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I have a toroidal transformer ac 230v with x2 25v.
I am trying to power an old ham amplifier witch takes 28v 8amp (transistor) (blw77).
I have now linked the transformer to an 8 amp bridge rectifier with a 35v 10000μF capacitor and are getting 35.7v dc.
What kind of voltage drop would I get as the transistor (blw77) should only take 28v 8amp.
I don't mind it not being on full power as I do not want to blow it up.

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This is a tricky one. The max rating of the BLW77 is realistically 35 volts but lowering the voltage reasonably under 35V to (say) 32 volts by a resistor is going to dissipate a lot of power in the resistor at 8 amps current.

Power in resistor will be V.I = (35.7 - 32).8 = 29.6 watts.

Dropping it to 28 V is going to dissipate about 62 watts so I believe your best bet is to construct a buck regulator to drop anything between 30V to 40V down to a regulated 28 volts. Maybe one of these: -

enter image description here

You can set the output voltage to be 28V by adjusting the potential divider on the Vfb pin. For alternative try Linear technology and use their technical search engine for narrowing down results. TI also have a great range of buck regulators too.

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the lowest voltage of ripple means deltaV=Approx = I/(2*f*C) for full wave bridge rectifier. If the supply freq. is 60Hz delta V= approx. 7.7 Volts..means Max V=35.7,,Vmin=29Volts. VTIngole

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IF your volts under load is too high measured by dummy resister load 28 volt 8 Amp ie 3.5 ohm then you better not connect your expensive transistor PA.Now you could do a really close in discrete buck convertor with S TRAP switching because the orthodox easy to buy buck chips probably wont have the voltage headroom and could go unstable on say below 31 volts. SO maybe the simplest thing to do is place inductance in the input you can use paralleled lamp ballasts in series to reduce the volts to 28 and you should find that the ballasts will generate a tenth as much heat as a resistor AND it will make the transformer run slightly cooler OR if this is still too much you can wind a turn on your toroid with any 10 amp insulated wire and connect to buck the secondary reducing the volts maybe you will need more than one turn FINALY is your ripple going to be low enough to not transmit hum remember that more capacitance may slightly raise dc volts and choke filter schemes will reduce it

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