# DC Circuit Problem

Discussion in 'FAQs' started by N Gauger, Jul 31, 2004.

1. ### N Gauger1:20.3 Train Addict

I have a "Store Bought "led flasher" circuit - it works perfectly with 2 D cell batts but when I try to hook it to my 12V Regulated Power Supply (With dropping resisters in series) it doesn't work

I get 2.6 V from the batteries... but...when I take the reading from the "power Supply - it keeps varying between 1.3V & 4.6V (But It's regulated)

When i disconnect the circuit.... the power supply reads 3.2 V After the Resisyters and 12.2V before the dropping resisters.
2. ### Fred_MGuest

Are you sure you are using the right dropping resistor? Sounds like it's too many ohms. Voltage is dependent on ohms and amps when using a dropping resistor, the regulated supply means nothing after the resistor. When your flasher circuit is in an off state the voltage will be higher than when it's in drain (on) state. Your drain state is about 1.3 volts as you measure it, so it appears to me from what you are saying the dropping resistor you are using is twice as big as needed. Fred
3. ### N Gauger1:20.3 Train Addict

well - when I put a led in the circuit I get 3.2 V but you might have something there...

I wonder if the "Flasher circuit turns on & off". instead of staying "on" & making the leds flash I'll have to try more readings.

What about the resistor.. does putting a resistor in series with a Regulated Power supply do something to the supply???

If I lower the resistor - wont it supply "too high a Voltage" to the circuit??? Ex. 6 Volts instead of 3????
4. ### Fred_MGuest

The regulated supply stay a constant 12 volts Mikey, but on the other side of the dropping resisitor the voltage is controlled by ohms law. The higher the amp load through the resistor the greater the voltage drop. The resistor you are using now, I figure, is about 470 ohms because you said it ran an LED and it prob. draws 20ma. at 3.2 volts. Now the flasher if its like most will have an IC like a 555 or such and will draw x current when off and x+y when they are on and powering the LED or bulbs. As a rule you need to compromise and use a resistor big enough to keep the IC safe when the unit is off, and small enough to allow it to come on. Most of the ICs used in such devices have a much voltage rating higher than the devices they control. This can sometimes be tricky. A better bet is to build a small regulator fot the 3 volt output. I built a few here http://www.all-model-railroading.co.uk/cgi-bin/ikonboard.cgi?;act=ST;f=3;t=138 . Or post the on amp draw and I will help you figure the size resistor I think you need, I'm betting a 220 ohm. Fred
5. ### krokodilMember

Hi Mickey

put a big capacitor to the power input. Such flasher circuits use to draw lot of current when the LED is on, so the voltage from the resitor will drop below 3 V. You can avoid this by a big (2000-5000uF capacitor). Or as already told you need a separate stabiliser.
6. ### Fred_MGuest

I wouldn't do that ET. The cap will charge during the off cycle and dump it's power instantly into the flasher when it goes on causing a voltage rush which is the number 1 killer of LEDs. During the off cycle it may charge all the way to 12 volts depending on the off time and resistance of the limit resistor and capacity of the cap. Fred
7. ### N Gauger1:20.3 Train Addict

Fred I think it would work, but ET's idea jogged my memory that I had Nothing in parallel with the circuit It's a regulated power supply - it actually "Adjusts" to voltage & current. The more "Junk" you add on to it - it adjusts the output Voltage to 12V

Like you said, Amps & Volts are dependant

Here's the original circuit that didn't work
The gray Circles are LED's

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8. ### N Gauger1:20.3 Train Addict

and Here's the one that works

Thank you all!!!!!

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9. ### cidchaseActive Member

???????????????

:cry: :curse:

goin' back to sleep now :sleeping:
10. ### krokodilMember

You are right, if you just add the cap.

Of course you need to check with your VOM the voltage on the cap. It may never exceed the maximum voltage on the flasher.

In such cases your VOM will give you more real reading as without the cap and you can easily adjust the resistor for proper operation.
11. ### cidchaseActive Member

I think this is what krokodil means.

I still can't figure out if the LED's that Mikey is flashing
are the same ones he's using as regulators, or are
they not shown? I'm a little slow today ...

