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Author Topic: 5 Volt Transceiver?  (Read 4230 times)
K0JEG
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Posts: 669




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« on: November 17, 2012, 10:52:09 AM »

Given the proliferation of small 5 Volt power packs for recharging phones and other USB devices, I got wondering if there are any small radios that are powered by 5VDC? What would be some of the limiting factors to 5V, assuming a high current draw was acceptable?
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KA3NXN
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« Reply #1 on: November 17, 2012, 01:42:05 PM »

The issue that I see here is that most if not all final amp transistors require a minimum of 12v for bias. with very few exceptions that are found in some VHF & UHF RF modules.


Jaime-KA3NXN
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WB6BYU
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« Reply #2 on: November 17, 2012, 08:57:26 PM »

I know there have been some HTs designed to run off of +3V, using a pair of
penlight cells.

There are a lot of chips these days that are designed for +5V or lower voltages,
partly because it allows the use of simpler battery packs.  The parts I deal with
at work are rated for nominal 3.6V, but have to operate up to 4.35V (when the
battery is on charge) and down to 2.8V (when the battery is low.)  There
shouldn't be any problem designing the low level stages.

There are power amplifier transistors available as well:  one issue is keeping the
saturated voltage across the transistor low enough to give good efficiency.
That's why most of the higher voltage transistors don't work as well at lower
voltages.  If you are designing your own radio, you'll have to earn about the
newer transistors instead of sticking to the old reliable parts such as the
2N3855 or 2N3553, and they may only be available in surface mount.

You can use several transistors in parallel if necessary.

One issue, of course, is the current required:  about 2.5 times the current
that a 12V radio would require.  Probably would work up to a couple watts
or so, but because many modern batteries are pretty small in volume the
capacity won't hold up very long if you are drawing a couple amps on
transmit.

A practical matter is matching the output impedance:  a 5W transmitter
has a 29 ohm output impedance at 12V but only 5 ohms at 5V.  Clearly the
former is easier to match (though the latter is commonly done for higher
power transmitters running from 12V, so it isn't impossible.)


An alternative approach is to use a 5V to 12V switching converter to
run a rig designed for 12V.  That's a fairly common approach for things
that run off of a pair of penlight cells.
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K0JEG
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« Reply #3 on: November 18, 2012, 05:39:58 AM »

Thanks for the very complete answer. You've pretty much confirmed what I expected, that there are components available but they haven't trickled down to the ham radio world yet. I know that power is always a trade-off, but I like the idea of a $25 solar panel with a 4000 ma battery running a small radio all day.
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KB1GMX
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« Reply #4 on: November 18, 2012, 08:41:30 AM »

 >>but I like the idea of a $25 solar panel with a 4000 ma battery running a small radio all day.<<

That does not have to be a 5V source!

I use a 12V 3W panel (Volkswagon surplus) and a 2000mAH NiMh battery to run a KNQ7A for
10W 40M SSB with plenty of power to spare. 

The last time I bought those panels I found 4 of them for 25$ at  a ham flea.  They can be used
in parallel and would net about .64A of charge current in bright sun (12W).

The real issue is what radio.  Most of the RF power devices for 5V or lower use are for
VHF/UHF FM use.  They are common enough and fairly cheap (RFparts has RFxxx series).
However you would then be on your own for design.  For receiving most radios rely on
the SA/NE602 which has a maximum voltage of 8V and runs well enough on 5-6V.

Most of the QRP radios (excluding the IC703, FT817, KX3) run on trivial amounts of power on
receive and at about 12V so a small battery generally does it.  You need more power for the
transmitter but that is intermittent peaks.  Going with 12v being a common standard with most
autos and such that its the common energy source is convenient.


5V was convenient for digital logic so it's basis is there not radios. 

Generally most HTs that run off penlite cells do so by trading much higher currents at the
expense of battery life.  I have several HTs like that.  The upside is penlite cells are
common and cheap.


Allison



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WX7G
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« Reply #5 on: November 22, 2012, 09:49:43 AM »

Connect three 5V battery packs in series and you're in business.
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N3QE
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Posts: 2288




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« Reply #6 on: November 22, 2012, 11:55:27 AM »

Given the proliferation of small 5 Volt power packs for recharging phones and other USB devices, I got wondering if there are any small radios that are powered by 5VDC? What would be some of the limiting factors to 5V, assuming a high current draw was acceptable?

There have been many QRP transmitters that use TTL chips (5V nominal) as the finals. Google "QRP" with "74HC240". Often the designs show the 74HC240 with +8V as VCC to get even more power out, but it will certainly work with 5V too.

A typical 5V USB charger power pack will go into current-limit mode around 500mA or so but that's plenty of current for the 74HC240 based transmitters.

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STAYVERTICAL
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Posts: 875




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« Reply #7 on: November 25, 2012, 01:24:43 AM »

I have tried using three 7 ampere hour Lion 5volt USB power packs in series.
They work, giving 15V , but as the previous post mentioned, are current limited to around 1 amp in my case.
This is ok for receiving, but they shut down in transmit.
The Lion cells could easily provide the current, but they are hidden behind a lot of protective circuitry, making it not worth the effort.

Good luck,

73 - Rob
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