for lighting circuits etc.

I believe the idea here is for using 1.5 volt lamps in a DC or DCC locomotive.

Another solution is to use a voltage regulator; the LM317LZ is a three legged regulator that without a control resistor offers 1.2 volts output; I use them with decoders that do not offer built-in regulation; you can power two 15ma lamps per regulator without additonal load compensation. Check the following link for further info:

http://tonystrains.com/technews/install-lamps-decoders.htm

TCS offers add-on voltage regulators for their X series decoders; the A6X drop in replacement for the Atlas/Genesis/Kato light board has an integrated voltage regulator.

BTW, the LM317LZ surface mount version is what's used on the latest Athearn Genesis DCC decoder/DC lighting motor board. The boards have four regulators (not all are always used).

[ShadowNix]

Actually it is 1.5V lamps in buildings/lightposts....

Brian

[larak]

Oh, would using a zener diode to clamp the voltage down to 1.5 work? 

Brian

Sure, if the polarity is correct. If backwards you get 0.7 volts. Don't forget the current limiting resistor or lamp in series and use a high enough wattage. Watts = Voltage * Current. (double it for safety).

Karl

 

[ShadowNix]

Here is what I ended up building (link below)... WORKS GREAT!!  The two diode deal worked, but it did not have enough current to supply all my lamps (maybe related to my old 1950's PS I was trying to use)... anyways, it only cost me like $19!!! Good luck.  Thanks for the help, everyone.

http://www.awrr.com/1point5voltPS.html

Brian

[Anonymous]

What about double or triple A batteries??

[Don Gibson]

Application 1. Headlight

Two diodes (in series) for single direction. 4 diodes or bridge rectifier for 2 direction.

Application 2. Passenger cars,

Single tailight - AA battery w. on-off switch.

 

[ShadowNix]

Good point Don.  I am using my power for buildings, but for passenger cars, an AA battery seems awfully simple (in an elegant way!)

Brian