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Why am I getting 8VAC on 2N3772 transistor emitter?

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Ron, across the load, the scope sees some AC. It is not a sine wave. It is circa 5 mVpp, and looks like it changes alternates quickly, but sort of evenly.M
 
New observation:

I installed a diode between the 7805 and base. The AC wave is gone. However, something else happens. When I ground the load, a noise appears, but after 4-5 seconds it dies down! What do you think?M
 
What are your thoughts?M
The wires of your breadboard are too long and have series inductance. If you used a small compact pcb like all REAL circuits then maybe adding a 0.1uF ceramic disc capacitor with short leads to the output and ground (like in the datasheet) will kill the oscillation.
 
The wires of your breadboard are too long and have series inductance. If you used a small compact pcb like all REAL circuits then maybe adding a 0.1uF ceramic disc capacitor with short leads to the output and ground (like in the datasheet) will kill the oscillation.

Is it not unrealistic that the long leads induce a perfect sine wave of 8VAC?

I noticed by placing a capacitator between the emitter of the transistor and GND, the ac is gone. Why did we not suggest this this initially?

Thanks.M
 
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I noticed by placing a capacitator between the emitter of the transistor and GND, the ac is gone. Why did we not suggest this this initially?

Because you've never posted the actual schematic, despite been asked to.

Assuming you're trying to get increased current from a 1A regulator, why don't you follow the suggested circuit from the datasheet?.
 
Because you've never posted the actual schematic, despite been asked to.

I am sorry, being inexperienced I thought the one I posted was good enough. I will be more detailed in the future.

I have looked at the datasheet several times, I must be overlooking something. Please tell me what that is.

Thanks.M
 
Before I connect the lead to the collector of the npn transistor, i only get 5V from 7805 via the base. Once i connect the main current to the collector, I get this AC current.
It's possible you have a faulty transistor and the collector-base junction is breaking down.
 
I am sorry, being inexperienced I thought the one I posted was good enough. I will be more detailed in the future.

I have looked at the datasheet several times, I must be overlooking something. Please tell me what that is.

If you don't post the correct schematic, then we can't really help, the datasheets clearly show you how to boost the output using a PNP power transistor, but as you've never posted your schematic, we've no idea if you're following that.

As already suggested, breadboards aren't terribly great - stability is a problem, as is poor contacts - personally I never use them.
 

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Adding a darlington transistor between the 7805 and gate of the power transistor solved the problem. Anyone have a clue as to why? Thanks.M
 
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Adding a darlington transistor between the 7805 and gate of the power transistor solved the problem. Anyone have a clue as to why? Thanks.M

No idea at all old boy!

Just adding random components to a half baked circuit is not something which is going to lead to success.
(A technique reminiscent of "mstecha" for those who have been here for a few years!)

JimB
 
so according to a data sheet for the 7805, you should have small (1-10uf) bypass caps between the input pin and the ground pin, and from the output pin to the ground pin. you should also have the 7805 heat sinked because of the huge amount of voltage drop from 24 to 5 volts.

if you are going to use regulators and other analog chips, you should read the data sheet first. go to www.datasheetarchive.com and put in the part number you want a datasheet for
 

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so according to a data sheet for the 7805, you should have small (1-10uf) bypass caps between the input pin and the ground pin, and from the output pin to the ground pin. you should also have the 7805 heat sinked because of the huge amount of voltage drop from 24 to 5 volts.

if you are going to use regulators and other analog chips, you should read the data sheet first. go to www.datasheetarchive.com and put in the part number you want a datasheet for

I have 2 x 10uF caps. The regulator case doesnt get hotter than 15*C, so i didn't use a heat sink.M
 
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The regulator doesn't get hot, so I didn't use a heat sink.
The calculation for the heating is simple: Volts x Amps= Watts.
The current in the regulator is very low because it is the base current of the transistor so the regulator does not get hot, but the transistor gets very hot with 8.5W so it will be burnt without a heatsink.
 
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