• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Leds to clamp data lines on usd

Status
Not open for further replies.

be80be

Well-Known Member
I ran into a little sag I don't have any 3.6 volts zener diodes and from what I read the USB data lines need a 3.6 voltage level I was thinking a have some white leds couldn't I use them to clamp the line to 3.6
 

Tony Stewart

Well-Known Member
Most Helpful Member
yes it's possible. , a 5mm white LED looks like 3V Zener with 15Ω ESR with a Vth around 2.85V at high impedance, but not sure about 100 pF requirement.

Low-speed and full-speed: VOD: 0.0 - 0.3 V VOH: 2.8 - 3.6 V

4.2.1 Full-speed

Full speed: Rise/fall time (10-90%) 4-20 ns

4.2.2 Low-speed

Low speed: Rise/fall time (10-90%) 75-300 ns.
 

kubeek

Well-Known Member
Most Helpful Member
Option A, connect the resistor to 3.3V.
Option B, use just a resistor to 5V and rely on the other side to do the clamping to the 3.3V rail.
Option C, use a divider and hope it doesn´t get too loaded and stays above the 2.8V.
 

be80be

Well-Known Member
None of A,B and C will work the usb chip is bit bang Usb and has to run at 5 volts so I got no 3.3 volt rail. Im going to use the leds they look like they would work and I have some that should do the trick.
Iet me spend a little time here they say that the data - and + line needs to swing from .3 to 3.6 volts for low speed but all I can find said it's 0 to 3.3 volts I have 3.3 zenners I'll try them but now I kind of like the led idea too LOL
 
Last edited:

dr pepper

Well-Known Member
Most Helpful Member
Hi Burt.
Leds dont have the same slope resistance as zeners, however you can use them as clamps, so long as you dont over drive them, and if the 3.6v is flexible by a few 1/10's of a volt.
 

Tony Stewart

Well-Known Member
Most Helpful Member
Actually they do have the same slope as Zeners in the same power rating.
The slope being ESR is ~ 1/Pd watt rating so 75mW diodes are approximately 15 Ohms.

All diodes have a wide tolerance on ESR.

However junction capacitance also increases with Pd so TVS's are better in this regard.
 

be80be

Well-Known Member
Thanks all I did read where they where used in this type of application now I got find them lol I moved into a new house and my stuff is a mess as normal.
 

dr pepper

Well-Known Member
Most Helpful Member
Doesnt clearly state what vcc is, if its 3.3 then the leds would just be there for noise pickup filtering.
One thing about leds compared to zeners is capacitance, leds will be a little higher.
 

Tony Stewart

Well-Known Member
Most Helpful Member
The main difference is that LED's are forward biased and cannot handle only 10uA and must not exceed >-5V ( reverse bias)
Whereas Zeners are designed to handle reverse current but LED's have same or lower ESR for same power level. e.g. 1W LED is ~1Ω @3V.


e.g. Maximum Zener Impedance [email protected] 1Ω typ 3Ω max at 1W rating for lowest voltage range.

Given 2 conditions, connected with 50 Ω load and no load, the variation in Vo will be great.


May I suggest two AlGaAs Red in parallel reversed in series to provide a 2V drop from 5V and achieve Vo= 3V max with an ESR of 25+15=40Ω is a decent match and will reduce any risk of latchup from overvoltage on the interface with a 3.3V chip and 3.6V limit for interface.

No R's required, but not short cct proof. to 5V you can add 10Ω in series. but this is an unlikely fault.

Otherwise using your schematic it's hard to balance max current, max voltage and matched impedance with connected and disconnected cable.
 
Last edited:

be80be

Well-Known Member
I think I'll try the 3.3 volt zener diodes I have. Im not thinking the best these days could you kindly show me. I'm thinking just replace the zener with two leds there not much current on the data lines they just don't need to reach 5 from the attiny
 

dr pepper

Well-Known Member
Most Helpful Member
You dont need 2 leds to replace one zener, just one, if you put the led forwards into the circuit, ie not reversed like a zener would be it should be fine, another thing with leds is that the reverse max voltage will be lower than a zener, however 5v shouldnt be enough to do them any harm.
 

Tony Stewart

Well-Known Member
Most Helpful Member
There are two conditions ;
50R termination connected
no load disconnected.

Requirements are 20mA max in LED and 3.6V max into data + or - and 2.8V min
As long as these are satisfied it is OK.

With two 2V reversed LEDs current is 60mA so scrap that idea.

Optimal solution requires a series R1 from driver and series R2 to LED such that no connection load drive current is 20mA or 100 Ohm's total (minus 25R+15R = 60 Ohm's shared by R1& R2 ) with 2V drop allowing 0.4V drop on R2 to LED yields 0.4V/0.02A= 20 Ohms and 40 Ohms to driver R1, the. when connected Vo = 5V* (50/(50+40+25)= 2.17V which is low but may work.

Otherwise use a 1W LED with 1Ohm ESR and calc R1=> 50/(50+R1+25)*5V=2.8V then R1=29 ohms and I (no load )= 3V/54 ohms =56 mA
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top