Powering a Yaesu VX-7R from the external DC jack
This post is rather esoteric, probably of no interest to 99.9% of readers but I'm sure it's going to get some hits from Google. Anyway, I've been wondering about powering my Yaesu VX-7R handheld transciever from something other than the lithium-io battery pack, say a bunch of AA batteries. That's useful partly for emergency usage and partly as a cheap backup since an extra Li-on pack costs at least $50. There is an official AA battery pack for the radio but it only uses three AA batteries and while using it the transmit power is cut down to the L1 level - a few hundred milliwatts instead of the usual 5W max. Also I knew if you plug a battery charger power supply into the DC jack it would charge the Li-on pack but again transmit was limited to the lowest output level.
So I dug around in my box of junk and found an old wall wart with the right kind of plug on it which turned out to be 4.0mm OD and 1.6mm ID with tip positive wiring. Then I hooked it up to a bench power supply and experimented with the radio's behaviour through a wide range of voltages - from 0V to 16V - to see how it behaves.
A table of the results is given below but basically I was pleasently surprised by the results and was able to conclude that it is perfectly possible to power the VX-7R from a AA battery pack with no additional regulation so long as it's voltage is between 6.5V and 9.2V. My suggestion is you use six alkaline or NiMH batteries - the only downside of the latter is they have a lower voltage, especially under full load - so the battery level indicator will usually indicate the batteries are flat when transmitting. But really since the indicator is expecting Li-on voltage profile you should just ignore the indicator anyway.
There are a few other modes of radio behaviour with DC input voltage ranges from 3.5V to 16V - but Vin of 6.5 to 9.2V seems the most useful and is ideally supplied by six AA (or C or D) cells. The other interesting input level is a supply between 10V and 14V - this not only charges the internal battery (if present) but also allows you transmit at full power. Basically what you get from a 12V car adapter is ideal for this mode of behaviour (with a regulator to avoid any spikes when the car is cranked and low voltage disconnect to avoid killing the battery) - or you could hook up directly to a 12V lead-acid battery (again with a low voltage disconnect).
So here are my testing notes. All transmit currents are measured at 462Mhz (70cm range) with the stock rubber duck antenna.
The DC input jack is 4.00mm outside diameter, 1.6mm inside diameter with a positive (inside) tip.
Connected to the DC-input without a battery the radio will turn on when the voltage reaches about 3.8V and off again when it falls below 3.5V. At this lowest operating voltage the battery indicator is three bars flashing.
At 4.5V the battery indicator goes to three bars non-flashing. Tx power can be set to L1 or L2 and draws 0.20A when transmitting at L2.
At 4.9V the battery indicator is still a solid three bars. Tx at L2 draws 0.35A
At 5.3V the battery indicator goes to no bars (empty) and flashing. L3 Tx power is now selectable but on transmit it shows L2.
At 6.0V you can now set the maximum Tx power but again goes to L2 on transmit. 0.93A is drawn.
At 6.5V the battery indicator stops flashing showing a solid empty indicator but you can now transmit at full power drawing 1.17A.
At 6.77V get one bar
At 7.00V get two bars
At 7.25V get three bars
At 7.43V get no battery level indicator shown (as with fresh batteries) and 1.47A is drawn on Tx
At 8.00V Tx draws 1.74A
When the voltage reaches 9.2V the radio turns itself off - and at 9.4V it turns itself on again. I believe this behaviour is to avoid some unpleasent behaviour in the power regulation circuit around 9.3V.
At 9.4V the radio draws 1.97A on Tx
At 10.0V if the lithium-ion battery is inserted the radio will start to charge, display "Now charging". Charging behaviour seems a little flakey right at 10V so 10.5V or higher is suggested.
At 12.0V the radio draws 1.5A on Tx, with the Li-on battery installed it drew 0.27A while charging.
At 14.0V the radio draws 1.3A on Tx
At 14.5V Tx at max power is no longer po ssible - L3 is the max where it draws 0.90A
At 16.0V Tx at L3 draws 0.86A
I didn't test input voltage any higher than 16.0V to avoid any risk of damage, but I suspect (hope) there is some input clamp for the DC input. Don't take my word for it!
