Electronics > Projects, Designs, and Technical Stuff
Building a bench power supply
stev.dk:
Getting more and more serious with my home electronics lab, i need a bench power supply. I don't really need it, like a car needs gasoline, cause i got tons of single PSU's ranging from 3.3V to +100V DC (i even got some AC onces), but they take up a lot of space, and the cord is all over the place.
I want a power supply that can handle output voltage up to 24VDC @ 2-3 Amps.
Some time ago i scored a ring type transformer at work. Its a 230VAC --> 24VAC 80VA. In theory, i believe, should give me about 3A. Im waiting for my fluke 87 to arrive, so i can make some accurate measurements, but my cheap meter says 27,4 VAC and 36,6 VDC - rectified with 4x1N4007 diodes, and a 1000uF cap and 1 Ohm resistor across output to gnd.
Im thinking of getting a bunch LM78XX's (05, 09, 12, 24) i have some heatsinks for the TO-220 package. Im not sure if i need a 3.3V rail, but if i do i'll need to find something to create that. I might want to do a negative rail for 5,9 and 12V, but im not sure yet.
If possible, i want to create a current readout (yeah i know everything is possible, especially the things that is in use by almost every manufacturer, BUT if i can get my head around it :-/O)
I'll update this thread when i advance the project :-)
mariush:
That's a nice transformer, but not really suitable for a bench power supply.
It's probably 24v AC rms, 80 VA (3.3a) - that means after you rectify it to DC you get about 24v x 1.414 - 2v (on rectifier diodes) = 32v max. and the maximum current would be about 0.62 x 3.3a = 2A
If you'll use a 7805 to get 5v 2A from that, you'll have to dissipate (32v - 5v ) x 2A = 54 watts. That's a ridiculous amount of power wasted, i don't even thing the 7805 is even able to do it.
If you don't care much about ripple/regulation, you can use a LM2596 or a mc34063 with external transistor to get something like 8v 3-4A, then use a 7805 or a pair of 7805 to get 5v 2-3A. This way you'd only have (8v-5v) x 3A = 10 watts, which is easy to dissipate.
1n4007 is only good for 1A, so in a bridge rectifier configuration they can only do 2A. You're better off with a beefier bridge rectifier, something rated for 10-16 amps, they're super cheap.
Linear power supplies are usually made with transformers that have multiple taps, usually about 6-10v dc per tap, and as you adjust the voltage the taps are connected or disconnected using relays to get a voltage slightly higher than needed.
For example, if you want 12v, two 10v windings are connected in series to get 20v and 8v x current value is dissipated on pass transistors.
Here's an example of a 18v 2A power supply, using two transformer taps, a separate winding for the relay which enables the second tap or not, and two separate small windings for +/-8v and +/- 5v for the multimeter chips :
transformer, power transistor, linear regulators for opamps , relays, multimeter chips
opamp to adjust voltage and current, current limit leds , output ... all jellybean components on this one and the previous part
multimeter chips (one for voltage, one for current) ... icl7106 is with lcd segments , icl7107 is with normal led segment (so you could use 7 segment digits)..
this last part is the only slightly more difficult part to make (and the chips are more expensive) but you don't really need it, you can get less accurate ready made meters from ebay. if you do make them, you need some film capacitors and more accurate resistors and you must read the chip datasheet to understand how it works etc..
stev.dk:
Thanks for the reply. When i got the transformer i wanted to use it for a small guitar amplifier. I see the point about the transformer not working for this purpose. I think i'll put it back in the box, and wait for another project suitable for it (small guitar amp?).
Is it possible to use multiple 220V -> 12V transformers, if a single transformer with the taps required is not available?
The LM7805 should be able to do 35V up to 1.5A (according to the datasheet).
I know about the diodes, i was just using whatever i had laying around :)
So if im not going to use this transformer, and these volt/ammeters are so cheap (found them on ebay at the price of a worn out t-shirt - i KNOW they're not accurate, but i only need a guideline. For accuracy i'll be using my fluke 87) i might consider doing a switch mode variable PSU.
mariush:
Yeah, guitar amp is a good ideea. I suggest one with a tda2030, it's fairly easy to do : http://www.st.com/web/en/resource/technical/document/datasheet/CD00000128.pdf (see page 8, figures 17 and 19) .. just make sure you give it maximum 36v (use some diodes in the bridge rectifier with large voltage drop)
Yes, you can use multiple 12v transformers.
Technically you can use 35v input on a 7805, but the total power dissipation is internally limited and can not exceed a certain value. Linear regulators like 7805 dissipate the voltage difference as heat, and they have a maximum limit, and even so, you'd need ridiculously expensive heatsinks to remove the heat if you want both high current and high input voltage.
So you can have 35v in, 5v out but at something like 10-50mA, if you demand 1-2A you're out of luck.
For example see : http://www.fairchildsemi.com/ds/LM/LM7805.pdf
Thermal Resistance Junction-Cases (TO-220) 5 °C/W
Thermal Resistance Junction-Air (TO-220) 65 °C/W
Operating Temperature Range LM78xx -40 to +125
Then on next page it says V out = 5v at Po = 15w .. that's a hint that the regulator may internally limit itself to 15w power dissipation if I understand correctly. So at 35v input - 5v output you have 30 watts so the maximum current it could output and stay within 15w power dissipation would be something 15w / 30v = 0.5 A
So if the maximum power dissipated is 15w and you feed it with 35v, you're limited to 0.5A of current.
BUT, even so, let's do the math on what heatsink you'd need for that...
assume room temperature is 25c ... the chip operates up to 125c as it says in datasheet so we have 100c to work with.
100c / 15w dissipated = 6.6 degrees c / watt
The 6.6 c/w is lower than the resistance junction to case air, so we definitely need a heatsink.
value needed = value calculated - resistance junction-case - resistance of material between heatsink and case (usually about 0.1 for thermal paste, 0.3-0.5 for silicon pads or something like that)
so heatsink needed = 6.6c - 5c - 0.1 = 1.5c/w
You're looking at about 10$ worth of heatsink :
http://uk.farnell.com/fischer-elektronik/sk-571-10-sa/led-heatsink-standard-extruded/dp/1850077
http://uk.farnell.com/fischer-elektronik/sk-04-100-sa/heat-sink-100mm/dp/4621529
With a fan, you can probably manage something cheaper, let's say 4c/w heatsinks, but then what's the point, you still have only 5v @ 0.5A, you're better off getting a better transformer in the first place.
stev.dk:
I appreciate the math and information given about the heat dissipation, cause i know next to nothing about that. I do however have a box filled with different kinds of heatsinks from old computer motherboards and powersupplys ect. I can never get enough heatsinks ;)
I dont know if i'll be needing more than 500mA on the 5V rail, and i know i have a heatsink suitable for the job. But i think i'll save my ring type trasformer for another project and get one with multiple taps, cause i have never worked with those (in diy projects from scratch at least not). And the main reason for building stuff like this is to learn.
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