DC power supply schematic help needed

[bobon721]

Hi! New guy here!
I'm designing a DC power supply. Here are the specifics:
1. 230Vrms 60Hz AC source
2. 5V, +/-15V, 1.2V to at least 20V variable outputs, all grounded, 3A max current overall
3. resettable short circuit protection using relays
4. pass transistor to increase current output of voltage regulators to the required max current
I need some tips on how to design the short circuit protection as well as the pass transistor. I've attached a screenshot of my schematic. I'm still unsure of the transistor, resistor, and relay specifications so I think there's no reason for me to include them.
By the way, I'm also having trouble with the step-down transformer. How do I make it deliver 3A max current to the whole circuit without problems?
Thanks in advance.

 

[shrtrnd]

Find information on crowbar circuits, and consider that. Most of the good power supplies I work on have them.

 

[davenn]

Hi! New guy here!
I'm designing a DC power supply. Here are the specifics:
1. 230Vrms 60Hz AC source
2. 5V, +/-15V, 1.2V to at least 20V variable outputs, all grounded, 3A max current overall
3. resettable short circuit protection using relays
4. pass transistor to increase current output of voltage regulators to the required max current
I need some tips on how to design the short circuit protection as well as the pass transistor. I've attached a screenshot of my schematic. I'm still unsure of the transistor, resistor, and relay specifications so I think there's no reason for me to include them.
By the way, I'm also having trouble with the step-down transformer. How do I make it deliver 3A max current to the whole circuit without problems?
Thanks in advance.

Hmmm this is almost identical to what Squeak07 posted a week ago... you are not him under a new login are you ?

anyway as to the last part of your question....

By the way, I'm also having trouble with the step-down transformer. How do I make it deliver 3A max current to the whole circuit without problems?

you have to decide what current you want each power rail to be able deliver. Then determine if there would be times when each rail was being used and nearing their max current load. Then choose a transformer that could supply that total amount of current
in this case you have 4 rails and if say 3A max per rail there's 12A total that could possibly be drawn from the transformer.
I would allow another 2A of headroom so that the transformer isnt working at its maximum.

Dave

 

[security_man]

I have a tip for the transformer, if you feel you really need a transformer with high current output, I have a tip that sure it works but is difficult to make, get an old transformer from a Microwave oven, then remove the secondary winding (removing the secondary winding gives a lot of hard work, you will need the help of a vise, and cutting tool (ex: Dremel whit cutting disks), after removing the secondary, rewind the secondary with the wire you feel is the most appropriate to get the number of turns you need and tick enough to support the current you need. By the way, a microwave oven transformer is also a little heavy and large. This is just an idea.

 

[davenn]

I have a tip for the transformer, if you feel you really need a transformer with high current output, I have a tip that sure it works but is difficult to make, get an old transformer from a Microwave oven, then remove the secondary winding (removing the secondary winding gives a lot of hard work, you will need the help of a vise, and cutting tool (ex: Dremel whit cutting disks), after removing the secondary, rewind the secondary with the wire you feel is the most appropriate to get the number of turns you need and tick enough to support the current you need. By the way, a microwave oven transformer is also a little heavy and large. This is just an idea.

Cheaper and much less stress just to buy one

Hey security_man ya love those microwaves dont ya
Have you ever considered one as a hi power 2.450 GHz radio transmitter ?

Dave

 

[bobon721]

Hmmm this is almost identical to what Squeak07 posted a week ago... you are not him under a new login are you ?

anyway as to the last part of your question....


you have to decide what current you want each power rail to be able deliver. Then determine if there would be times when each rail was being used and nearing their max current load. Then choose a transformer that could supply that total amount of current
in this case you have 4 rails and if say 3A max per rail there's 12A total that could possibly be drawn from the transformer.
I would allow another 2A of headroom so that the transformer isnt working at its maximum.

Dave

Dunno who that is. I'm definitely new here. Are there more info in that guy's thread? Can you please provide a link?
About the current flowing on each power rail, there's no specific value for the current required. As long as there's 3A max current after the rectifier, the regulator (this is where the pass transistor comes in), and through the load if only one power rail's loaded. That means it should be less if the other power rails are loaded too.

 

[bobon721]

I was talking about a Multisim 11 schematic here. I can't afford to buy the components and actually build the circuit if the schematic fails to deliver desirable results. It'd be a big waste of money if the components break. This is a course project by the way.

 

[davenn]

you still have to accept that ther possibility of more than one rail being used at once and if so then have a transformer rated to handle a reasonable amount of current so that whatever rails are being used at a given time will be able to supply the needed current to their loads

Looking closer at your PSU cct there are some things missing....
1) ... you will have to fuse the DC lines out of the rectifier, to protect it and the transformer from excess current drain.
2) ... you have NO smoothing caps after the rectifier on any of the DC rails. 0.33uf at the input to the individual regulator chips isnt going to be enough. You should be looking at least at 1000uF and up to 3300uF.
3) ... move those 0.33uF to the outputs of the regulator chips


Dave

 

[(*steve*)]

And the 3A fuse on the mains is unreasonably large.

 

[bobon721]

Here's my new schematic on the 5V rail. I used the tested short circuit protection my instructor shared with us. He said we could use it as long as we made the proper adjustments. I made some adjustments to it so it would give me my desired results.
Quoting my instructor: "During power up, the circuit does not provide output. The tact switch by the collector and emitter of Q2 is the reset/set switch. It is a momentary switch. One will have to hold this switch for a while to turn ON Q1."
I set the relay to close its switch when at least 1mA of current passed through its coil and reopen when the current reached 3A.
The LED lights up when at least 1mA of current passes through it. I've tried to make it light up at 2A and 3A. At 2A it did light up but at 3A it never did since the current passing through it never reached 3A.
My problem now is that the output voltage is 5VDC with short but regular fluctuations. How do I remove the fluctuations while keeping the current output?
I intend to apply this to the +/-15 and variable voltage outputs. What changes do I need to make by then?
By the way, we've been told to use a 12-0-12V 3A transformer for this power supply.

 

[Resqueline]

What's the reason for all those 1 ohm resistors? You're practically shorting out the power supply. Applied in real life a lot of parts would burn..
And how much current is Q4 contributing, when the 7805 contributes 1A?

 

[bobon721]

Now that I retested it the regulator's current output still exceeds the 1A max it should have. Q4 contributes around 600mA according to the simulation. Total current output's around 2.4A. The 1ohm resistors were formerly either 0ohm or unrated in my instructor's "tested" short circuit protection. As I said before, I used it and made changes to make it provide my preferred output. Easiest way for me was simply adjust the resistor values and not remove them entirely.

 

[Resqueline]

Ok, so the 7805 is contributing1.8A. How much is the input voltage to it?
Calculate: how much current can flow through Q1 & Q2 (if they conduct fully), and how much current will flow through the LED? Will any of them survive?

 

[heya_everyone13]

Hi

What simulation did you use? I am also working in power supply and I did not find the transformer in the simulation that I use. thanks for the reply