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USB 5V to 1.5V

Thread starter #1
I'm looking for a simple way to use the 5V USB power and step it down to 1.5V in order to power a hand-held Audio player/recorder. I'm not sure PWM is the way to go or not, but would not a simple Zener regulator do??


The major problem with using a zener diode is that you're current limited by the value of R in this case. You can reduce R to supply more current, but that means the zener diode would have to dissipate more power when less load is presented.

How much current/power does your device use?

You may have to resort to using a LM317 which can regulate down to a lower voltage, or as you said the PWM which would give you the highest efficiency - and may be the only way if you need more current than the USB port can supply.
Thread starter #3
Any idea how much current the USB can supply. I think it is lower than 1A.

I will see if I can check the current draw from the hand held device.

Actually I was worried that PWM devices might generate a ripple that might cause noise in the audio... maybe i'm wrong.
Linear has several linear regulators that will do 1.5V out. www.linear.com and go to power management, positive regulators, view all, then sort by output voltage increasing.

Most are teeny tiny packages but I think they make one in a TO-220 or similar style package.
You can get 100ma out of a USB port without requesting more, if you want more you have to send a request to the computer and then it will let you draw up to 500ma. IIRC.
Thread starter #6
Just tested my hand held device.

With the music fairly loud it was drawing:

95mA with LCD light on.
70mA with LCD off.
Navigating turns the light on and a temp surge from 70mA to 115mA was seen.
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I've seen way too many implementations where they simply send all the current from the local PS to the device and mark off whether that particular node is powered or not (and sometimes, incorrectly).

If you design the PWM PSU you should choose very high switching speeds and it will make noise production a non-issue. After all, USB power is the output of yet another PWM supply, not like that's going away...

With a linear solution, regardless if you use a chip or zener solution, you'll get less than 33% efficiency. But if you only need 100mA (for bus-power) or 500ma (local PSU) then it's probably good enough. Sounds like you need around 200mA or so to deal with surges thus you will need the local PSU and bus power is insufficient.

So 5V - 1.5V = 3.5V drop by resistor.
3.5V=200mA * R
R=17.5 ohms. Round to 15 ohms. P=v^2/r=3.5*3.5/15=.8 watts wasted here, round up to requiring a 1W device.
Zener diode dissipation worst case: recalculate current with rounded resistor:
3.5V=I * 15 = 233mA
power dissipated = V * I = 1.5V * 233mA=350mW, make sure you get a 500mW zener diode. Looks like it is feasible to get something like this working with available parts.

(If you needed 1A of 1.5V, you'll have to use a switch mode PSU. With a properly designed SMPSU you can get 1300mA or more at 1.5V without too much difficulty from a local powered USB port. (around 250mA with bus-powered) This sounds like overkill though based on your observations, unless you want to be able to run off of bus power.)
Well I've never seen a 1.5V zenner diode before.

Two rectifiers in series will provide 1.2V - 1.4V whihc should be enough.

I would use an LM317 if I wasn't bothered about efficiency, i.e. I'm running it from a desktop computer.

I would use a switching regulator if efficiency is more critical, i.e. I'm running it from a laptop.
Don't you mean Schottky rectifiers?

Silicone is always between 0.6V and 0.7V.
Yeah, Schottky's are a semiconductor-metal junction device. I usually treat them as .4V but as always it's a curve, so .3 to .5 depending on current.

The only reason I'd recommend the switched capacitor solution over a linear is that, if he is limited to 100ma from 5V, the switcher will give him more current at 1.5V than a linear solution.
Does it?

An LM317 will go all the way up to 1.5A, although at those sorts of currents a switcher does tend to make more sense.
I think it would. Unless I'm being retarded again :) Shouldn't he be able to get 1.5V @ 200ma from a capacitor switcher with an input of 5V @100ma assuming 60% efficiency?

Of course, if the USB port will just allow him to draw whatever current he wants, then I agree a linear would be a better way to go. These capacitive switchers are only good for a few hundred mA I think anyway. Any more and you have to go inductive.

mvs sarma

Well-Known Member
iONic said:
I'm looking for a simple way to use the 5V USB power and step it down to 1.5V in order to power a hand-held Audio player/recorder. I'm not sure PWM is the way to go or not, but would not a simple Zener regulator do??
Don't call it any more Handheld-- it is now PC tied!!

For a 1.5V , why at all bother so much and finally loose its mobility?

Use a duracell which will last for say 20hrs of intermittent use. If forcontinuous listening, transfer them to the PC as wave or MP3 files witha suitable software.
Thread starter #19
I've been reading that the USB power standard, (100mA unless requested by the USB device), is not often followed.

So I connected a Luxeon Star III to the power of the USB port on my laptop and
.....WOW! It lit up well! After hooking up a meter to it it was drawing 1300mA,
with a 1k resistor....maybe 200mA. Either way more than the "allowed" 100mA.
A 1k resistor in series with the LED gave 200mA? I don't believe you!

What's the LED rated to?

Did you use a series resistor when it drew 1300mA?

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