Using 6 fresh AA Duracell alkaline batteries I got 9.70V open circuilt voltage and 9.35V with the radio on in receive (no audio) mode. The radio drew 0.12A in this mode, and 2.6A on transmit at maximum power. After a few brief periods of transmit the battery voltage had fallen to 9.1V in receive mode, and between 6.9 to 7.5V during transmit (there was some observable no-Tx recovery period before it would rise to 7.5, falling to 6.9). The battery indicator would still show one bar during transmit (and even with LiOn use only it will typically show two bars).
You should also note that plugging in the external power supply will cause the radio to turn off and on again. Unplugging it will do the same - assuming the battery is installed, otherwise it will remain off of course.
One other thing - you could probably shrink wrap your own 6 AA NiMH battery pack to fit in the back of the radio where the Lion battery usually is since three AAs fit nicely in that space. It would stick out a bit but would probably be more convenient than a completely external pack. However you'd probably leave the battery susceptable to water damage since the LiOn power connectors inside the battery space are not weather proof. You'd also need to figure out a way to get the AA power to those connectors and create a 6 AA NiMH charging circuit. I think I'm just going to stick with my 6 AA battery box which lets me insert either alkalines or NiMH batteries I have charged elsewhere.
posted by The Editor @ 12/05/2007 03:58:00 PM
8 Comments:
Interesting experiment with this handy. It shut down and back again even if you just connect the charger. But I think the radio is very nice!
Thanks for this interesting article, I have a question. Let's say I will connect an external 7.4V li-ion battery to the DC plug. The original Yaesu battery will stay in it's place. Will the radio drain power from the external 7.4V source or from the Yaesu battery? Big and cheap 7.4V packs are available on ebay...
I haven't been able to test if there is any current at all drawn from the internal battery when the external power is less than the charging threshold (10V) - my belief is no, it draws power only from the external source.
This is based on that if you have ANY external power connected and you reduce it to a lower and lower voltage the radio eventually turns of. I expect the design is such that there is a relay or solid state switch to disable the internal battery unless it goes into charging mode. That is why the radio power cycles whenever you connect/disconnect the external power - it is part of the isolation process.
So - assuming your external battery pack is less than 10V which 2-cell Li-ion series packs are then you will only draw current from the external pack. The internal pack can be removed if you want - it would be really nice if there was a dummy back available - you could probably make one by canabilizing an old used up Li-ion pack to remove the cells.
Just make sure you use a Li-ion pack with a discharge control PCB to ensure that the Li-ions are not over discharged (many people say not below 3V per cell, others say not below 2V per cell). When connected to the external power input the VX-7R will NOT do that for you.
If you had a three cell Li-ion you'd have enough juice to recharge the internal pack and power the radio. So long as you didn't discharge below 9.5v you'd probably be okay, a freshly charged four cell back would start with too high a voltage - it would think it was conencted to the external charging adapter, and support only low power Tx. Then after it was discharged a bit (3.6V per cell) you would start being able to use the radio at full power Tx and charge the internal battery.
I think you're basically best off sticking to 2-cell battery packs, but like I said, make sure you never over discharge them!
I just wanted to say thank you for your post.
Esoteric, yes, but I've been scratching my head about running a VX-7R on AA's for months. I had rigged up an 8xAA holder, but it would power reset the radio when transmitting on high, for some reason.
After reading your post, I understood that, although that problem was curious and unresolved, 9 volts would work just great, and the radio would treat it as simply a very highly charged battery pack rather than a 12V source.
I tested that with an 8xAA holder with two of the cells shorted, and it seemed to work okay.
I ordered some 6xAA battery holders. They came in today, and I just tried one, and it worked perfectly.
Thanks for taking the time to write up your post, it gave me a way to be confident in my radio.
