Power supply

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epilot

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I am using a 555 IC as an oscillator at 40 KHz.
The circuit is working ok with my adjustable regulator power supply which I bought (100$).
Today I decided to use a transformer instead of my power supply for my oscillator.
I used a 7815 regulator chip and a KBP02 bridge rectifier (yes I connected a 1000uF & a 100nF capacitor across the rectifier outputs) but the output is not a catharsis output!
I have connected a 1000uf and a 100nF capacitor across the outputs of the 7815 chip too but there is not a pure output yet!.
Don't know what to do?
I want to get as stability as I can but don't know what to do?
The transformer is a 18 V,500mA

Any suggestion would be very appreciated
 
I can't possibly imagine what you're doing wrong.

Have you checked all connections?#

What's wrong with the output?

Have you looked at it on an oscilloscope?
 
If you could see the output on a scope it might help. The 1000 microfarad capacitor may have failed - or possibly a diode in the bridge. The 7815 might have a problem as well.
 
Hero999 said:
I can't possibly imagine what you're doing wrong.

Have you checked all connections?#

What's wrong with the output?

Have you looked at it on an oscilloscope?
Click to expand...

yes I have cheked all th connections and the components.

I thought perpahs there is a problem with my adaptor, but I had the problem while I used another one.
Perhaps the problem is due to my oscillator which is working at that freq(40 Khz)?
 
stevez said:
If you could see the output on a scope it might help. The 1000 microfarad capacitor may have failed - or possibly a diode in the bridge. The 7815 might have a problem as well.
Click to expand...

I used from a 2200uF capasitor after the brige, I had a small improvement but not much.

do you guys think that a regualtor chip like 78##(and a bridge diode beside 2 caps) are able to genarate a very good wave withought ripples at its output for a transformer or I need some thinh else?
 
What I am seeing on my scope (at the output of the regulator chip) is a smoothed DC voltage. But I can see the ripples too. I am pretty sure that the problem is caused due to those ripples. I need a FLAT DC voltage. I think it is possible using inductors to get a flat voltage but don't have any idea about the values and circuitry?

P,s I thought a 78## regulator chip is able to smooth the input voltage somewhat but it seems I was wrong!?
 
What do you see exactly? ripple? noise? The 78xx have good ripple rejection and line regulation. If you have two probes you may check if each diode is working.
Did you place a decoupling capacitor as close as possible to the 555?
 
eng1 said:
What do you see exactly? ripple? noise? The 78xx have good ripple rejection and line regulation. If you have two probes you may check if each diode is working.
Did you place a decoupling capacitor as close as possible to the 555?
Click to expand...

The first pic is AC component when the 555 is oscillating, and the second pic is the AC component when I removed the 555.

Yes there is a decoupling capacitor inthe board.
 

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How many V/division and s/division in the first picture? and what is the ideal frequency you expected?
 
eng1 said:
How many V/division and s/division in the first picture? and what is the ideal frequency you expected?
Click to expand...


Sorry, what do you mean by "what is the ideal frequency you expected"?!

Well the whole circuit current draw is about 550mA(yes There is an amplifier at the output of 555 where its output is just 200mA), I just realized that if i could reduce amp draw by removing the 555 or amplifier I will have a smoother supply, so it seems the power supply is not able to work at large current draws(550mA!) I used a 2200uF cap in parallel with the input of the regulator(parallel to 1000uF input cap) and the wave form was a bit imporved.
According to high current of the supply storry, do you think that the solution is using a very large cap (5000uF or larger) or there is a better solution??
 

Higher the current, higher the amplitude of those "spikes". When the 555 is oscillating, the current demand is a periodic waveform and it reacts with the impedance of the lines. I would add decoupling capacitors (1uF, 100nF, 10nF...) across Vcc and ground, as close as possible to the pins of the 555. BTW what is the amplitude?
What about adding a BJT to the output of the 555?
 
Is your transformer rated at 18V RMS, or is that the peak voltage?
Keep in mind that it is only rated at 500mA. It is not too commonly known that a full-wave bridge rectifier with a capacitor-input filter (as you apparently have) draws about 1.6 times as much RMS current as the available output current, i.e., with 550mA DC load, you need a transformer rated for at least 900mA. Some documents I've seen say the factor is 1.8 instead of 1.6.
I think you are overloading your transformer.
 
Last edited:
You need to post a schematic.
 

Thanks Ron.

Yes, you are right. Since I was not able to find a 18V 1A transformer in the markets so I went for a 9Vx2(double) 500mA transformer. I thought the transformer is able to deliver a 550mA current and perhaps it gets warm a bit, but it seems I was wrong.

Well I find a 12Vx2(double) 1.2A in the market, yes you was right and the circuit is working ok without any noise or ripple but the 7815 cuts the voltage after 5 minutes because it gets hot, yea I am using a radiator on it but 24V of the transformer seems to be too much for a 7815 regulator.
 
Your 24VAC transformer makes an unregulated DC of 32.4V! It is much too high for a 7815 to keep from getting too hot.

A 7815 needs a minimum input of 18VDC which can be made from a transformer with a total output of 14VAC.
 
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