D+ & D-, varistors to protect usb lines on a cell phone?

Thread Starter

mikewax

Joined Apr 11, 2016
126
hey guys,
so it seems that most usb devices intended for use with a laptop or desktop host have either varistors or clamp diodes on the D+ and D- lines to protect against spikes.
but my device is intended strictly for cell phones. it draws its power from the host, and the only noise it generates is from a transistor that is switching a 1.5 amp load at audio frequencies, like less than 2kHz.
i've seen guys on youtube connect their devices without these precautions and publish their schematics on the net, so maybe its ok to do so.
anybody have experience with this?
thanx, mike
 

Papabravo

Joined Feb 24, 2006
12,878
They publish specifications for a reason, but you're free to do anything you want. So if you think you know what you are doing -- go for it. That's how progress happens.
 

Delta prime

Joined Nov 15, 2019
160
i've seen guys on youtube connect their devices without these precautions and publish their schematics on the net, so maybe its ok to do so
Of course it's okay to do so.
For example; I just quoted a particular part of your question that I saw fit. Because that is my agenda or my application. There are many schematics for a particular application or purpose. Pick three very simple circuits you can breadboard from the internet. Scratch that, just look at the circuits you will find at least one was never meant to work. In a roundabout way again progress is made!
 

Papabravo

Joined Feb 24, 2006
12,878
MY biggest fear in taking apart things mechanical is not being able to put them back together correctly. Same with PC boards until I saw a tech take a knife to one. I do lots of things to PC boards now that I would never have dreamed of doing once upon a time. Such is the path of progress, and now my budget can stand an occasional binning!
 

Delta prime

Joined Nov 15, 2019
160
So if you think you know what you are doing -- go for it
Hehe sorry... did it again.
You exude wisdom. You have answered many of my own questions that I have not even thought of so... what's a budget? Just kidding and taking the moment to say you're a rockstar my book!
 

Thread Starter

mikewax

Joined Apr 11, 2016
126
yeah you guys have a good point. Time will tell. The way i figure it, a usb device that is connected to a plugged-into-the-wall computer should have some protection, but with a cell phone ya can't plug in my device and plug in a charger at the same time.
much obliged :)
 

MrSoftware

Joined Oct 29, 2013
1,674
It's always good to follow the spec, that ensures nothing goes poof, including the cell phone. The USB specs are freely available online and there's lots of good info if you google around. Also to get 1.5A from the port you may need to do some negotiation.
 

Thread Starter

mikewax

Joined Apr 11, 2016
126
It's always good to follow the spec, that ensures nothing goes poof, including the cell phone. The USB specs are freely available online and there's lots of good info if you google around. Also to get 1.5A from the port you may need to do some negotiation.
that's true, but you know how it is, the datasheets are based on a standard protocol of measurements and tests, and you always have to extrapolate the data for your own electrical situation. In my case i have a device that is powered off the phone and the only high Hz component is the uC. The device is not connected by a cord since the PCB is soldered right to the pins of the usb plug. It's a REALLY tiny device.
as for the current, yes it has to enumerate and yes it has current control, i figured that out beforehand.
anyway if nobody around here has actually tried what i'm doing and says no it don't work then i'll just try it and see....
 
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