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Product certified (tested) up to 4.2 V @ max of 1.2 A from 5.0 V @ 2 A supply

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Aug 27, 2022
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Have a SA (spectrum analyzer) powered via 5 V USB supply. Battery management IC hosed, NOT available. Product is certified - tested up to 4.2 V @ 1 A.

What is the best option?
Use a 1A # _____________ diode (forward voltage drop)?
Use a 1A 4V - 4.2V Zener diode?
Or ..........?

Needless to say space is 'limited'!


Mrs. RA

There's a question whether your USB is able to provide 1A. Newer types can.
Is it 'old' A type, or newer B or C?

What Amperes does your device draw from USB? The device may not be designed to draw high current.
If it contains a battery then caution is needed. It's essential that you do not send too much current through the battery nor overcharge it. Or do you intend to remove it?

An ordinary silicon diode is handy for dropping 0.6 or 0.7 V in series. Say, 3A 50V. (Example, Radio Shack cat no. 276-1141.)

If you wish greater voltage drop, consider a higher voltage or current rating. Such a diode may offer slightly greater internal resistance.
While experimenting you might try adding 1/2 ohm resistor in series.


THANKS for your prompt reply. Sorry if my info was not clear - sufficient. The BMS reduces the supply (5 V) to 4.2 V at a maximum current draw just over 1 A (depending on function chosen i.e. the SA's power bus is supposed to be 'hefty' enough to power an LNB/line amp) if needed.

I first used two 300 mA parallel silicon diodes in series with two identical diodes (also in parallel), but the voltage drop was too low. The SA would not complete it's self test.

The tiny SA is so tiny (.75"? T X 5"? X 4"?) so a 5 W resistor would be unlikely to fit. Needless to say there's NO space on the PCB for a SMD (voltage regulator, etc.). .6 V to .7 V 'should' work, but my assorted diode supply is painfully small. The nearest possible supplier is 78 miles one way.

Your help is greatly appreciated. More ideas?

Remain WELL & be SAFE!

Mrs. RA

Voltage regulation via transistor. Bias voltage 4.8 V provided by string of 3 red led's (or zener diode).

Load is placed at emitter leg. It receives 4 V regulated, over a wide range of load resistance.

NPN biased to 4.8V regulates voltage 5V supply load 4 ohms gets 4V.png

--- Updated ---


Or since you evidently have 4 diodes, make a string of 1 diode and 1 ohm resistor. Duplicate this so that you assemble 4 such strings in parallel. The diodes drop about 600mV. The resistors also subtract a fraction of a volt, while making sure no diode carries more than 250 mA.

Attach load in series. Output voltage wanders up and down a bit as load changes. However this is a simple method to drop a volt or so.
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Is the battery a dangerous Lithium-Ion type? The 4.2V is its maximum allowed charged voltage.
To prevent a fire and explosion, you need a proper charger circuit to replace the Battery Management IC.

Sorry about the delay in my reply! First. no battery is involved. The ultra miniaturized spectrum analyzer can be powered via it's Li battery or via USB connector. Since the BMS (battery management system) became inoperative the battery cannot be charged and it also served as a 5 V regulator - conditioner the USB line is also inoperative. The battery will be removed. The objective is tap the 5 V USB line, lower the 5 V line to a maximum of 4.1 V with maximum current of 1.25 A. (the spectrum analyzer can provide power for an in line LNA if needed).

Due to miniaturization there is no room for a PCB/several components such as several resistors / 3 A diodes. I looked, but could not determine if there is a 4 V / 4.1 V 5 W Zener diode. If not, how would two Zener's be

My calculations based on 4.1 V with 1.25 A current calls for a 3.28 Ohm 5.125 W resistor. I'm reasonably confident there is no stock 3.28 Ohm resistor so if I use a 10 & a 5 Ohm resistor in parallel 3.3 Ohms should be close enough using 5 W resistors?

Is there a better option? Odds are I live in a VERY rural area so will have to order whatever is suggested, so PLEASE cite a source that should stock the needed part(s).

Thanks for your interest, attention, AND assistance. Be WELL & be SAFE!

Mrs. RA

Your load appears to be 3.3 ohms.
My simulations show that a mere fraction of an ohm is needed to drop 5V to 4.2 V @ 1.2 Amperes.

Here are 4 simple methods using a diode or resistor or combinations.
The rightmost arrangement needs careful adjusting so that you balance stress on the two diodes. The resistors can be custom lengths of thin wire, for example nichrome. Or a few feet of 22 or 24 or 26 gauge copper.

reduce 5V to 4_2V 1_3A (4 methods diode resistor).png


Thanks for the detailed options. As is too often the case, I may be misunderstanding your understanding of how little space is in the 'Tiny SA' spectrum analyzer? Even with the battery removed the total available size is 6.69" X 4.72" X 2.56". The maximum available space is less than a sewing thimble! So a, "... few feet of #22, #24, or #26 wire .... probably would NOT fit.

I always encourage people to ASK rather than not 'know', so I'll ask! Looking at your 4 diagrams (options) it appears 1.3 or 1.2 A is ?total current flow? Since considerable current flows through the 4.1 / 4.2 V tap off how - why do you label so much current flow through the dropping circuit to ground? I've not measured average current flow through the SA. I'd guess 350-700mA so what am I failing to take into consideration?

For the 3.3 Ohm resistor(s) in diagram #2 the parallel combination of a 5 & 10 Ohm 1 W resistors should suffice/work. That only leaves the diode. Will any 1 W Silicon diode work? At least I have several on hand.

AGAIN, thanks for your interest, assistance/help AND patience! Remain WELL & be SAFE!

Mrs. RA

Wherever you see a 3.3 or 3.9 ohm resistor in my simulations, that is your spectrum analyzer (load).

Are your diodes 1N4001 family or similar silicon type? With 700mA current draw the voltage drop from a silicon diode is liable to be insufficient. Probably you need to drop 0.3 or 0.4 V additional.

Notice you can solder three 1 ohm resistors in parallel to get a 1/3 ohm resistor. (These can be little bitty 1/8 W size.) Such are the experiments available to you as you try to produce a custom supply voltage.

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