(Although I'll stick with FT-60R's with integrated AA packs for emergencies)
jblitzen - glad you found the post useful. I expect the considerable voltage drop on full power transmit took your 8 AA pack into the 9-10V twilight zone causing the reset. Eight cells is a tempting number but even without Tx voltage drop the cells will eventually get into that zone during receive mode as the cells discharge. At that point the radio wont turn on any more.
In any case as we both figured out unless you are trying to recharge your li-ion as well as power the radio, 6 batteries is the magic number. If you really are wanting to keep the internal battery topped out then 10 AA alkaline cells will probably work - although there is a chance the voltage will start out too high to allow high power Tx.
You could also use four AA cells and a boost mode DC-DC converter set for 7.2V, or 8 cells and a buck mode converter for 7.2V. I don't recall what efficiencies you can expect but I think something around 90% is possible and the converters can be made really small too.
Another good backup power source if weight is no issue would be a small lead-acid cell along with a small solar panel and charge controller. A 5W panel/controller setup is pretty cheap and assuming you are not on Tx all day would probably give you non-stop normal usage on most days. Of course they also make lithium-ion solar packs too...
Thanks for posting this! I have a question. You said:
"At 12.0V the radio draws 1.5A on Tx, with the Li-on battery installed it drew 0.27A while charging."
If I'm calculating correctly, then at 12.0V/1.5A, the VX-7R is pulling 18W.
But at 7.43V/1.47A it is only pulling 10.92W?
Why would that be? Are my calculations correct? Is the VX-7R trying to charge the battery?
My interest in this topic was because I was looking into the feasibility of running my VX-7R off of a solar panel out in the field if need be. I'm trying to figure out how big a panel I need.
Also, if the battery is discharged and you plug in the charger and turn on the radio, will the battery charge while being used?
Thanks again for this post. I found it to be very informative.
I would have to go back and retest to completely verify those results but I'm pretty sure the observation of power consumption increasing as voltage goes up is valid - as best I can tell I didn't have the Li-ion battery installed for most of the measurements except where explicitly stated. I don't honestly recall measuring external current draw while transmitting and with the battery in - it is possible it stops trying to charge the battery, or it just continues to draw the same amount (.27A at 12V). I'd have to patch into the battery connections to figure that out.
My theory is that the voltage regulator they are using is a linear one so that as Vin increases it simply burns off extra power as heat. If not then they have a buck-boost regulator that is getting less efficient. Since buck or boost regulation involves oscillators you'd probably want to keep that out of a radio right? Hence my money is on a linear power regulator.
With the Li-ion battery installed and Vin at anything over 10V it is charging. I think this is the intended slow charge behavior when you are using a 12V power input such as a car adapter. If you go to 14.5V and above the radio no longer transmits at full power - this is what you are getting with the wall-wart DC adapter. At this point I think it is doing a "fast charge" - I can't confirm this because I didn't measure the no-TX power draw with the battery in and Vin >= 14.5V, I wish I had. If I get a chance I will make some measurements with and without Li-ion battery at Vin > 10V and update the article.
As for solar charging - if you want to use the radio with max Tx power you'll have to keep the input below 14.5V, But as Li-ion charging intelligence seems to be internal to the radio then you don't want to use a solar Li-ion battery charger. You should probably want to get a variable DC to 12V voltage regulator. There are some nice 12V power supplies for car based PCs that have just such behavior - they are probably overkill, a simple linear DC-DC regulator circuit is fine if you're willing to do a bit of soldering.
If you don't actually need to use the radio while charging it would be much more efficient to take out the battery and hook it up to a 7.4V Li-ion charger and power that from your solar panel.
Really useful post, thanks !
Just adding the obvious that (i) both the car charger cord and the wall wart PSU supplied with the radio are nominally 12v rated and (ii) the optional 2.5 hour fast charger must pull the full rated current (500mA) from the wall wart to charge the battery pack (1300mAh capacity) in it's advertised 2.5 hours.
Watching the radio's inbuilt voltage display shows a battery voltage of 8.6V hot off the charger. This settles down to the rated 7.4v after a little use or rest, so the unit ought to be able to support full transmission power at 7.4v. Two nice big protected lithium rechargeable cells with a DC plug should make a handy backup.
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