# Three 1.5 AAA vs. 4.5 wall wart

Discussion in 'General Electronics Chat' started by TexAvery, Mar 12, 2010.

1. ### TexAvery Thread Starter Member

Oct 7, 2009
58
0
I am powering two LED fans with a 4.5 VDC 600 mA. wall wart instead of three AAA batteries.
The fans spin a bit faster compared to using three AAA batteries that the fans were originally designed for. After 3 hours the LED's burn out.
I think the current is too high compared to three AAA's.
If this is correct how can I reduce the current?

BTW... I added a zener diode but now the voltage is 3.7 VDC.. too low to make all the LED's light.

Last edited: Mar 12, 2010
2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
There is likely high ripple on the wallwart, you are only seeing the DC component. Add a large capacitance (for testing purposes) and measure the DC.

Remember, it is the load that dictates how much current is pulled. If there are pulses hidden in the pulses then they will be current spikes.

What is the value of the zener?

The zener is the best evidence that you aren't using filtered DC.

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The wall wart outputs roughly 4.5v when there is a 300mA load applied. The problem is likely that your LED fans require far less than 300mA current, so the output voltage of the wall wart is too high, resulting in your LEDs getting fried.

What does the output of the wall wart measure with no load on it?

How much current does one of these fans use when running from three AAA batteries?

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I like Wookie's answer better overall. Have you measure the voltages while running this?

5. ### TexAvery Thread Starter Member

Oct 7, 2009
58
0
4.5 Vdc 1600 ma. wall wart (I am worng about the 600 mA. original post)
Diode = 1N4148
When diode is in series voltage=3.7
Each fan current must be over 200mA. because my toy multimeter only goes to 200mA. and it reads OVERLOAD
with a rapid beep-beep

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728

Jul 7, 2009
1,585
141
You can get a suitable multimeter for less than 10 bucks from Harbor Freight (I've seen them on sale there for \$4).

8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It is best to measure current by measuring the voltage drop across a 1 Ohm resistor.
If you measure current directly, you stand a very good chance of blowing your meter's fuse - or the meter itself.

If you place a 1 Ohm resistor in the current path, and measure the voltage drop across the resistor, you get a direct correlation from Volts to Amperes.
This is because Ohm's Law states:
I=E/R, or Current in Amperes = Voltage / Resistance in Ohms
If R=1, and I=E/1 ...

PackratKing likes this.
9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Well, the Harbor Freight meters DO have a 10A setting, but you have to plug the positive lead into the 10A jack instead of the V/Ohms/mA jack. That's pretty typical for any meter though.

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
The cheap fans were made in ***** which is a country where junk is made. You are lucky that they lasted 3 hours.

11. ### hwy101 Active Member

May 23, 2009
91
28
Ha Ha Ha...so true, i miss the days when electronic devices were actually made in north america, they were built to last, then that cheap junk from overseas stated to flood the market and now people are just dumping that crap into our landfills because it quits working in less than a year.