hello everyone.
i´m doing a voltage source and i want to get approximately 30V-500mA on Vout with the smallest ripple as possible. : i got a transformer 12 0 12 V and a 1000V 2A bridge rectifier and 4700uf 35V electrolytic capacitor. The thing is:
after rectifying it it should give me 24 * sqrt (2) = 33.9V which is more than I need. but the worst thing is that the transformer came to me badly regulated and it gives me 14.4V in each wire so I get 28.8V so after rectifying it I get approximately 38.4V which is a very large voltage. I need as close to 30V as possible. rectifying with the diode bridge that I have and after going through that capacitor it gives me as little ripple as I need but I have to lower that voltage somehow and I don't know how. I tried connecting a 2n3055 transistor and a 30V 200mA Schottky diode to it as shown in the picture, but doesn´t work. only lowers me to 38V.maybe putting a linear voltage regulator (it could be an LM317 (1.2V-37V. 1.5A)) but I don't know if it would work. what do you think?
thnks!

 

Your 35V capacitor is not high enough of a voltage rating. You need something like a 50V capacitor to start with.
A LM317 regulator will work to take the 38.4V and reduce it to 30V at 500mA. This assumes the transformer can supply more current than 500mA. You still need a heatsink, as you wil be dissipating (38.4-30)x 0.5A = 4.2 Watts of heat.
Once you start drawing current, the voltage from the transformer may start to drop. That can affect the LM317 regulation if the input rectified voltage drops to close to 30VDC

 

That 38.4 volts may be a problem for the LM317, maybe you want to have a look at the TL783 which is good for 125 volts in.

 

That 38.4 volts may be a problem for the LM317, maybe you want to have a look at the TL783 which is good for 125 volts in.

LM317HV.

 

That 38.4 volts may be a problem for the LM317, maybe you want to have a look at the TL783 which is good for 125 volts in.

Why? It has no connection to ground!

 

An LM317 would be my call, as the I/O differential it sees is low and it incorporates overcurrent and overtemp controls.

However your first circuit with the emitter-following 2N3055 will work, except you have specified a SCHOTTKY diode, whereas you should have specified a ZENER diode, as the zener breakdown provides the regulation. Off the top of my head, a 30V 5 watt zener and a 56 ohm 2 watt resistor feeding the 2N3055 should deliver what you ask. The 2N3055 seems overspecified and using something with a higher gain would allow much reduced dissipation in the zener andresistor. And paralleling the zener with a capacitor will improve regulation.

 

I tried connecting a 2n3055 transistor and a 30V 200mA Schottky diode to it as shown in the picture, but doesn´t work. only lowers me to 38V.

You said "schottky -diode?, diode doesn't do anything in that position, try Zener diode and you have emitter follower circuit which should bring it around ~29.4Volts, adding series diode you can compensate the transistor B-E drop if you use 30V zener. R14 should be rated for pass current of the Zener.

 

TL431 and transistor? (Might need a Darlington to get enough gain)

 

Why? It has no connection to ground!

If you have a capacitor across the output, you might have excess voltage when the circuit is first switched on -it takes some time for that capacitor to charge. I think that's why they make an HV version.


The LM317, LM317HV, and TL783

 

..... excess voltage when the circuit is first switched on......

That's a reasonable point, but I see that this circuit is in the National Semiconductor library - perhaps the startup is too transient to be a problem: