## Need to reduce 4 volts to 3 volts

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
j611
Posts: 11
Joined: Wed Aug 27, 2003 1:01 am
Contact:

### Need to reduce 4 volts to 3 volts

I am trying to reduce a 4 volt battery to a 3 volt source. I have used a 1 ohm 5 watt resistor, which drops the voltage to 2.9 volts, but the resistor is eating up the battreies current. I think the resistor was using up around 1 amp of current. Can someone help me design a voltage regulator circuit that will do the job and not use so much current?
Thank you.

russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

### Re: Need to reduce 4 volts to 3 volts

j611
Posts: 11
Joined: Wed Aug 27, 2003 1:01 am
Contact:

### Re: Need to reduce 4 volts to 3 volts

Hi Russ,
If you mean how much current does it draw, I am guessing 1 amp or even lower. I am not real sure at this moment. I can measure it when I get home and let you know tonight.

haklesup
Posts: 2978
Joined: Thu Aug 01, 2002 1:01 am
Location: San Jose CA
Contact:

### Re: Need to reduce 4 volts to 3 volts

Resistors do not use up current. The current through the resistor would be the same as that through the load. What the resistor does consume is battery capacity (units of mAh) which translates to reduced battery life. <p>To measure the current easily (without opening the circuit for a meter) measure the voltage across the resistor and the resistor value then divide I=V/R. Since you say you are dropping 1.1V across a 1 ohm resistor (assuming it is accurately 1 ohm) you have 1.1A flowing through your load. If you are using a 2200mAh capacity battery, this resistor will shorten useful charge time from 2 hours to 1 hour.<p>A resistor is never efficient and certainly will not regulate the output should the battery voltage drop when it starts to discharge. However, you cannot beat it for simplicity and low price.<p>If you require good regulation, you should probably go the route of a variable voltage regulator. With just a few resistors you should be able to get the voltage you want. The LM338has a minimum input voltage of 4.2V which is marginal for your application. There are probably other regulators with specs better matching your application.<p>When Russ asked what was the load I presume he was shortcutting a long list of questions like; Can it withstand overvoltage, undervoltage, does it require regulation and to what precision, is the current constant or variable and to what range etc. By simply telling us what it is, we can better make those assessments ourselves and maybe give alternate advice (or at least that's what I would have meant)<p>[edit: strike that last part, I just connected it with your other post about the Flourescent light]<p>Chris<p>[ September 17, 2003: Message edited by: haklesup ]</p>

jollyrgr
Posts: 1289
Joined: Thu Jan 03, 2002 1:01 am
Location: Northern Illinois
Contact:

### Re: Need to reduce 4 volts to 3 volts

By far the simplest way to do this would be to add one or two silicon diodes in series. Each would drop about 0.6 to 0.8 volts depending on current draw. Something like a 1N4001 to 1N4007. Or, if there is not much current draw, a red LED would do the trick.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

haklesup
Posts: 2978
Joined: Thu Aug 01, 2002 1:01 am
Location: San Jose CA
Contact:

### Re: Need to reduce 4 volts to 3 volts

What was I thinking, that calculation with the capacity was totally wrong.<p>If your dropping resistor is 1 ohm and the current is 1.1A and the voltage across the load is 2.9V that makes the load equivelent resistance = 2.6 ohms.<p>Obviously, increasing the series resistance will reduce the current in the loop thus using up the battery capacity more slowly and extending the useful time of the battery. <p>No matter what you use to drop the voltage it will consume 25% (1V out of the 4V) of the power thereby reducing battery life by that amount as compared to the ideal situation of having a load that uses all 4V or a 3V source. Since you have neither and the load probably does not need close regulation, the resistor should be sufficient.<p>The Diode is better than the resistor (for small currents)because the voltage drop is more constant (but not exactly constant) as the current through the diode changes. However, the voltage drop across a diode at 1A will be considerably more than the 0.7V threshold. The 1 ohm resistor has a lower resistance than the forward resistance of most diodes anyway.<p>FYI the battery capacity of a typical alkaline AA battery is about 1800mAh. Two in series is the same, two in parrallel will double it.<p>I find it hard to believe that you can light a fluorescent lamp from 3V without an inverter and that would take more than one xistor and require a transformer of some sort. I haven't seen a Flourescent lamp for use under 6V or 12V is common. <p>Search for "fluorescent lamp circuit" or "cold cathode driver" or some combination of these words.<p>I agree with Edd (other post) that you may have damaged the semiconductor by a temporary reversal of polarity. The 3 terminal device may not be a transistor but a regulator or other special IC. I would expect a lamp circuit (especially one designed to run on batteries) to tolerate a 1V overstress. Even a 2n2222 can handle 4V (C to E)but forward bias the B-E or B-C junction to 4V and put 1A through it and you will have popcorn. Battery operated equipment should tolerate reversal of the batteries though.<p>For a replacement, if you cannot read any markings, you may have to reverse engineer the circuit so that you (or we) can make an educated guess.<p>Is this one of those new camping lanterns Coleman Micro Lantern Most I have seen need 4 or 8 batteries. To get 6 hours out of that lamp it would need to consume 300mA. At 6V that uses 1.8W. The 1.1A you measure may be a result of the burned out transistor or the fact to get 1.8W out of a 3V source you need 600mA but on two AA batteries this would barely last over 3 hours. Does this agree with the performance of your lamp. at 1.1A the battery would be dead in just over an hour.

Chris Foley
Posts: 146
Joined: Mon Jan 13, 2003 1:01 am
Location: Chicago IL
Contact:

### Re: Need to reduce 4 volts to 3 volts

LTC3411 pdf Datasheet<p>Good luck.
Chris<p>[ September 17, 2003: Message edited by: Chris Foley ]</p>

toejam
Posts: 253
Joined: Wed Dec 05, 2001 1:01 am
Location: n.c.
Contact:

### Re: Need to reduce 4 volts to 3 volts

bigger batteries

chessman
Posts: 292
Joined: Tue Jan 14, 2003 1:01 am
Location: Issaquah, WA
Contact:

### Re: Need to reduce 4 volts to 3 volts

Two AAs :p

dyarker
Posts: 1735
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

### Re: Need to reduce 4 volts to 3 volts

Sounds like the 1 Ohm resistor is parallel with the load, instead of in series. THAT will eat batteries!<p>Did you measure the current yet? And, what KIND of load?
Dale Y

### Who is online

Users browsing this forum: gerty and 8 guests