Frequency change with voltage change on 555 timer

Thread Starter

Rittter

Joined Dec 5, 2015
60
I constructed a 555 timer, LM555CN, on a breadboard to generate 1Hz, 50% duty cycle, 9VDC supply, R1 4.7K, R2 150K, C1 4.7uF, using a standard wiring diagram. Nothing unusual. It worked just great. Soldered a duplicate and tested it with 9V. Worked great. Applied 12VDC to each units as a test before continuing with the rest of the circuit and the resulting frequency went from 1Hz (with 9V) to 4KHz (with 12V). I have read that a few people have had this problem but have not found a definitive solution. Any help on this would be appreciated.
 

MrChips

Joined Oct 2, 2009
24,202
One can only guess what is a standard wiring diagram. We know what you mean but in order to avoid miscommunication, show your circuit diagram.

A circuit schematic is the language of electronics for communicating ideas. Use it.
 

OBW0549

Joined Mar 2, 2015
3,566
Applied 12VDC to each units as a test before continuing with the rest of the circuit and the resulting frequency went from 1Hz (with 9V) to 4KHz (with 12V).
Then something is terribly wrong-- defective component, wiring error, poor solder joint, or something else catastrophic. In going from a 9 volt supply to 12 volts, I wouldn't expect more than a percent frequency change, at most. Something is wrong.
 

wayneh

Joined Sep 9, 2010
17,153
Then something is terribly wrong-- defective component, wiring error, poor solder joint, or something else catastrophic. In going from a 9 volt supply to 12 volts, I wouldn't expect more than a percent frequency change, at most. Something is wrong.
Ditto. Check and recheck every connection. It would also be a good idea to add a 0.1µF decoupling capacitor across the power rails near the chip.
 

MrChips

Joined Oct 2, 2009
24,202
All of the above. Something is drastically wrong. To debug the problem we want to change one thing at a time.

Step 1 - Check and recheck every connection.
Step 2 - Change to LMC555
Step 3 - Install 10μF in parallel with 0.1μF across the power rails.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
The decoupling capacitors are scheduled next, but had to make sure the basics worked. The circuit works well with a the nine volt battery.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
It is bizarre, but I have read that others have run into the same situation and there was no solution on the posts.
 

dl324

Joined Mar 30, 2015
13,122
It is bizarre, but I have read that others have run into the same situation and there was no solution on the posts.
Timing is based on ratios of VCC (1/3 and 2/3), so changing from 9V to 12V should have had little, if any, effect. It must be something else.
 

OBW0549

Joined Mar 2, 2015
3,566
I've been doing some reading, data sheets and other material online, and I'm becoming highly suspicious that the problem may be caused by the lack of decoupling caps.

The bipolar (i.e., non-CMOS) version of the 555 timer is notorious for having a very large shoot-through current during high-to-low and low-to-high output transitions. These short current spikes, as much as several hundred milliamps, can cause large voltage transients on the Vcc pin if it is not decoupled properly, and can result in erratic operation. The TI LM555 data sheet recommends, at a minimum, a 0.1 μF ceramic capacitor and a 1 μF electrolytic capacitor in parallel connected across the Vcc and GND pins as closely as possible to the IC.

The CMOS version of the chip, the LMC555, eliminates or at least drastically reduces the current shoot-through problem.
 

RichardO

Joined May 4, 2013
2,271
One more random thought. Maybe the timing cap is breaking down on the higher voltage. Try another cap and see what happens.

By the way, I assume you are using an electrolytic capacitor for the timing cap. I am not fond of using electrolytic caps this way They have a very large capacitance tolerance and very high leakage which can cause very unpredictable frequencies in the 555 timer circuit.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
THAT IS THE ANSWER !!!!!!!!!!!
It was the absence of the decoupling capacitors ! It now works with both 9v and 12v.
I thought that with no load on the 555, those Caps would not be needed yet. Wrong again.

Thank you all for your time and input. Much appreciated.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
One more random thought. Maybe the timing cap is breaking down on the higher voltage. Try another cap and see what happens.

By the way, I assume you are using an electrolytic capacitor for the timing cap. I am not fond of using electrolytic caps this way They have a very large capacitance tolerance and very high leakage which can cause very unpredictable frequencies in the 555 timer circuit.

Your assumption is correct. Are you suggesting ceramic caps for this application ?
 
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