Electronics > Beginners
lab power supply
Jwillis:
What is the voltage rating of your electrolytic capacitors and The voltage of your transformer secondary.
If you measure the AC voltage of the secondary you take that measurement and multiply by 1.4 to get the rectified voltage With filter caps . So for example if your transformer is putting out 12VAC . 12 X 1.4 = 16.8 VDC after rectification and filter caps . 16V capacitors will be right on the edge of destruction at that voltage. Try to double the capacitor voltage rating over the rectified voltage. Don't just take the voltage of the rectifier and try to match the capacitors to that voltage because as soon as you put a filter capacitor in circuit that voltage will rise potentially exceeding the rating of the filter caps your installing . If you can salvage old televisions there are usually some big high voltage ones in those . For your simple power supply a couple 220uF or 470uF 200 - 400 V capacitors would be more than adequate.
If your going to parallel transistors you need to bias them to prevent load hogging and thermal runaway. Usually this can be done simply by putting a resistor at the base and a very low resistor ( 0.1 ohms is plenty) with a high wattage (5W) at the emitter of each transistor. Even in a single transistor circuit, biasing will keep that transistor from exceeding its rated current capability if the power supply is over loaded by shorting .
glinjik:
hi thank you for your reply the caps were 4 x 1000mf 50v transformer is 20.02 so with you calculation 28.02v should have been ok i was trying to get as close to the capacitance as possible had in series i thought the capacitance was really important ie got loads of big voltage caps but with low capacitance 22omf 400v, 150mf 450v, 100mf 450v etc i the emmiters have 0.5 \$\Omega\$5w ceramic resistors, should i put a big chunky diode at the output to prevent shorting? thanks for your reply hope you dont mind my noob questions it was kinda fun to watch those caps pop though lol
glinjik:
this is what ive done minus the dead caps :palm:
Doctorandus_P:
You can also have a look at:
https://hackaday.io/search?term=powersupply
for inspiration.
Some of the projects there looks suspiciously like Dave's old project, such as:
https://hackaday.io/project/23249-digital-battery-operated-powersupply
Also, think about adding a switched pre-regulator.
You can build one by modifying a cheap SMPS module from Ali / Ebay / China.
The basic way to do it is to add a PNP transistor to your circuit with the emitter to the power input, and the base connected to the output voltage.
(With added resistor for current limiting and 1 or 2 diodes in series to increase the voltage drop.
The collector of the PNP transistor then gets connected to the voltage sens pin of your SMPS board.
Such SMPS modules are cheaper then the big aluminimum heat sink it replaces.
Stuff like this is also so easy to find on the 'net.
How about a picture search for schematics?
https://duckduckgo.com/?q=eevblog+power+supply+schematic&t=h_&iar=images&iax=images&ia=images
glinjik:
--- Quote from: Doctorandus_P on December 17, 2019, 11:01:27 am ---You can also have a look at:
https://hackaday.io/search?term=powersupply
for inspiration.
Some of the projects there looks suspiciously like Dave's old project, such as:
https://hackaday.io/project/23249-digital-battery-operated-powersupply
Also, think about adding a switched pre-regulator.
You can build one by modifying a cheap SMPS module from Ali / Ebay / China.
The basic way to do it is to add a PNP transistor to your circuit with the emitter to the power input, and the base connected to the output voltage.
(With added resistor for current limiting and 1 or 2 diodes in series to increase the voltage drop.
The collector of the PNP transistor then gets connected to the voltage sens pin of your SMPS board.
Such SMPS modules are cheaper then the big aluminimum heat sink it replaces.
--- End quote ---
hi thank you for your reply but i dont want to put modules in, im learning an awful lot from doing this so to buy modules would be pointless if you know what i mean but i did see somewhere about adding pnp transistors ill have a look at hackaday in a bit but thank you
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version