Drop down voltage

[AtomSoft]

Hey guys i have a printer power supply i want to use from a old HP printer. It gives out 16v (625mA) and 32v(940mA) (and gnd of course heh)

The power i need is 28v @ 20mA...

Any thoughts on how to step this down? I tried about 6 diodes in series but heh im retarted.. it dropped to about 30v lol so you help would be nice!

 

[ericgibbs]

Hey guys i have a printer power supply i want to use from a old HP printer. It gives out 16v (625mA) and 32v(940mA) (and gnd of course heh)

The power i need is 28v @ 20mA...

Any thoughts on how to step this down? I tried about 6 diodes in series but heh im retarted.. it dropped to about 30v lol so you help would be nice!

hi atom,
How about a 3.9V or 4.3V zener diode in series, add a small resistive load on the 28V end to 0V, to keep the zener happy, say a 5k6.

 

[AtomSoft]

Oh i think i may have those zeners!!! yes! ill give it a try ... thanks... also can i simple use a BRIGHT LED with a 1k resistor as my constant load?

 

[AtomSoft]

I only have a 3.3v zener... I think 29v would be fine... just have to limit current... a 1.5k should give me about 19mA

 

[ericgibbs]

I only have a 3.3v zener... I think 29v would be fine... just have to limit current... a 1.5k should give me about 19mA

hi atom,
Add an ordinary diode in series with the 3.3V zener.
Dont use a series resistor as the voltage will change as the load current changes.

 

[AtomSoft]

ah ok... I was thinking about that extra diode in series to drop it.. kind of like creating a 4v zener correct?

 

[AtomSoft]

also how did you come to 5.6k resistor in parallel? What calculation would i do to determine the output current?

 

[ericgibbs]

also how did you come to 5.6k resistor in parallel? What calculation would i do to determine the output current?

hi,
The 5k6 is not in series with the zener, its from the zener Anode [28V end] to 0V, so its across the 28V to 0V.
A load of 5.6K is about 5mA across 28V.
OK.?

 

[AtomSoft]

yeah i fully understood that heh...

ok since 28v / 5.6k = 5mA thats all lol thanks

 

[MrAl]

Hi,


You can use diodes or a zener or both, but as Eric pointed out you need to have a load of at least some resistance on the output or you will not see the true voltage drop. This is because diodes need current flowing though them in order to drop voltage; at least some current. 5ma sounds like a good start. You can then measure the output voltage and get a more realistic reading.

 

[AtomSoft]

yeah i understand thanks a bunch for clearing it out tho...

Here is my outcome right now... looks good?

 

[MRCecil]

Another option you might consider is a LM317L (TO-92, SOIC or 6 bump SMD). It's rated at 100ma & 40V max, so with 28V @ 20ma it would be fine with 4V of overhead. Just add a cap and 2 resistors and that's it for under a buck.

 

[AtomSoft]

i would have to order the LM317L ... but would be better... but waiting sucks! lol ill make another design with it in mind...

 

[MrAl]

Hi again,


OH yes, the ever popular and illustrious LM317. A very good idea. So much for so little parts count. It requires so little support parts i've used it in applications that require voltage adjustment (via pot) even though the output regulation didnt have to be that good. Just have to watch power dissipation, but yes with only 4v overhead it shouldnt be too much of a problem. I would bet no heat sink required at less than 100mw.

 

[AtomSoft]

This is for testing on my table anyway.. I would need a boost circuit later on... Have to boost 3.7v to 28v lol going to be fun

 

[AtomSoft]

Here is a picture of my board so far...

 

[AtomSoft]

did a breadboard test and get 28.72v which should be fine.. thanks guys.. I can now make the PCB... This is to power (test) LCD Backlight...

I used a 500ohm resistor to LCD so 28v --- to 500 ohm --- to LCD... and GND and uses a nice 12mA nice light!!! yes thanks guys..

 

[AtomSoft]

Heh here are some images.. I got a video of backlight on but havent upload and probably wont lol