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I have a DC adapter 5 V, 350 mA. I'm using this adapter to be fed into a 3.3 V regulator which powers up my bluetooth device and a few LEDs.

My question is: what happens if I source 300 mA from this adapter or if I use all of 350mA or even exceed it? Will it run out of juice or will it get hot?

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got a datasheet? manufacturer part number? photo? – vicatcu Jun 11 '12 at 23:09
no datasheet. The adapter is my very old phone's charger – David Norman Jun 11 '12 at 23:19

3 Answers

up vote 5 down vote

Best-case scenario, the adapter has a foldback circuit, which causes it to drop the output voltage substantially and shut down your attached equipment. This is unlikely unless you've specifically bought and paid for an adapter with this added feature.

Worst-case scenario, the adapter heats up until it catches fire and burns your house down.

Likely scenario, the adapter runs very hot and eventually fails, after years, months, or days, with or without damaging your attached load.

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Thanks photon, I dont think this adapter has the foldback circuit as its a charger from my phone. Its alcatel. – David Norman Jun 11 '12 at 23:21
By foldback circuit, are you talking about a resettable fuse? – Ben Voigt Jun 11 '12 at 23:35
@Ben - read this answer – stevenvh Jun 12 '12 at 4:03
@DavidNorman, Yes, when I said it's "unlikely" to have foldback I really meant "you don't have it." However having a fused circuit (or a regulator with thermal limitting) is somewhat more likely and would lead to a similar near-best-case scenario where you don't start any fires or damage whatever you're powering from the supply. – The Photon Jun 12 '12 at 4:12
@stevenvh: Unfortunately that answer doesn't describe how to get that sort of characteristic. I guess a high-bandwidth (fast response time) thermal cutoff, combined with load capacitance (which power supplies have for ripple reduction anyway) would act like that. – Ben Voigt Jun 12 '12 at 4:18
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up vote 3 down vote

Most voltage regulators have a current limiter, so it probably will run out of juice. Additional safety measures usually includes thermal protection, which either shuts down the output voltage completely, or further reduces output current.

If the output is unregulated (rare nowadays) the transformer will get hot, and go kaputt causing a short, worst case scenario a fire.

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thanks man, I have been having problems with my wifi module and usb to ttl dongle. Thought power was a concern, thus the question but I guess power is not the problem – David Norman Jun 12 '12 at 4:26
up vote 1 down vote

  • As long as the 3V3 regulator that you are using is rated to withstand the maximum voltage that the adaptor outputs then from no load up to any value <= Iout_max the adaptor itself should be very fine.

  • The 3V3 regulator will dissipate power of
    Power = V x I = (Vin - 3v3) x Iin.
    If Vout is a steady 5 Volts then at 350 MA the regulator will dissipate
    (5V - 3.3V) x 0.35A ~= 0.6 Watts.
    You need to ensure that there is enough heatsinking of the 3V3 regulator to handle the maximum dissipation experienced. Modest heatsinking will usually be enough to handle that level of dissipation.

A phone charger may have constant current mode when overloaded.

An electronic supply, which is what you have, based on it being very light weight, will be regulated, so that Vout ~+ Vrated, across most of the load range.

At 0% to 100% of rated load it should run well.

If you load it very heavily it will dissipate up to all energy internally or about 5+V x 350 mA or say up to 2 Watt. Most electronic supplies have overload protection and will progressively shut down if they get too hot. In the worst realistic case , if very heavily overloaded, Vout will drop below nominal and you MAY be able to make it catch fire but most manufacturers try to not let that happen.

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thats true and besides, It doesnt have a transformer becauz its very light. I assume its a resistive or a capacivitive supply inside the adapter – David Norman Jun 11 '12 at 23:38
@BenVoigt - as it was stated and intended to be read the statement was correct. However, as your comment shows that what I intended to be read was obviously not as clear as I thought, I have amended my answer to make my point clearer. Thanks. Note that the original question and my answer relates to how the power supply would fare under various loads. If the supply Vout regulation is poor at light load then it will be designed to handle this AND the dissipation will be low as current is low. As current increases Vout will drop. A 350 mA supply will be designed to operate OK at 350 mA. – Russell McMahon Jun 12 '12 at 2:39
@David - It does have a transformer, otherwise it wouldn't be safe. But this works at a very high frequency and therefore can be kept very small. – stevenvh Jun 12 '12 at 4:23

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