[Moved]: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

Hello Everyone,
I have a project that I’m working on which involves installing a USB power source in my 2003 vehicle. I searched ebay and found a nice looking socket from China which was advertised to provide 5VDC @ ~1.55 A for each port. I went ahead and purchased it and installed it. I required me routing out one of my dash panels. The installed product looked very nice and clean. I plugged in my phone and my phone external power icon showed a connection. But there was a problem.

I found my phone was randomly making the tone that it makes when the external USB power jack is disconnected and reconnected. I got a bit suspicious decided to check the load current vs voltage drop using an older potentiometer. I rigged some USB male and female connectors so I can splice into the circuit path. I basically slowly minimized the resistance of the POT, connected 1 –DMM as a current meter and the 2-DMM to measure the voltage across the USB port. I found something that was bit concerning.

As I lowered the resistance on the POT, I saw the current rise and voltage drop. The product was actually only able to provide 150mA before dropping below 4.75V (the low end of the USB spec). I contacted the seller and basically they want to send a replacement. I’m a bit sketchy now because I don’t think this is a defect but a poor design.

I was determined to have my 5VDC USB jack in my car so I decided to take a look at a linear voltage regulator. In this case I purchased some LM1084IT-5’s from Digikey. I tested these under the same load circuit and drew 2A @ 4.99V then my potentiometer started smoking. The regulator didn’t heat up to the touch either.

My question:
Is this the proper way to validate the current specification? Are linear power supplies and switched power supplies tested in the same manner? If so, this just means that these types of Chinese electronics are way over spec’d on ebay.

Any insight or advise is greatly appreciated.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

What specs? They usually dont have specs on Ebay
Maybe they meant surge current for 1ms. ...arg.

Yes the Load regulation is essential the inverse of the output ESR where 50% load regulation uses a load equal to ESR of source.

Active Load
1) Use a power Darlington transistor and bias that easily with any 10k pot with gain from 1k to 10k.
2) Add a current sense R ( 100mV at max current. using AWG 30 if none around.
If you fold in half magnet wire and twist, the inductance tends to cancels out.
3) keep circuit small and low inductive as possible measure Isense voltage a > few mm away from ends ( Kelvin method)


0.25V drop with 0.15A means ESR =1.67 so for 1% load regulation, load must be 100x or 167 Ohms or 5/167 = 30mA which is typical for a super small LDO.


I suggest you either blew it with ESD or you have been misled on specs.

With cig lighter USB adaptors on the store pegs for $3.99,
and wire-in aux cig lighter sockets hardly more than that,
I wonder why all the effort.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

SunnySkyguy said:

What specs? They usually dont have specs on Ebay
Maybe they meant surge current for 1ms. ...arg.

Yes the Load regulation is essential the inverse of the output ESR where 50% load regulation uses a load equal to ESR of source.

Active Load
1) Use a power Darlington transistor and bias that easily with any 10k pot with gain from 1k to 10k.
2) Add a current sense R ( 100mV at max current. using AWG 30 if none around.
If you fold in half magnet wire and twist, the inductance tends to cancels out.
3) keep circuit small and low inductive as possible measure Isense voltage a > few mm away from ends ( Kelvin method)


0.25V drop with 0.15A means ESR =1.67 so for 1% load regulation, load must be 100x or 167 Ohms or 5/167 = 30mA which is typical for a super small LDO.


I suggest you either blew it with ESD or you have been misled on specs.

Click to expand...

There appears to be some confusion. Below is a screen capture of the product in reference. I'm not sure if this is relevant considering this is a very basic test.

- - - Updated - - -

dick_freebird said:

With cig lighter USB adaptors on the store pegs for $3.99,
and wire-in aux cig lighter sockets hardly more than that,
I wonder why all the effort.

Click to expand...

We have those scattered all over our glove box. Problem is when two larger devices are connected they can not supply enough current.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

chiques said:

There appears to be some confusion. Below is a screen capture of the product in reference. I'm not sure if this is relevant considering this is a very basic test.

Click to expand...

I suggest you either blew it with ESD or you have been misled on specs.
With this additional information, it appears to have been damaged.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

SunnySkyguy said:

I suggest you either blew it with ESD or you have been misled on specs.
With this additional information, it appears to have been damaged.

Click to expand...

Oh, I didn't realize these things were vulnerable to ESD after they have been assembled. My bet is that the specs are off. Thanks for the opinions.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

tear it down and find out.

Re: 5VDC Load Testing Question- Project: Adding USB 5VDC to my Vehicle

SunnySkyguy said:

tear it down and find out.

Click to expand...

Good idea ��