Thursday, August 18, 2011

5V noise generator

An increasing number of electronic devices can be powered or charged from a USB socket. To charge them independently of the computer they usually come with a small switched mode wall-wart. Finding enough wall sockets for these broadband noise generators - in addition to all the wall-warts that already exist in the ham shack - can be a challenge. It would be nice if ham radio power supplies had a few 5V DC outputs.

When I started using my HTC Touch Pro smartphone as an APRS iGate I wanted to avoid using the HTC charger as I knew it created a few warblies on some of the HF bands. I wondered if I could power the phone from the shack supply instead. In the junk box I found a car charger for an old satnav that had the same mini-USB plug and fitted the cigar lighter type socket on my Diamond GSV3000 power supply. That seemed to do the job very well and didn't appear to generate any RF noise (though the noise level here is now so high that the incremental difference made by one more switched mode device is hard to detect.) The only annoying thing about it was that it sticks rather a long way out of the front panel of the GSV3000, which is a bit of a nuisance on my narrow operating desk.


My next thought was that it would be nice to have more than one 5V outlet, perhaps in USB format, to occasionally charge other devices without requiring the computer to be on or plugging in another wall wart. Browsing eBay I saw a Griffin Dual Power USB Car Charger. This would give me two 5V USB outlets. And it was also designed to be flush with the top of the cigar lighter socket so it would look like an integral part of the GSV3000 once installed.

When it arrived I was quickly disappointed. As soon as I plugged a USB cable into one of the outlets, even with no load attached, broadband HF noise jumped up about 3 S-points. The device also got surprisingly hot even though nothing was drawing any current from it. I guess I should have anticipated that  a device that size would use some kind of switching regulator and that this might result in noise.  For the typical user wanting to power or charge two USB devices from their car, the noise is unlikely to be a problem. So I guess this particular solution to eliminating wall-warts is a non-starter.

6 comments:

Casey Bahr said...

Would it be possible to simply plug in a USB hub to the original Griffin adapter and get the 5V from that? I guess you might have to use a powered hub, which would require another wall wart though...

Glenn DJ0IQ and W9IQ said...

Hi Julian,

The USB spec allows 0.5 amps at 5 volts per USB connector. When you start with a 13.8 volt supply and use a linear regulator to get down to 5 volts, you generate up to 4.4 watts of heat per USB connector - about 36% percent efficient.

If you use a switching regulator you can get 80 - 90% efficiency but the small form factor involved does not allow for proper harmonic suppression. Some of the commercially available, mains powered USB chargers have decent filtering.

At the inconvenience of another wall wart, you could start at 7.5 volts into a 7805 linear regulator. This could handle 2 USB jacks and only generate 1.25 watts per jack. A pretty simple project since you already have a board with two USB connectors mounted...

- Glenn DJ0IQ and W9IQ

ES2DC said...

Hello,

it could be just defective regulator especially if it gets hot with no load. Had similar experience with same kind of equipment bought from DealExtreme. One went really hot and cut off 5V output in couple of minutes. Opening the case revealed someones horrible soldering skills and definitely miswired ciscuit (shortd caps wit tin blob over them). Other chargers of same brand seem to work well with no problems, I guess its just a matter of luck.

Julian Moss said...

I wonder if this is one of those products where there are a lot of fakes? Some eBay sellers have these Griffin chargers on sale very cheap. I bought this from a UK seller but it didn't come with any retail packaging.

Unfortunately to try to find a good one could turn out to be an expensive lottery, as I found when trying to obtain a usable short stubby V/UHF HT antenna. It's not worth the cost of sending the bad ones back for a refund.

Ones and Zeros on the Interweb said...

Why not use an ATX power supply to obtain your 5v supply? I use a couple of these in the shack in lieu of buying a proper Ham power supply. They were of course free, requiring only to be taken out of old PCs that were being scrapped. I have been using them for more than a year and one of them runs the FT-857 more or less 24/7 on WSPR. I am sure you will know that the ATX supply provides a variety of voltages including 5v.

Regards

Ones and Zeros on the Interweb said...
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