I guess the current through that LED on the end with
no resistor is now zero

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12. ### Fred_MGuest

That circuit will most likely blow the IC as it will act as a capactive discharge. The 4 LEDs don't flash. The IC is an IC flasher unit with the LED on it from what Mikey tells me. He was trying to run it and 4 leds though a 480 ohm resistor. He just didn't have enough power to run them all. That's why it works in the second, he has one 480 ohm resistor running the 4 leds and another running the IC flasher assembly. Fred
13. ### rclineMember

To: N Gauger - Mikey, unless you are dead bent on useing leds. Give this one a try. It worked really well for me. I have built several and use them for tower lighting, street and house lighting, even auto turn signals. :thumb:

[size=+2]Functional description[/size]

[size=+1]The module turns on or off eight different lights (or groups of lights) in a pre-programmed sequence. The sequence is started with a pulse from e.g. a k83 decoder or a manual push-button. The first pulse starts an "on" sequence, the second an "off" sequence and so on. The module has four different programs, selectable via jumpers. Each program has separate "on" and "off" sequences.[/size]

[size=+2]Building and programming the module[/size]

[size=+1]I built the module on a Veroboard. I do not think it is worth the effort to make a real circuit bord for just one or a few units. The design is based on a PIC processor, the PIC16C505. The program I developed for Peter Funck's module can be found here:[/size]
[size=+1]The sequences in this program are:[/size][size=+1][/size]

[size=+1] Sequence 0 (RB5 and RB4 low). Total 20 s:[/size]
[size=+1] [/size]
[size=+1] On: L0 (5s) L1 (2s) L2 (4s) L3 (1s) L4 (3s) L5 (4s) L6 (1s) L7[/size]
[size=+1] Off: L4 (2s) L2 (5s) L6 (3s) L1 (2s) L7 (1s) L0 (5s) L5 (2s) L3[/size]
[size=+1] [/size]
[size=+1] Sequence 1 (RB5 low, RB4 high). Total 11 s on, 13 s off:[/size]
[size=+1] [/size]
[size=+1] On: L0 (2s) L1 (1s) L2 (2s) L3 (1s) L4 (2s) L5 (2s) L6 (1s) L7[/size]
[size=+1] Off: L4 (1s) L2 (3s) L6 (3s) L1 (1s) L7 (1s) L0 (3s) L5 (1s) L3[/size]
[size=+1] [/size]
[size=+1] Sequence 2 (RB5 high, RB4 low). Total 20 s:[/size]
[size=+1] [/size]
[size=+1] On: L4 (2s) L2 (5s) L6 (3s) L1 (2s) L7 (1s) L0 (5s) L5 (2s) L3[/size]
[size=+1] Off: L0 (5s) L1 (2s) L2 (4s) L3 (1s) L4 (3s) L5 (4s) L6 (1s) L7[/size]
[size=+1] [/size]
[size=+1] Sequence 3 (RB5 and RB4 high). Total 13 s on, 11 s off:[/size]
[size=+1] [/size]
[size=+1] On: L4 (1s) L2 (3s) L6 (3s) L1 (1s) L7 (1s) L0 (3s) L5 (1s) L3[/size]
[size=+1] Off: L0 (2s) L1 (1s) L2 (2s) L3 (1s) L4 (2s) L5 (2s) L6 (1s) L7[/size]
[size=+1] [/size]
[size=+1]If you are familiar with PIC programming you can change the sequences in the source code, assemble it and get a new .hex file. To simplify the programming for those who are not familiar with PIC processors, I have written a DOS program hexgen.exe that generates a .hex file based on input from the keybord or from a text file.[/size][size=+1][/size]

[size=+2]Power supply[/size]

[size=+1]Power is supplied via the yellow and brown wire, see below. The module can operate with 10 - 20 V AC or DC.[/size]
[size=+1]Note that the power supply must be isolated from other equipment on the layout![/size]

[size=+2]Control[/size]

[size=+1]A program sequence is started by a voltage pulse on the control inputs, blue and yellow wires, see below.[/size]
[size=+1]Blue is + and yellow is -. The voltage of the pulse should be 5-30 V and the duration minimum 20 ms.[/size]

[size=+2]Output[/size]

[size=+1]There are four 3-pole flat cable outputs, each carrying two outputs. The outputs are the outer wires, and the[/size]
[size=+1]center wire is a common return wire. The return wire is the positive end.[/size]
[size=+1]Each output can be loaded with 0.5 A at 25 C (derate with 4 mA/C at temperatures over 25 C).[/size]
[size=+1]The total load on all 8 outputs together must not exceed 3 A.[/size]

[size=+2]Program select[/size]

[size=+1]Four different programs can be selected via jumpers on two pin headers, see below.[/size]

[size=+2]Schematic:[/size]

[size=+2]Component placement and connections:[/size]

[size=+1]Seen from solder side.[/size]
Copy this and and place in word and then resize the drawings. Lots of luck,
rcline - little choo choo
14. ### rclineMember

Oh !! I forgot to say that I did not design this unit, I just use the tar out of them !!

rcline - little choo choo
15. ### rclineMember

Oh !! I forgot to say that I did not design this unit, I just use the tar out of them !!

rcline - little choo choo
16. ### krokodilMember

Hi CIDCHASE

you were right with the capacitor, exactly this is what I would add to the LED circuit.

I also agree with your second question (probably we are all slow now in the hot summer ), I do not know what are those elements in series with the resistor (Lamps?). They cannot be LEDs for different reasons.