undervolt-resetting of a Yeasu VX5r from weak power supply

[tom]

I have an HT that draws 1.9 amps when transmitting, and the voltage can't
sag more than 2v or the HT does an undervoltage reset. I just bought a
samlex 12v, 4amp power supply to run my HT but there's a problem. The
samlex 12v output voltage begins to sag as soon as the mic is keyed and the
amount of sag increases steadily until 3-4 seconds into the transmission the
HT does an undervolt-caused reset because it's input voltage has sagged 2v.
I might not be able to return the samlex to the shop and replace it, and I'm
wondering if there's something that can be done to enable me to use it some
way, somehow to power my HT. The HT will work fine on a reduced
transmission power setting but then I can't reach repeaters. I have another
wall -wart plug in adaptor that puts out 1700mA at 12v.
-Is there some way I can combine the two power supplies together?
-In parallel maybe? Parallel would probably work if they were batteries
because I know in series the voltage would exceed the max of 16v.
-How about some kind of arrangement with a transformer?
-If I get a lead-acid, 12v, 10amp/hour battery, how complicated acircuit
would I need to charge the battery from the samlex? is this a possibility?

 

[mike]

I have an HT that draws 1.9 amps when transmitting, and the voltage can't
sag more than 2v or the HT does an undervoltage reset. I just bought a
samlex 12v, 4amp power supply to run my HT but there's a problem. The
samlex 12v output voltage begins to sag as soon as the mic is keyed and the
amount of sag increases steadily until 3-4 seconds into the transmission the
HT does an undervolt-caused reset because it's input voltage has sagged 2v.
I might not be able to return the samlex to the shop and replace it, and I'm
wondering if there's something that can be done to enable me to use it some
way, somehow to power my HT. The HT will work fine on a reduced
transmission power setting but then I can't reach repeaters. I have another
wall -wart plug in adaptor that puts out 1700mA at 12v.
-Is there some way I can combine the two power supplies together?
-In parallel maybe? Parallel would probably work if they were batteries
because I know in series the voltage would exceed the max of 16v.
-How about some kind of arrangement with a transformer?
-If I get a lead-acid, 12v, 10amp/hour battery, how complicated acircuit
would I need to charge the battery from the samlex? is this a possibility?

If you're a ham, you need to learn how to figure this stuff out.

I can't tell from your post whether you measured stuff, or just guessed.
If you can't measure volts and amps, your first priority is to buy or
borrow a meter that will.
So don't get offended If I go too slow. Divide and conquer is the strategy.

First, check for operator error. Is there a current limit control
somewhere in/on the power supply that's mis-adjusted? It's possible to
have wire that's too small and a couple of inline fuses contributing to
the voltage drop. Measure the voltage under load at the power supply
terminals and as close to the handheld terminals as you an get.
Also consider the possibility that if your handheld battery is dead, the
charging current plus the transmit current may exceed 4A. I'd say, not
likely, but worth a think.

Next thing I'd do is check the power supply. Use a big resistor or a
car headlight or a couple of tail-lights to put a 4A load on it.
Measure the current and voltage. If you get rated current at rated
voltage, it's probably good. If not, fix it. If you just bought it,
take it back.

Does the VX5 have normal battery life when run on batteries? That shows
that the final is probably not blown drawing excessive current.

But it is possible for the final transistor breakdown voltages to have
been reduced by overheating and draw too much current on 12V.
Verify the external power supply load by running it off a car battery
and verify with your ammeter that it doesn't draw more than 1.9A. If
it's drawing more than 4A, it will drag down your power supply and
probably go up in smoke. Maybe a 2A fuse during the test to reduce the
smoke.

Ok, so they're both good...must be interaction.
Put the 4A lights on the power supply. Run the HT on internal batteries
and waive the rubber ducky around the leads to the lamps while
transmitting. If the lamps dim, you know you have RF getting into the
power supply. Try output bypass caps and ferrite cores on the wires.
If that don't fix it, you'll have to get out the radio amateur handbook
and learn about keeping RF out of your power supply. This should not be
a problem with any modern commercial power supply.

It's almost never a good idea to parallel power supplies. It's always
a BAD idea when one of them is broken. If your 4A supply ain't broke,
you don't need to parallel. If it is broke, you need to fix it.

If your parallel lashup gets more broke and puts out 20V, you'll have a
handheld pile of burnt junk.

mike



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[tom]

I just got back from the shop --- I up-gunned the power supply to a 7-10
(amp) model and it works fine now, at all power settings and antenna
variations, without a single glitch. This model of yaesu has a transient
current spike, according to the shop's guru, that is similar to what happens
when you start an electric motor. At the first instant, the current drawn
is much higher than the rated draw, but just briefly. Some power supplies
can cope, others can't. What's interesting is that I tried a new, hi-tech,
completely solid-state, 8-10amp rated unit (that was 1/4 the size of the
samlex and twice the cost), and it reset every time! But the cheaper, lower
tech, lower rated, older fashioned 7-10amp samlex works fine --- because the
solid state unit can't handle the 'amp-transient' or whatever you call it
(whatever the proper term is, I'm pretty sure it isn't "amp-transient"!).
At least, all of this is according to that shops electronics guru. However,
til I hear another explanation that's a better fit for the facts, I'm going
with the "amp-transient" theory. Anyone got a better one? Mike?

 

[mike]

I just got back from the shop --- I up-gunned the power supply to a 7-10
(amp) model and it works fine now, at all power settings and antenna
variations, without a single glitch. This model of yaesu has a transient
current spike, according to the shop's guru, that is similar to what happens
when you start an electric motor. At the first instant, the current drawn
is much higher than the rated draw, but just briefly. Some power supplies
can cope, others can't. What's interesting is that I tried a new, hi-tech,
completely solid-state, 8-10amp rated unit (that was 1/4 the size of the
samlex and twice the cost), and it reset every time! But the cheaper, lower
tech, lower rated, older fashioned 7-10amp samlex works fine --- because the
solid state unit can't handle the 'amp-transient' or whatever you call it
(whatever the proper term is, I'm pretty sure it isn't "amp-transient"!).
At least, all of this is according to that shops electronics guru. However,
til I hear another explanation that's a better fit for the facts, I'm going
with the "amp-transient" theory. Anyone got a better one? Mike?

Sounds like you have the explanation. Probably could have worked around
it with a big electrolytic on the output of the power supply.
I sure don't like the idea that the handheld can fold up a 10-amp power
supply. That's gotta be a reliability problem for the handheld.
Think I'll cross the VX5 off my wish list.
mike

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[cl]

Sounds like you have the explanation. Probably could have worked around
it with a big electrolytic on the output of the power supply.
I sure don't like the idea that the handheld can fold up a 10-amp power
supply. That's gotta be a reliability problem for the handheld.
Think I'll cross the VX5 off my wish list.
mike

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I'm wondering if perhaps it was RF that got into the power supply and caused
it to shut down or go flaky enough to cause the drift in voltage. I find it
hard to believe that a Handheld screwed up a power supply like that "just"
on current draw. RF getting into a power supply can do some weird things.

cl