Easiest way to drop a few volts from my wall wart?

[nirvanas silence]

Some of my wallwarts are putting out an unusually high voltage (13.5VDC is 17, 15VDC is 19.5!) and it's getting frusterating.  My wall is putting out 117VAC so that's not the problem.  Now my nintendo is overheating and we can't have that!  So the wallwart I have apart now is 14.5VAC secondaries going into a bridge, then a 2200uF cap on the output.  That's it.  What's the easiest way to bring it down a few volts without a regulator.  Also, its a 1A power supply.  Thanks from Mario and Me.

[mikeb]

Without a regulator?!?  And there's no regulation on those supplies? Hmmm .... use a voltage divider and take the voltage feed from the function. You'll need to use wire-wound resistors to dissipate the heat, methinks.

Mike

[nirvanas silence]

Time for some math.  I had a feeling the voltage divider was going to be needed but 1A is going to be quite a bit of heat.

[nirvanas silence]

I need some really inefficient diodes.....

[mikeb]

I doubt the entire 1A will be drawn through the divider .... still think the voltage regulator option would be better, you'll get overcurrent protection thrown in for nothing....

Mike

[nirvanas silence]

The circuit draws 600mA which helps a bit.  Just my luck... I go through my junk box and find a 650mA 9.75VDC adapter and it runs at  11VDC, just enough under to not work.  So I'm basically screwed right there.  I tried a voltage divider calculator and its calling for some large resistors, which means alot of heat.  So I'm going to have to go the voltage reguklator route which I really don't want to.

[Paul Perry (Frostwave)]

Drop 2V at 600ma?
that is calling for 3.3 ohms, which you get from three 10 ohm resistors in parallel.
Power in the drop is only 1.2 watts, so you can use three half watt resistors.

[changes]

how can you do that with a voltage regulator???

[stm]

Take it easy!  

Wall warts are designed to provide the rated voltage AT the rated load.  If you happen to measure them without load you usually see 20% or so more voltage.

Never trust resistors or voltage dividers to drop down voltage accurately. The final output depends on the load.

Easiest solution:  just put as many silicon diodes in series as needed. Consider each diode will drop around 0.6V.  For 600 mA you should use 1N4001-1N4007 (anything in between)-- no way to use 1N4148!  Also, make sure you measure the voltage with the acutal load, as you will see it drops.

In summary, I would suggest you put three to four diodes in series to start and then connect thw load.  Measure voltage entering the load and see if you need to remove any diodes.

Regards,

STM

[Paul Perry (Frostwave)]

Never trust resistors or voltage dividers to drop down voltage accurately. The final output depends on the load.

I agree entirely, but I assume that the voltage had been measured under load. If it wasn't for the prevailing culture of 'cheapness' here, I would have suggested springing $5 for the 'right' transformer :? but, I'm a dumpster diver myself!
Incidentally, it is always possible that adding resistance after a plug pack might cause some circuit misbehavior, if there is not a lot of power supply fileter in the unit being powered.

[Paul Marossy]

Check out thrift stores on a regular basis. You can find all sorts of unwanted wall warts in all kinds of voltages for 50 cents to a dollar or so each. A lot of the wall warts that I use were obtained this way and work perfect for the application. Just make sure that you filter the power supply in your device well...

[mikeb]

With all the time spent faffing about here on the forum and mucking around with a half-arsed solution you'd be still better off using a voltage regulator .... life's too short.....

Mike

[Paul Marossy]

Agreed. A voltage regulator is the easiest solution. But, I still like to get wall warts at the thrift store. I use them for things other than just stompboxes - stuff like adjustable power supplies, oscillators and other stuff. These things are just sitting around in thrift stores because the cordless phone they went with broke (or whatever the were connected to...). I'm just suggesting that thrift stores shouldn't be overlooked as a source of some electronic parts.  Whenever I can use a wall wart instead of batteries, I will do it. I would spend a freaking fortune if I had to constantly buy batteries for all the stuff I have built...

[RDV]

http://www.national.com/ds/LM/LM117.pdf
The schem on page 14 with the ripple protection is a winner. Or instead of the 5k pot, use a 500 ohm trimpot and a 1.1k resistor in series.

RDV

[nirvanas silence]

Drop 2V at 600ma?
that is calling for 3.3 ohms, which you get from three 10 ohm resistors in parallel.
Power in the drop is only 1.2 watts, so you can use three half watt resistors.

No, I'm trying to drop 6V+ which is the problem.  I have about 19.5-20V and I need about 13-14V.

The problem is it was 2AM and I wanted to test out a regulator and new part I installed in my nintendo.  So no electronics stores and I'm too impatient to order online.  Not to mention a regulator inside of the wallwart isn't physically going to fit in this case.  

I just happen to come across another wall wart so thanks anyway guys.  

P.S. All voltages were measured with a load.

[mikeb]

I'm just suggesting that thrift stores shouldn't be overlooked as a source of some electronic parts.

Completely agree. Most people don't know how to fix even basic stuff like a broken wire that prevents an adapter from working, and it is better to rescue things than 'buy new' ..... Mother Earth will thank you for it!

Mike