How Can I Drop DC Voltage?

[markrvp]

I'm currently hacking 3 Jakks handheld games into a cabinet (see the project HERE).

The AC to DC converters I have are outputting 7.2 volts with a load applied.  I assume that I can put something (resistor, transistor, diode - don't laugh I'm not an electrical engineer) in line that will drop the voltage to 6v.

Can anybody tell me specifically what I need to drop DC voltage 1 volt and how to wire it in?

[fl0yd]

A resistor will do it. 

One easy way would be to use a potentiometer.  You could hook it up to a volt meter and turn the knob until you were at your desired voltage.

[Dire Radiant]

The even easier way would be just to get a selectable voltage power supply from Radio Shack.

[fl0yd]

The even easier way would be just to get a selectable voltage power supply from Radio Shack.

Mark had stated that he's using a converter in a different post, but getting different voltages than he expected.

Mark's quote:

I'm using 3 different power supplies.

[markrvp]

The even easier way would be just to get a selectable voltage power supply from Radio Shack.

Fl0yd is right.  I can't believe how innacurate these things are.  I was blown away when I set in on 6v and it ouput 9 volts.  I went to Radioshack and bought a dedicated 6v unit.  I had the clerk meter it and it came out 7.8 volts.  Even with a load it's still hot.

Luckily it hasn't fried my handhelds, but the video signal seems a little hot.  I wouldn't think that would matter, but it apparently does.

[btoddkelley]

You can't meter a regulated dc power supply and find the LOAD voltage. A meter only measures unloaded output. Hook it to something, power it up and then meter it. It will be very close to advertised voltage. 

Todd

[rdagger]

You can get an adjustable voltage regulator for about 50 cents at an electronics store.  Radio Shack sells them for around $2.

[ShinAce]

The best thing to use is a 5 volt regulator. It has one pin for input(keep this one above 7 volts), another pin for output(it will give you 5 volts, regardless of load), and a GND pin.

It carries the part number 7805 for the 5 watt version, or 78L05 for the half watt version. You might need a heatsink, so just go with the 7805 to try and avoid that.

And yes, you can get the load voltage by simply applying a meter to a regulated output. It's regulated, so even without a load it is giving 5 volts. Besides, a voltmeter is a tiny load.

[Druin]

It's the unregulated supplies that you can't meter without a load.  You might have a wall adapter that says 12volts and you meter it and see 14 or 15 but when you plug it into something it shows close to 12.

I was going to suggest a 7805 but I thought the target was 6 volts.
There does exist a 7806 too but it's harder to find I think.

The adjustable voltage regulator that's popular is LM317, just needs a few resistors to set the desired output.

The problem with just using a Pot to form a voltage divider and scale the output, you may have problems with it being an unregulated response, the load may "load" it way down and you might not get the current you want also, it can be tricky when trying to power something off of a voltage divider. IT's ok to just scale voltages to send to another non-loading circuit as a reference input voltage like an op amp that needs a set point or something. 

The other thing that hasn't been mentioned yet is just simply a bunch of diodes in series.  Each diode will drop the input voltage by some amount (text book number is around 0.6 volts per diode for silicon but I've seen much less drop before).  I once tried to drop a voltage down with diodes and I had to string a whole pile of them in series to get the final result, but it's easy to do if you have a bunch and don't mind chaining until you measure what you wanted to see.  If I remember correctly I had a 9 volt battery and I wanted to get 5 volts out of it so I had who knows how many diodes in series with the + terminal. 

[JMDickson]

Putting a resistor inline should not drop the voltage of a well regulated supply.  I usually use diodes if I need to drop the voltage slightly.  Most standard silicon diodes have a .7 v drop, so one or two of those inline should get you right where you need to be.  Most devices that run off of batteries will tollerate the voltage being a bit off, as different battery chemistries produce different voltages.

You could also try an LED, as long as the handheld can run under 20 ma or so.  You will likely need a current limiting resistor if the handheld does not provide enough resistance.

General purpose silicon . ~.7v drop
IR LED ~ 1.2v
Red LED ~ 1.6v
Green LED ~ 2.1v
Blue LED ~ 3.8 - 5v

[markrvp]

Thanks everybody.  Here is the Radio Shack LM317T adjustable-voltage regulator.



How do I wire it in?  What resistors do I use (and where do I solder them in)?

[btoddkelley]

Thanks you Druin for catching that. It is UNREGULATED supplies that drop under load. So much for becoming an EE!

Todd

[ShinAce]

If you want to use a regulator to get 6 volts, you should have at least 8 volts on the input.

In your case, the easiest would likely be a diode or 2 in series.

[Druin]

Yeah I just looked up Texas Instrument's data sheet on the 317 and the recommended input voltage is at least 3 volts higher than output, so either switch the adapter to 8v or higher up to 37 volts, or just try some diodes in series.

[shadowdrak]

It wasn't exactly clear to me from the post, are we talking about a regulated or unregulated supply?