How to bring down 5vdc to 4vdc without using regulators?

[iDk]

hi. whats the best thing should i do to bring down the 5vdc (USB supply) to 4vdc without using any regulators, just by using only discrete components?

thanks.

 

[ME]

USB is normally lower than 5V.

 

[iDk]

hi. i just really want to make it 4vdc. if ever i would put a resistor in series, what would be the value and power rating?

 

[ME]

How much current are you going to draw?
And how stable should the 4Vdc be?
What are you going to use the 4Vdc for?

 

[iDk]

Re: 5vdc to 4vdc?

probably the maximum current consumption is around 500mA and for voltage must range from 3.8vdc to 4.2vdc.

thanks

 

[ME]

This is impossible to do the way you describe.
Read this topic: www.elektroda.pl/eboard/searchtopic51364-500ma.html+usb

You can only draw 100mA until the device is configured.

https://www.beyondlogic.org/usbnutshell/usb2.htm

 

[Tornado]

Re: 5vdc to 4vdc?

You can use 2-5 Schotky diodes in series the forward drop for
each diode is between 0.2-0.3V and varies from manufacturer
to manufacturer, start with 1N5819s they are max 1A.
Because the V drop on diodes is stable enough from
few mAs to max current, you don't have to use resistors
which they dissipate a lot of heat.

Regards
Tornado

 

[iDk]

@ME:

thankss. sorry i forgot i want to use this to power up a mobile phone. maybe the 500mA assumption will only be attained when the phone is in Tx mode. thats only my assumption that the mobile phone would draw a 500mA maximum current.

@Tornado:

thanks for the tips and info. i'll try your suggestion. but as of now i'll start looking for those diodes.

 

[ME]

If you want to connect a phone to the USB plug I reccomend you use a voltage regulator to be sure not to damage the phone. Use a LDO (Low Drop Out) voltage regulator.

 

[iDk]

Re: 5vdc to 4vdc?

thanks. if ever a regulator is the safest way is there any schematic that you can recommend?

thanks

 

[ME]

Re: 5vdc to 4vdc?

You can for example use National Semiconductor LP2989ILD-4.0 or LP2989AILD-4.0:
**broken link removed**

Parametric Table:
Ouput Voltage: 4 V
Output Current: 500 mA
InputMin Voltage : 4.31V
InputMax Voltage: 16 V
Dropout Voltage: 0.31 V
On/Off Pin: Yes

You can order samples online.

But remember the USB port will only deliver 100 mA until the connected device gets recogniced as a high current device.

Power (VBUS)
One of the benefits of USB is bus-powered devices - devices which obtain its power from the bus and requires no external plug packs or additional cables. However many leap at this option without first considering all the necessary criteria.

A USB device specifies its power consumption expressed in 2mA units in the configuration descriptor which we will examine in detail later. A device cannot increase its power consumption, greater than what it specifies during enumeration, even if it looses external power. There are three classes of USB functions,

Low-power bus powered functions
High-power bus powered functions
Self-powered functions
Low power bus powered functions draw all its power from the VBUS and cannot draw any more than one unit load. The USB specification defines a unit load as 100mA. Low power bus powered functions must also be designed to work down to a VBUS voltage of 4.40V and up to a maximum voltage of 5.25V measured at the upsteam plug of the device. For many 3.3V devices, LDO regulators are mandatory.

High power bus powered functions will draw all its power from the bus and cannot draw more than one unit load until it has been configured, after which it can then drain 5 unit loads (500mA Max) provided it asked for this in its descriptor. High power bus functions must be able to be detected and enumerated at a minimum 4.40V. When operating at a full unit load, a minimum VBUS of 4.75 V is specified with a maximum of 5.25V. Once again, these measurements are taken at the upstream plug.

 

[iDk]

thanks again for a quick reply. this is already available at local electronics store?

 

[ME]

This is not a common device, I don't think you will find it at any local electronics store, but you can order samples from National.

If you only need 200 mA or 100mA you can find other devices from National.
There is a lot of other manufactures of LDO's too.

4V is not a common supply voltage, 5V and 3.3V is more common and easier to find. You can also use a variable LDO instead of a fixed one, but it requires external resistors to set the voltage for 4V.

 

[ME]

You can also use the MAXIM MAX1857:
**broken link removed**
Just build a circuit as the one shown at figure 2 in the MAX1857 data sheet.

It's usally very easy to obtain free samples from MAXIM online:
**broken link removed**

 

[lj]

Re: 5vdc to 4vdc?

you can use adjustable ldo

There is a lot of manufactures of LDO's

maxim nsc ti

 

[iDk]

hi. i have just tried to read some notes on the manual of the mobile phone. the maximum battery voltage is 5.4vdc. my decision on dropping the 5vdc usb supply voltage to 4vdc is in order for me in safe limit because the normal battery rating of mobile phone like nokia is 3.6vdc. and also i saw on the schematic of a dongle which is being powered thru USB the positive side they put in series a 1A diode (1N4001 i think?) then this will serve as a power supply of the mobile phone. is that safe enough for a mobile phone just by putting a 1A diode in series?

thanks alot for your replies.

 

[ME]

Yes, it is safe enough if the voltage is within the limits of the phone.
An standard diode like a 1N4001 vil result in a voltage drop of about 0.6 - 0.7 V.
You can buy a 1N4001 in every electronics store very cheap.

 

[iDk]

Re: 5vdc to 4vdc?

thank you very much for all the help and also to others who help and share their ideas. i'll use the 1N4001 diode to clear my doubts.

thanks to all.

 

[Tornado]

Re: 5vdc to 4vdc?

If you don't need 1V exact then 1N4XXX diodes are OK.
I suggested 1N5819 because you can adjust the voltage
drop closer than any other diode type.

Regards

 

[iDk]

Re: 5vdc to 4vdc?

@Tornado:

how can i know the voltage drop of a certain schottky diode? like MBR0250L for example?