using diode forward drop to reduce voltage?

[mr_doyle]

i was wondering if there's something wrong in this procedure: i have about 22/23 volts DC coming out from a power supply, and i want to lower  this down to 18/19 volts.
what if i use a fe diodes in series? their forward voltage drop would "eat" a few volts, and it would be simpler than using a 7818 or similar...
What do you think?

TIA,
D.

[bancika]

You probably could use diodes (IIRC one would eat about 0.5V), but it's better to use regulator in combination with two filter caps (220uF or something like that)

[Joe]

You'd need a lot of diodes (~8), and the voltage will change depending on the load. It would be easier to bias a power transistor with a resistor (say 22K), and take the output from the emitter, but it's still not regulated.

[brett]

Hi.
You can lose 0.7V per diode (use 1n4004s or 1N4007s).  If the current is high (>0.5A), watch their power rating and keep them away from electrolytics and anything else that doesn't like heat.  To keep riple to a few %, you'll need large caps (at least 2200uF per amp is a good rule of thumb).

Other options:
for low ampage (<1 A), a large cap and a 7818 would be a good way to go.
for medium ampage with good regulation (1A up to about 10A), the 7818 needs to work through a power transistor (the 7818 datasheet has the details), OR
You can parallel a couple of 7818s if rigged up with low-value resistors (0.2 to 0.5 ohm). There are threads here and links on the web about doing this.  You lose some of the precision in the regulation.
for high amperage (>10A) you need some serious advice 
good luck

[Peter Snowberg]

LM317 

[R.G.]

LM317  icon_biggrin

Oh, sure. Just pick the easy, simple, logical thing to do. 

[Eb7+9]

i was wondering if there's something wrong in this procedure ...

dropping voltage with diodes is a standard trick in IC design - I used two strings of four 1n4148's in series to drop +/-9v down to +/-6v for my Rocktron X100 which pulls about 36mA of current ... you can use Ge (200mv) and Si (700mv) diodes, LED's (1.2v~1.8v) and diode connected transistors ... if you're pulling lots of current (over 50mA) then you'll need power diodes, still works fine ...

[gez]

Plus, the humble zener.  I've used them like this, but they can introduce noise sometimes.

[mr_doyle]

Wow, thanks for your inputs guys!

D.

[Paul Perry (Frostwave)]

One trouble with dropping a voltage using series diodes (whether foward biased power diodes or a zener) is that you are subtracting a fixed voltage from an unregulated source, so if the source voltage changes (maybe from varying load) then, the output is LESS regulated that when it was unregulated in the first place!
Of course, this may not matter here..

[gez]

One trouble with dropping a voltage using series diodes (whether foward biased power diodes or a zener) is that you are subtracting a fixed voltage from an unregulated source, so if the source voltage changes (maybe from varying load) then, the output is LESS regulated that when it was unregulated in the first place!
Of course, this may not matter here..

Yeah, I've had that problem, though it's more to do with zener voltage drop varying with current.  A simple RC filter helped (if you can get away with the voltage drop across the resistor), the cap acting as a reservoir.  Not ideal, but no extra current is wasted as is the case with regulators, and if rock solid supply isn't critical then it can be an effective, albeit crude, way of doing things.