555 timer problem and simple smps

AnalogKid

Joined Aug 1, 2013
10,987
So, you've said that the zener voltage "climbed" to 15 V, meaning that it was not steady at 15 V. Also, sawtooth, angle, and vertical are not nearly descriptive enough to describe all of the details that would give us information to form an answer. 555 pins 3 and 8, and Q1 collector - Without scope shots, you will not get useful answers.

ak
 

Thread Starter

Salvador123

Joined Oct 3, 2015
22
well sadly my scope is kinda old with a small screen , my electrician budy called it the "schoolboy's scope" , and I dont have a good camera around so taking shots from the screen right now is next to impossible.
well ok the waveform , let it be , it's a sawtooth almoust a tipical one as you would expect it to be so i doubt theres much to tell just from that alone. actually right now it doesnt concern me the most , what concerns me is will the waveform have enough current to drive the further stages.i will find that out now.

the supply is now kinda weird , i now have about 3.6 Kohm of resistance (I say about beacuse I use those old but robust wirewound ceraic resistors in parallel) now the voltage goes up to about 15.5 volts and after a while climbs slowly to 15.83 and stays at that point.
when i saw that in the multimeter i thought the 555 would go toast anytime soon , but nah , it stays as if nothing is wrong.I assume that once I will attach a load to it's output pin the supply will sag, so normally there will be a little less than 15 volts.

oh and yes I'm not sure why but what's the highest output amplitude in volts the 555 should give? I'm having less than two volts , maybe , just maybe 2 volts peak to peka but sems more like 1.5.
what could be the cause of this or is it normal for the Ic?
 

AnalogKid

Joined Aug 1, 2013
10,987
2 Vpp from a 555 powered by 15 V is not right. The output is semi-shorted, Vcc is not correct, or the device has partially failed. Again, are you sure Vcc is stable throughout the cycle?

ak
 

Thread Starter

Salvador123

Joined Oct 3, 2015
22
yes im sure , i keep my meter at ground and Vcc, it shows the 15.83 volts at the end of its slow climb from 15.00

the connection is as i have showed in the multisim screen shots. not sure what else can be wrong , after all it's just an IC with a few parts around it.

just did a couple of tests with the amplifier stage connected, except the output transistors.Now the signal from the 555 gets through C1 onto the base of Q1 (mje340) but then that's all , both at the collector or emitter of q1 there is a flat DC line. firslty it's probably because of the low output of the 555, secondly maybe the base resistors for Q4 are too low value ? it makes the transistor conduct all the time?

Also , maybe i should get rid of both r4 and r9 , the resistors coming from the +v line to both q1 and q4 bases? , it would definately help in terms of extra heat but would it also help in to boost my signal?
 
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AnalogKid

Joined Aug 1, 2013
10,987
A meter gives you the average DC value, not anything indicating the quality of the power source. A meter can not tell you if the zener is pulling out of regulation every cycle. There could be an 18 kHz sawtooth wave as Vcc.

ak
 

Thread Starter

Salvador123

Joined Oct 3, 2015
22
ok , i guess i would have to measure the +vcc line with a scope.
also the capacitor i used for the frequency of the 555 is wima it has written on it 1000/100 and on top of the red cube there is 2.5 and that's all it says , I assume it's 2.5nF but maybe im wrong and maybe this is causing some problems?

for my application should i take the +ve resistors away from the bases of the transistors leaving just the one connecting the bases to ground ?
 

Dodgydave

Joined Jun 22, 2012
11,285
Disconnect C1, and see what the 555 output looks like, then reconnect each stage one at a time, i still say you're using the wrong chip for smps.
 

Thread Starter

Salvador123

Joined Oct 3, 2015
22
I agree Dave, it is the wrong chip for an smps , I just though since i had these NPN BJT'S lying around I could make a simple amplifier of them as what an smps sort of is, and since their bjt's and so on i just needed something to drive them , could use an op amp and a pc audio card for all it matters.
maybe i should just build an astable multivibrator from two mje340 and drive the rest of the transistors with it ?
I think the circuit is right just that the 555 output is too low at this point in time to drive it.

Also I guess I will just make a smps based on a osscillator chip like the SG and a driver chip like the IR2110 got some IRPF 460 transistors , but let's leave that for another thread.

right now what interests me most is why i cant make a 555 run two npn transistors. :d
 

Alec_t

Joined Sep 17, 2013
14,280
the 555 in this configuration outputs not a square wave but more of a sawtooth
Are you absolutely sure you have the 555 oriented correctly and are checking the output voltage between pin 3 and ground?
Running the 555 at 15V is close to its limit. You don't have a decoupling cap across the zener, so any voltage spike on the 325 input voltage might raise the zener voltage enough to exceed the maximum rated voltage (18V) of the 555. Perhaps it has already died.
 
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Thread Starter

Salvador123

Joined Oct 3, 2015
22
well sadly my inspiration on this one kinda died so I took the 555 board and the amplifier board off and put it away to get some nice dust , I will first build a working smps from a schematic using some dedicated mosfet driver IC and a bunch of mosfets i have on hand.

I will get a small traffo that outputs some low voltage , rectify that voltage and check again later to see what was wrong with the 555 thing or just get myself a 9v battery to use as power supply., will come back soon to this thread to report on what i saw with the 555 chip.
If I will get luck managing the 555 having a nomral output maybe I will try one day again to build myself a switching amplifier circuit.
 
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