Hi,
I've modified a 555 circuit[1] I found online to drive a LED with pulses in
order to reduce the brightness. I expected this to be more efficient than using
a larger resistor value. An attempt to verify this produces confusing results.
The 555 circuit is compared with just a LED and a resistor such that the LEDs
in both circuits appear equally bright. Then I did some measurements of the
current at the 9V battery with my digital multimeter and made a SPICE model of
both. I don't think my DMM is a true-RMS kind so probably useless for this but
I did it anyway.
The SPICE results[2] show a pulse width of 0.0388 ms at 10.75 mA + a 3mA offset
and a period of 1.4225 ms so I reccon the RMS current is 10.75 mA * 0.0388 ms /
1.4225 ms = 0.29 mA not counting the offset. So that's where the numbers below
come from.
The results are as follows for the SPICE model, digital multimeter at direct
current and at alternating current setting respectively.
1) Is there anything I can do with the 555 multimeter results or are they
useless for this? I have no clue what to make of them.
2) Why is the offset current (3mA) in the SPICE model so high? If an NE555 has
a voltage divider made of 3 * 5k resistors wouldn't I get a leakage current of 9V
/ 15k ohm = 0.6mA?
3) Is my interpretation correct that just a LED with a 33k resistor is more
efficient than the 555 circuit?
Ps. On my breadboard I put a 100n capacitor and a 2.2u polarised capacitor
across Vcc and gnd of the 555 that are not present in the SPICE model as I
thought it wouldn't matter for the idealised 555 that LTSpice has.
[1] http://postimg.org/image/wem3qmll5/
[2] http://postimg.org/image/bvf4t1zel/
Edit: The original circuit can be found here:
http://www.learnabout-electronics.org/Oscillators/osc44.php
"Fig. 4.4.8 Improved Duty Cycle Control" at the bottom of the page.
The SPICE model for the 555 circuit:
I've modified a 555 circuit[1] I found online to drive a LED with pulses in
order to reduce the brightness. I expected this to be more efficient than using
a larger resistor value. An attempt to verify this produces confusing results.
The 555 circuit is compared with just a LED and a resistor such that the LEDs
in both circuits appear equally bright. Then I did some measurements of the
current at the 9V battery with my digital multimeter and made a SPICE model of
both. I don't think my DMM is a true-RMS kind so probably useless for this but
I did it anyway.
The SPICE results[2] show a pulse width of 0.0388 ms at 10.75 mA + a 3mA offset
and a period of 1.4225 ms so I reccon the RMS current is 10.75 mA * 0.0388 ms /
1.4225 ms = 0.29 mA not counting the offset. So that's where the numbers below
come from.
The results are as follows for the SPICE model, digital multimeter at direct
current and at alternating current setting respectively.
Rich (BB code):
SPICE DMM DC DMM AC
NE555 3 mA DC + 0.29 mA AC 4.65 mA 0.063 mA
33k resistor 0.25 mA 0.2 mA -
useless for this? I have no clue what to make of them.
2) Why is the offset current (3mA) in the SPICE model so high? If an NE555 has
a voltage divider made of 3 * 5k resistors wouldn't I get a leakage current of 9V
/ 15k ohm = 0.6mA?
3) Is my interpretation correct that just a LED with a 33k resistor is more
efficient than the 555 circuit?
Ps. On my breadboard I put a 100n capacitor and a 2.2u polarised capacitor
across Vcc and gnd of the 555 that are not present in the SPICE model as I
thought it wouldn't matter for the idealised 555 that LTSpice has.
[1] http://postimg.org/image/wem3qmll5/
[2] http://postimg.org/image/bvf4t1zel/
Edit: The original circuit can be found here:
http://www.learnabout-electronics.org/Oscillators/osc44.php
"Fig. 4.4.8 Improved Duty Cycle Control" at the bottom of the page.
The SPICE model for the 555 circuit:
Rich (BB code):
"ExpressPCB Netlist"
"LTspice IV Version 4.21o"
1
0
0
""
""
""
"Part IDs Table"
"R1" "4.7k" ""
"R2" "180k" ""
"D1" "1N4148" ""
"D2" "1N4148" ""
"C1" "100n" ""
"D3" "D" ""
"U1" "NE555" ""
"V1" "9" ""
"R3" "820" ""
"C2" "10n" ""
"Net Names Table"
"A" 1
"N003" 5
"N004" 7
"C" 9
"N002" 14
"0" 16
"B" 21
"N001" 23
"NC_01" 26
"Net Connections Table"
1 1 1 2
1 2 1 3
1 6 1 4
1 7 3 0
2 1 2 6
2 3 1 0
3 2 2 8
3 4 2 0
4 3 2 10
4 4 1 11
4 7 2 12
4 7 6 13
4 10 1 0
5 5 1 15
5 7 5 0
6 5 2 17
6 7 1 18
6 8 2 19
6 9 2 20
6 10 2 0
7 6 2 22
7 9 1 0
8 7 4 24
8 7 8 25
8 8 1 0
9 7 7 0
Last edited: