Vusb and the 500ma fuse

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Axiom Greg

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Hello,

From looking at the Teensy schematics, it appears that for Teensy 3.2 and less, the 500ma fuse comes before the Vusb pin whereas later models put the fuse after the Vusb pin (see diagram). Does this mean that if I have a 2amp USB wall adapter, I should be able to draw more than 500ma (and perhaps the full 2A) with Teensy > 3.2?

I have both new and old Teensy and I want to power my project from USB > 500ma. So is it best to tap into the USB 5V prior to entering the Teensy? (that would at least seem to be true for Teensy <= 3.2 if I understand correctly)

Thanks!

Teensy USB.png
 
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For so much current, your best option is usually to connect wires directly from your power supply to the thing(s) which draw so much power.

Don't try to pass more than half an amp through the poor little PCB traces on Teensy. Use a real wire.
 
For so much current, your best option is usually to connect wires directly from your power supply to the thing(s) which draw so much power.

Don't try to pass more than half an amp through the poor little PCB traces on Teensy. Use a real wire.

The Teensy will be mounted inside an enclosure and I'm looking to keep it "permanently" connected to USB. Under normal use, it will be powered from the wall, but I also want to retain the ability to plug into the computer in order to tweak the program. Notwithstanding the fuse (for the older Teensy's), I understand the internal traces have their limits too, so along the lines of your suggestion, I think my best bet may be to tap into the USB cable itself for powering the other things either via a DIY cable hack or better yet a cleaner solution that incorporates a USB cable splitter and female breakout board (image below).

Thank you for your (prompt) response!

USB.png
 
Consider the USB spec says you should not draw more than 100 mA until after the USB host has completed enumeration (or at least the set config part), and only if your configuration says you will use more than 100 mA. Teensy's default config claims 100 mA will be used. The USB host is supposed to check whether you're getting power through an unpowered hub and not send the set configuration command if the hub can't give you enough current.

Even then, the USB 2 spec says 500 mA is the maximum allowed.

Not all USB cables are created equal. The ones PJRC sells have #24, which can handle 500 mA. Many of them have #28 wire. You should check that cable you're planning to use.

Using just slightly over 500 mA can probably work, but if you're planning to go well over the official USB spec, expect problems. It very well may work fine with some PCs and cable and hubs, but fail in difficult to predict ways with others.
 
Thanks for the info, I just ordered the PJRC #24 USB cable to be sure.

I think I may have a simple plan to handle my two use cases which involves strictly separating "power USB" from "signal USB".

1) Development mode - Connect and power the Teensy via "PC-USB". Use "Wall-USB" ONLY to power the peripherals.

2) Operational mode - Power both the Teensy and the peripherals from "Wall-USB".

This would involve adding a jumper to my PCB allowing me to connect/disconnect Teensy power to "Wall-USB" (case 1 above, jumper removed, case 2, jumper in place).

Note: The "peripherals" are about 50 neopixels cycling through spectral colors. I estimate total Wall-USB power would be around 1.5A and since I plan on using FastLED, I can impose a limit as follows:

// limit my draw to 1.5A at 5v of power draw
FastLED.setMaxPowerInVoltsAndMilliamps(5,1500);

(Wall-USB rated at 2.1A)

In retrospect, I think I may have just outlining a general method of using external power, but still allowing USB to co-exist. However, the jumper may be overkill because after revisiting this link:

https://www.pjrc.com/teensy/external_power.html

I just realized that this is a solved problem in that I may be able to do all this by simply cutting the jumper which isolates USB power from external power (or just use a custom made "signal cable") for case #1. In addition, it's also causing me to rethink my desire to use "Wall-USB" since any appropriate "wall wart" would work here (in addition to eliminating the chance of inadvertently plugging the USB power only connector into a USB host). But I think the moral of the story for my application is not to mix and match USB for both power and programming.
 
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I just realized that this is a solved problem in that I may be able to do all this by simply cutting the jumper which isolates USB power from external power (or just use a custom made "signal cable") for case #1. In addition, it's also causing me to rethink my desire to use "Wall-USB" since any appropriate "wall wart" would work here (in addition to eliminating the chance of inadvertently plugging the USB power only connector into a USB host). But I think the moral of the story for my application is not to mix and match USB for both power and programming.
I had great success with Texas Instruments’s TPS2113A autoswitching power multiplexer.
Pololu - TPS2113A Power Multiplexer Carrier with USB Micro-B Connector.
 
I had great success with Texas Instruments’s TPS2113A autoswitching power multiplexer.
Pololu - TPS2113A Power Multiplexer Carrier with USB Micro-B Connector.

Thanks for the suggestion, I had wondered if something like this exists. However, from a cursory read of the description, it's not entirely clear to me if I can achieve the following:

1) When USB is plugged into the the TPS2113A, USB power and data from that connector is passed through to the Teensy and the Teensy is NOT connected to external power.

2) When USB is unplugged from the TPS2113A, external power is supplied to the Teensy.

(in my case, external power is the same voltage as USB so voltage level cannot serve as a trigger)

According to the description:

TPS.png

When VSNS is high, then IN1 (USB power) will be applied. When USB is unplugged, I believe I would want VSNS to go low in order to switch to IN2 (external power), so what I think I really want is the presence of IN1 to be the trigger for VSNS (i.e. connect IN1 to VSNS). Does that seem correct to you? Thanks.
 
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