# power for resistors on 555

Discussion in 'The Projects Forum' started by lotusmoon, Jul 4, 2013.

1. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
I am making a LED flasher with 10 set frequencies on a 555 timer circuit. I have been looking through resistors to see which to use. they have different power ratings in watts. The power supply for the circuit is 15 v with output of 200ma with I think makes it 3w. what I am unsure of is if this full power is actually going through the resistors?
I set up the circuit and used a multi meter to try and measure the amps but could not get any readings across the resistor or across the whole circuit. I and unsure why not. but could someone help find which power rating I need on my resistors

2. ### tracecom AAC Fanatic!

Apr 16, 2010
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If you are asking about the resistor in the RC network, .25W is enough; .125W is probably ok, too.

3. ### wayneh Expert

Sep 9, 2010
12,156
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The timing resistors would normally not need to be more than standard 1/4W, as they carry very little current. Without your schematic it's hard to say more.

Jun 14, 2013
203
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5. ### wayneh Expert

Sep 9, 2010
12,156
3,063
I missed this the first time through. The answer is a solid NO. The capacity of the supply is like the size of a garden hose. Once the hose is big enough to supply your spray nozzle, additional size doesn't matter - the nozzle (like your circuit) controls the flow.

If 0.25W resistors are easy for you to find, they'll be just fine. For any resistor, you can confirm this by calculating the power dissipation P = I^2*R = ∆V^2/R. You should specify the power rating at at least 2X the expected dissipation to avoid overheating and shortened life of the resistor.

Last edited: Jul 4, 2013
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6. ### Wendy Moderator

Mar 24, 2008
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I would use 4.5VDC, I suspect the 555 will get pretty hot with 9VDC.

You can vary the volume with a simple pot like this...

7. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
thanks bill , i am lighting LED's and as the 555 would not comfortable light them all. I was going to connect the 555 to them with an npn transistor also with a resistor on the output terminal 3 to lower the power through the 555.
I was going to run the whole thing at 15 volts so as to be able to have many LED's in series. With the resistor on terminal 3 I figured most of the power would go through the LED's and not the 555 timer, but I could be wrong.

8. ### Wendy Moderator

Mar 24, 2008
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How much current do you plan on using for the LEDs? A 555 can power a lot of conventional 5mm 20ma LEDs by itself, it is the total current that matters. I would limit the 555 to 100ma, it is rated for 200ma drive. So if I needed 120 ma I would still use the 555 directly.

9. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
I am thinking of 61 running at about 30mw = about 1.8w. At 15 volts i think the 555 output drops by a couple of volts to 13 volts. 1.8w/13v= 140ma
If i use an npn I would have the whole 15 volts and could run more LED's in series. ?

10. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
I am thinking of using less LED's. this now works out that the 555 is working at 64% of its full capacity 200mw at 15volts. I have taken into account that the voltage will probably drop to about 13volt at the output.
is this to much for the 555, will over heat or have a short lifespan?

11. ### LDC3 Active Member

Apr 27, 2013
920
160
Since there is about 130mA of current going through the 555, it probably will heat up some. Also, the amount of heat will be determine by the duty cycle of the pulses. It really shouldn't decrease the lifetime since it is below the specifications of the 555.

12. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
thank you. what is it about the duty cycle that would heat up the 555, I am having the duty cycle as close to 50/50 as possible. do higher frequencies kHz also to heat it up?

13. ### LDC3 Active Member

Apr 27, 2013
920
160
The frequency has little to do with heating. It has more to do with the length of time that the current is flowing since P=I^2*R. The higher the duty cycle, the longer the current flows and more heat is produced. If your duty cycle is 90%, more heat will be produced than if the duty cycle is 60%. The 555 can cool (some) during the off periods.

14. ### wayneh Expert

Sep 9, 2010
12,156
3,063
Since the 555 sinks and sources current in this circuit, does the duty cycle really matter? Current is always flowing, only the direction changes.

15. ### LDC3 Active Member

Apr 27, 2013
920
160
Current only flows one way through a diode (or LED).

16. ### wayneh Expert

Sep 9, 2010
12,156
3,063
Ah, I've been confused about what the load is. I guess it's not a speaker (why is there a link to a piano project?), and is either the base of an NPN transistor or directly driving LEDs. So current is just one-way.

17. ### LDC3 Active Member

Apr 27, 2013
920
160
That circuit was to show how to connect an NPN to drive the LEDs. This also has current flowing in only one direction since the reverse has 0 voltage potential.

18. ### lotusmoon Thread Starter Member

Jun 14, 2013
203
4
That you that all seems good as my duty cycle is 50%