Pretty Basic - dropping 3V to about 1.3v -1.6v

iONic

Joined Nov 16, 2007
1,650
I have a 3V wall adapter (350mA) that I want to drop the voltage on to equal somewhere in the range of a single cell 1.5v battery. Thus I can accept anywhere from 1.25v to 1.60v. I suspect a diode or two could do this, but was wondering if there was any other "easy" option. The device I am powering is a portable(not any more) .mp3 player that operated on a single cell bat. and consumes aprox. 125mA max with high volume and back-light on. This device will be playing for 8-10hrs/day every night, so batteries, even rechargeable ones are really not an option.

Thanks

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MikeML

Joined Oct 2, 2009
5,444
LM317 with the ADJ terminal grounded or jacked up by 0.25V? Need to check the dropout voltage, though...

spinnaker

Joined Oct 29, 2009
7,835
IMHO diodes will be your easiest option.

iONic

Joined Nov 16, 2007
1,650
I have tried three diodes. Vdrops of .65v, .65v, .22v = 1.52V leaving 1.48V available. Measured the open circuit voltage at about 2.0v -2.2v and does not power the device.

The LM317 is adjustable and its Vdo is 1.2 V(min)
Vin = 3.0V, R1 = 240, R2 = 22 for Vo = 1.365V

Ya think Caps are necessary .1uF input, 1uF output???

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marcf

Joined Dec 29, 2014
257
0.65, 0.65, and 0.22v?

I think that you should investigate why the 3rd diode is only has 0.22V across it.

What is the output of the 3V 'wall wart' under load, with say a 12ohm resistor across it?

Do you measure any AC voltage across the load?

If it is a good 3vdc 'wall wart', all three diodes should drop 0.6V under load.

Bordodynov

Joined May 20, 2015
2,542

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MikeML

Joined Oct 2, 2009
5,444
LM317 with the ADJ terminal grounded or jacked up by 0.25V? Need to check the dropout voltage, though...
@Bordodynov 's sim is showing that the LM317 is dropping out because the 3V input is just not quite high enough... I doubt that his Spice model is good enough to model this accurately.

This is from the TI data sheet:

At room temp and at ~150mA, for proper regulation, the LM317 needs a min of ~1.8V differential across it.

Any chance that the 3V wall-wart is actually higher? Is it regulated or just a full-wave rectified transformer with some arbitrary value of filter capacitor. If so, try putting more uF of filtering on it upstream of the LM317, and it might behave...

GopherT

Joined Nov 23, 2012
8,012
I have tried three diodes. Vdrops of .65v, .65v, .22v = 1.52V leaving 1.48V available. Measured the open circuit voltage at about 2.0v -2.2v and does not power the device.

The LM317 is adjustable and its Vdo is 1.2 V(min)
Vin = 3.0V, R1 = 240, R2 = 22 for Vo = 1.365V

Ya think Caps are necessary .1uF input, 1uF output???
If you can accept down to 1.25 V, why not use a bare wire for R2? An LM317 is simply a 1.25V fixed regulator.

And... If your load draws more than 10mA at 1.25 V, then you can eliminate R1 as well.

iONic

Joined Nov 16, 2007
1,650
I think that you should investigate why the 3rd diode is only has 0.22V across it.

What is the output of the 3V 'wall wart' under load, with say a 12ohm resistor across it?

Do you measure any AC voltage across the load?

If it is a good 3vdc 'wall wart', all three diodes should drop 0.6V under load.
It's a pretty cheep wall-wart, designed to power a "Game Boy"
Open Circuit voltage is 3.07V
Voltage across a 10ohm, 0.5W resistor read about 2.6V
I measured the voltage drop on the diodes using the "diode" setting on my multimeter
2 diodes @ .65v (45765) were the same, the third diode was a different part# (IN5819) which read .22v
Don't think there is much AC, .65mV on the AC setting on the multimeter

GoferT said:
If you can accept down to 1.25 V, why not use a bare wire for R2? An LM317 is simply a 1.25V fixed regulator.

And... If your load draws more than 10mA at 1.25 V, then you can eliminate R1 as well.
Yes, 1.25V will probably work fine. A 5ohm or even a shunt might work, given the poor input voltage.
I'm gonna scan my Vregs, I should have one with a lower VDO.

Update:
Just found a couple of new options.
1) MCP1700-1802E
LDO 150mV-350mV
Io 250mA
Vo 1.2V - 5.0V
Vi 2.3V - 6V

2) An old 5V 1A cell phone charger supply if needed

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ScottWang

Joined Aug 23, 2012
6,882
I was modified from my another circuit, but this one haven't try yet, if you want then you can try it, and the output is adjustable.

iONic

Joined Nov 16, 2007
1,650
I was modified from my another circuit, but this one haven't try yet, if you want then you can try it, and the output is adjustable.
View attachment 94601
Thanks Scott, but since much of my electronics parts are boxed up for a move I am opting for a simpler solution, not that your solution isn't simple, but that it takes finding all the parts. Since I found the MPC1700 I can get pretty much the exact voltage with 2 parts. And I even had it operating prior to my soldering......which I am in the middle of right now. So I think I'm at the finish line, I believe!!

ScottWang

Joined Aug 23, 2012
6,882
Thanks Scott, but since much of my electronics parts are boxed up for a move I am opting for a simpler solution, not that your solution isn't simple, but that it takes finding all the parts. Since I found the MPC1700 I can get pretty much the exact voltage with 2 parts. And I even had it operating prior to my soldering......which I am in the middle of right now. So I think I'm at the finish line, I believe!!
MPC1700 is fine for the voltage you want, but its rating current only 250mA and it is better using less 150mA or even less, I will using it less 100 mA, how much current do you need?

If the current is not enough then you can adding a npn bjt to provide the bigger current.

iONic

Joined Nov 16, 2007
1,650
MPC1700 is fine for the voltage you want, but its rating current only 250mA and it is better using less 150mA or even less, I will using it less 100 mA, how much current do you need?

If the current is not enough then you can adding a npn bjt to provide the bigger current.
My understanding is that the current is provided as needed, up to 250mA, and the voltage is fixed at 1.8V. By the way, I added a diode to the output of the Vreg to bring the voltage to 1.5xV. I am noticing, however, that there is a low hum when the supply is plugged in and passes through the speakers. This is not the case with the 1.5V battery.

Bordodynov

Joined May 20, 2015
2,542
See

I want to give a warning. Stabilizers "Low Drop" likely to generate under some load capacitance values!
When a voltage of 1.5 volts maximum output power greater than at 1.3 volts.

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ScottWang

Joined Aug 23, 2012
6,882
See
View attachment 94764

I want to give a warning. Stabilizers "Low Drop" likely to generate under some load capacitance values!
When a voltage of 1.5 volts maximum output power greater than at 1.3 volts.
Could you upload the asc files?
Thank you.

Bordodynov

Joined May 20, 2015
2,542
Circuit for LTspice:

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