Too many mA draw => USB reset?

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Too many mA draw => USB reset?

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Started 3 years ago by pico
Latest reply from

pico
Member

I'm trying to get a radio transmitter working on a Maple 5. It draws a max of about 12mA or so when active, < 1mA otherwise.

The problem I'm having is that when I switch the radio from standby to active in software, the USB connection resets (in fact, the Maple 5 may be doing a complete reset itself -- not entirely sure.)

Is this expected behaviour if you attempt to draw too much power from the 3.3V rail? I'm running it at 7.5DC Vin, so PD/(Vin - Vout) should be 0.37/(7.5 - 3.3) = 88mA total available. Board itself is supposed to take up about 30mA, leaving about 58mA to play with, I was thinking. I'm also reading a 10K thermistor and driving a transistor to actuate a SS relay.

The radio generates a low interrupt which I've got set up on D2.

I'm initialising with:

pinMode(IRQ_PIN, INPUT_PULLUP);
attachInterrupt(IRQ_PIN, radio_irq_vect, FALLING);

Any other ideas of what might be causing my symptoms? Does drawing too many mA sound plausible to explain these symptoms?

One other thing: It does this if powered by the USB connection, with or without the 7.5DC power supply being connected as well. (The reason I mention this is the USB would only supply 5V as Vin, so that should give me even more headroom to play with, I would have thought.)

Posted 3 years ago #
gbulmer
Moderator

pico - what happens when you test each one of those current consumers connected individually?

Pulling too much current might cause a reset.

Try adding e.g. an LED to show a full reset (delay in setup before turning the LED on, so you can see if there is a full reset)
There is a pin connected to the USB pull-up resistor, so accidentally changing it's state would look like a USB reset.

Have you got a multimeter (or something) to measure the current draw?
Disconnect from USB, and measure the current into the 7.5v DC pin.

Possible thoughts:
1. The radio transmitter is being picked up on a track, and triggering reset
2. The SS relay is connected incorrectly and is shorting something

Posted 3 years ago #
crenn
Moderator

What type of transistor are you using to drive the SS relay? BJT?

I'd suggest isolating the radio itself and seeing if you can reproduce the issue. Can you provide us with a circuit diagram (scanned drawing works!) of how things are hooked up?

Posted 3 years ago #
pico
Member

Ooops... found the problem, a bit embarrassing. I'm not going to admit to _too_ much, but should say that I don't think the reset pin (which is the one _next_ to the 3v3 rail pin) makes for the most reliable power supply for your peripherals!

Thanks for your help, guys.

BTW, what do you think about my assumptions that powering via USB should give me more headroom than via the 7.5VDC external PS. Is it comparing apples and apples or not?

Posted 3 years ago #
Anonymous
Unregistered

I have noticed that if I draw too much power from the Vin Pin while hooked up to external power, the board will reset, or at least the charge LED goes from Green to Red.

Normally on an Arduino, the Vin pin is a straight shot to whatever is coming into the board. I checked the schematics for the Maple, and there is a diode in-line with the power before you get to the power jumper block. Bypassing this diode seems to alleviate the problem, so I am guessing that 400mA@5V is just too much for that diode. :/ Instead of de-soldering the diode or bridging it with solder, I simply built a 5V power supply and attached it across Vin and GND so that I can power the rest of my stuff directly from the power supply and not from the Maple.

If you power from USB, you do not involve this diode, so using an external 7.5VDC PS may cause you the same grief. You can hook up directly to Vin/GND and set the jumper to EXT so that you don't run into problems when attaching a USB cable to your PC. That is how I do it.

Posted 3 years ago #

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