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Topic: Zener vs. Voltage divider (Read 2713 times) previous topic - next topic

imagiro1

I want to try connecting a USB cable to the Arduino using V-USB. On the schematics, they use a 3.6V Zener diode. I agree that's the easiest solution, except I don't have one, ordering one takes a while, and I can't source one locally. Could it be replaced with a voltage divider? (3.9k/5.1k) would give me 3.8V. Thanks in advance.

jackrae

Basically, in a word NO because the efficacy of a voltage divider is load dependant and the device you propose to attach does not represent a constant load.

Rather than a zener, which by virtue of design demands a non-useful base-load to be applied to the supply, the ideal supply is via a voltage regulator, either linear or switchmode.

MarkT


I want to try connecting a USB cable to the Arduino using V-USB. On the schematics, they use a 3.6V Zener diode. I agree that's the easiest solution, except I don't have one, ordering one takes a while, and I can't source one locally. Could it be replaced with a voltage divider? (3.9k/5.1k) would give me 3.8V. Thanks in advance.


I can't see any schematics in this post ?!
[ I won't respond to messages, use the forum please ]

billroy

Don't try this at home, but I had good luck using 3.2 volt (and higher) blue LEDs in a V-USB rig instead of the Zeners.

-br

imagiro1

Figures I would forget to include the schematics. I ended up finding a whole seller who was willing to sell me a few of them. Thanks for the responses.

http://vusb.wikidot.com/hardware

dhenry

Most white leds are ~3.6v and you can just use those.

cjdelphi

what if you used a voltage divider to get 3.3v

then feed the voltage div out to a transistor (via the base) 5v on the collector, and emitter should give a regulated 3.3v out??

vasquo

A voltage divider is not a voltage regulator.

The zener diode will "clamp" the same voltaqe *constant* across it's terminals (assuming there's enough in the supply voltage).

The voltage divider's voltage will vary *proportionally* to the supply voltage.

I believe one should have a stock of 7805, 7812, 7905, 7912, 317s, 337s and 3.3 volt regulators in their pile of parts. You can get these parts very cheap in TO-92, or TO-220 packages. 

cjdelphi

I have a bunch of regulators and zener 3.3v diodes.

but if i use a Zener Diode in the same way, there is a slight fluctuation, as the resistance changes, the voltage of the diode changes with it.

now, let's suppose the 2 voltage divider was fed a regulated source eg 5v, used the correct voltage divider, you should get a nice stable 3.3v out using nothing more than
a transistor to boot the signal current...

why not? infact i'll try a circuit

cjdelphi

5v power source is easy to find, if you get desperate hack open an old usb cable.....

should be easy to find.




cjdelphi

with exception to my child like drawings,....

um, v-usb, ah, i doubt the transistor would be even needed and why would a voltage divider not work...

westfw

Are you looking at the power supply, or the clamp diodes on the data lines?
For the latter, I'd think that a voltage divider would interfere with some of the USB early negotiation.
Quote
Speed Identification
At the device end of the link a 1.5 kohm resistor pulls one of the lines up to a 3.3V supply derived from VBUS.
This is on D- for a low speed device, and on D+ for a full speed device.


DirtBiker

The problem with a voltage divider is that it puts a load on the line at all times and changes the impedance characteristics.  The diodes do not.  They only come into effect if/when the voltage goes over 3.6V.  I think blue or white LEDs, forward biased (opposite direction to the zeners), would be an acceptable substitute.
Dirt Biker

zoomkat

Note that zener diodes require a certain minimum current thru them to operate at a certain voltage. If on the cheap, five diodes with .7v voltage drop across each placed in series should operate somewhat like a 3.5v zener.
Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

winner10920

Taking a quick glance at that page you need to level shift not supply power, you should either drop your vcc to 3.3v(easiest) or lookup proper level translators(needed for speed), or find proper zeners that can handle the speed

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