# Stable current on different voltage levels (555 timer)

Discussion in 'General Electronics Chat' started by Fuji, Mar 20, 2015.

1. ### Fuji Thread Starter Member

Nov 8, 2014
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The 555 timer's output works just fine on any frequencies I choose. My problem is keeping the output current (Pin 3) on a constant level. For example, 5V at 4mA of current is the default voltage and current of the 555 timers output. If I increase the voltage (6v, 7v, 8v etc..) or lower the the voltage (4v, 3v, 2v etc.) I'd still prefer to have 4mA of current in the output of the 555 timer despite the voltage changes.

Would anyone recommend me some type of IC that can accomplish this, if there is any?

Jan 15, 2015
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What are you trying to do? The 555 can source or sink current up to 200 mA on the pin 3 output depending on Vcc. As Vcc is increased the 555 is capable of sourcing more current. For example with Vcc = 5 Volts the maximum output voltage on pin 3 will be about 3.3 Volts and a max current of about 100 mA. If Vcc is increased to 15 Volts the maximum output on pin 3 will be 13.3 Volts and a max current of about 200 mA. Either case the 555 will only source as much current as required by the load. Where are you getting this 4 mA?

Ron

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3. ### GopherT AAC Fanatic!

Nov 23, 2012
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Please study ohms law. You get to pick two of the three variables (voltage, resistance or current), ohms law will determine the third. That being said, it seems like you want to change the voltage with the same load (resistance) and then you expect ohms law not to be followed (you expect 4 mA at all input voltage levels).

So, the problem is not circuit design or finding the right component, it is you needing to understand ohms law. The only other option was that you described what you want in a poor way and want something completely different than what we understood.

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4. ### ronv AAC Fanatic!

Nov 12, 2008
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Maybe he is driving an LED or something.
Here is a constant current circuit. Not real good regulation, but maybe what you are looking for.

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5. ### Papabravo Expert

Feb 24, 2006
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So can you accept that you get to pick two of the three things and the third is determined for you, or is that "unacceptable".

6. ### Fuji Thread Starter Member

Nov 8, 2014
100
1
Not sure if I wrote it correctly.

Your telling me that there is no way to current limit (current limiting resistor) on Pin 3 for any voltage?

Not talking about converters right now, but my DC DC converter (MC34063) can current limit any voltage I choose and keep it between 4 to 5mA of current on the output from a single resistor. So this is not possible with a 555 timer??? My problem is the slide switch im using. If I switch to a certain voltage, I need the same current output.

Last edited: Mar 20, 2015
7. ### Fuji Thread Starter Member

Nov 8, 2014
100
1
Thanks thats a very good circuit. I assume their will be a slight voltage drop on the output due to the 2N3904 transistors?

8. ### GopherT AAC Fanatic!

Nov 23, 2012
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A DC/DC converter can be set at a voltage with the additional instruction (do not exceed 5 mA). Let's say you set that DC/DC converter to 5 volts. Then you connected it to a 1000 ohm resistor. Well, ohms law will let that happen because 5v/1000 ohms is 5 mA.

Now, what if you connect to 500 ohms. Then a 5 volt supply through a 500 ohm ressitor would be 10 mA. But your additional instruction to the DC/DC converter was, do not exceed 5 mA. According to ohms law, the only thing the poor converter can do is stop putting out 5 volts. It will drop the output to 2.5 v.

Now let's look at your 555. It is not a power controlling device, it is a timing device. The only thing it does is put out pulses of various frequencies or delays at the same voltage (almost) that you out into it as a supply. No more, no less. No current limiting, just voltage similar to the power supply voltage.

9. ### GopherT AAC Fanatic!

Nov 23, 2012
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RonV,

What is the reason for R6 in your circuit above?

10. ### ronv AAC Fanatic!

Nov 12, 2008
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Without the short RC time constant the current can go up faster than the transistors can respond, so it just slows down the edges a little.

Fuji,
Yes, you will loose some across the transistor. It works better if you can put the load in the collector and use a high gain transistor like a BC547C.

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11. ### Fuji Thread Starter Member

Nov 8, 2014
100
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You can figure out yourself.

12. ### Papabravo Expert

Feb 24, 2006
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True, but I'm not the one with a problem.

13. ### Fuji Thread Starter Member

Nov 8, 2014
100
1
I never asked you in particular for help either. Lol.

Last edited: Mar 22, 2015