Power Supply Woes

[MrClamperSir]

Hello!! I am new to this forum and new to electronics. I am a tattooer of 16 years and I have been repairing (replacing parts) on antiquated power supply's for a couple years now. I don't care for the switched mode supply's because of the rippled output so I stick with the linear supply's.

Unfortunately for me I am one of the last of a dying breed when it comes to my preference of supply. That being said, I have built my own supply and am mostly happy with it. However, it runs a little too hot (most of my older supply's get real warm but his one is HOT). I'm hoping someone can help me verify the schematic, and see if there is a way to decrease the Vin to Vout while maintaining the current needed to drive my tools.

I'm thinking about lowering the volts on the secondary side of the transformer to 14V. But what about the amps? Is 1A too much or too little?

My machines run on a variable of 3V - 8V drawing 0.8A - 1.3A

Below I have attached the schematic drawing. I will try my best to answer any questions but like I said, I really new to this and self taught so bear with me please.

 

[davenn]

Hi there
welcome to EP


If you are only needing 3V DC, then 14V AC is a serious overkill as that is giving you almost 20V DC into the regulator chip
it will have to drop 16 V at say 1A = 16W ... yeah the regulator WILL get hot !!
get a transformer with a ~ 8 VAC out and a 2A capability and the reg chip wont have to work so hard
It is ALWAYS wise to have the PSU capable of supplying more current than what is needed, that way it's never working at its maximum

just looking at your circuit ... that transistor connection definitely doesn't look right for doing current pass
note that the base and emitter are short circuited

you can do away with the transistor and just use a LM 338 reg which is capable of up to 5A, instead of the LM317


Dave

 

[MrClamperSir]

Thank you for your reply.

I need a variable output the ranges from 3v-8v. Not sure if that changes anything you suggested.

I'll look into the LM 388 right now.

Was this not an option in the early 90's?

 

[davenn]

I need a variable output the ranges from 3v-8v. Not sure if that changes anything you suggested.

The LM338 is an adj reg ... just a higher current version of the LM317

not sure when the 338 became available ?

 

[MrClamperSir]

Right now I have the secondary side of the transformer at 25VAC. The regulator doesn't get too hot (that I'm aware of) but the transistor definitely does.

So if I move the secondary side down to 8VAC at 2A and use the LM 388 (I'm assuming mounted on a heat sink) this will be a better output/PSU to power my tools?

** These changes can be made while still using the same circuit at the same values??

 

[davenn]

So if I move the secondary side down to 8VAC at 2A and use the LM 388 (I'm assuming mounted on a heat sink) this will be a better output/PSU to power my tools?

yes 8 - 10 VAC secondary ( 10V would be best if available will allow for a bit of voltage drop under load)

actually .. just redid calculations ... definitely go for a 10V, 2A secondary rated transformer

heatsink definitely

** These changes can be made while still using the same circuit at the same values??

yes and there is no need for your R2 - 47 Ohms


Dave

 

[MrClamperSir]

Looking into the LM338 it looks like it's often used with the LM7812. It also looks like adj reg or a transistor

What's the difference? Is one better than the other?

 

[MrClamperSir]

Thanks Dave!! I've been alone in this endeavor for the past year and I've spent the last month banging my head on the wall!! I'm feeling energized o get back to it!!

 

[davenn]

Looking into the LM338 it looks like it's often used with the LM7812. It also looks like adj reg or a transistor

What's the difference? Is one better than the other?

use the TO3 metal cased one

Thanks Dave!! I've been alone in this endeavour for the past year and I've spent the last month banging my head on the wall!! I'm feeling energized o get back to it!!

that's good to hear


D

 

[MrClamperSir]

I'll try and wrap my head around building this circuit!!

 

[duke37]

If you use a boost transistor, it needs to be a pnp transistor such as a 2N2955.
The circuit you show has the pass transistor permanently turned off so I do not see how it should get hot.

 

[MrClamperSir]

If you use a boost transistor, it needs to be a pnp transistor such as a 2N2955.
The circuit you show has the pass transistor permanently turned off so I do not see how it should get hot.

Is there a way to test a component to see if it's pnp or npn? I have a transistor in a working circuit but the values (markings) are not present.

Also, how did you conclude that the pass transistor is permanently turned off? because I listed it as a 2N3055 when it should be a 2N2955?

 

[BobK]

Yes, an NPN transistor would be turned off as shown.

A PNP transistor would act as a forward biased diode, shorting out the regulator.

So you probably have a PNP transistor acting like a diode and no voltage regulation. The transistor would get hot.

Bob

 

[BobK]

Check this out:

http://www.ebay.com/itm/LT1083-Line...276682?hash=item2106e0310a:g:R1cAAOSwwE5WaRsm

7A capable.

You could replace the adjustment pot with a combination pot and resistor to get the 3-8V range you need.

Bob

 

[davenn]

Also, how did you conclude that the pass transistor is permanently turned off? because I listed it as a 2N3055 when it should be a 2N2955?

because as I told you in an earlier post, it is wired incorrectly ... the base and emitter are shorted out

 

[MrClamperSir]

Yes, an NPN transistor would be turned off as shown.

A PNP transistor would act as a forward biased diode, shorting out the regulator.

So you probably have a PNP transistor acting like a diode and no voltage regulation. The transistor would get hot.

Bob

I'm not sure how the schematic compares to the wiring of the actual circuit considering the schematic was drawn for me by a guy who had never seen the supply in person and I wired the circuit using my existing supply as an example. So I'm not sure if the transistor is PNP or NPN but I do know its getting really hot and I'm trying to figure out why.

 

[MrClamperSir]

because as I told you in an earlier post, it is wired incorrectly ... the base and emitter are shorted out

Yes, I see that now. Thank you. I went back the circuit and changed the schematic to represent what I was seeing in the circuit. Is this how it should look, or am I'm I missing something or.....?

 

[duke37]

Look up the data sheet for the 317, it normally gives details of how to boost the output.
R2 is not necessary in your circuit. Q1 just works as an emitter follower.
If you use a pnp transistor, then R2 should come before the 317 and the collector should be connected to the 317 output.
You can compare a known pnp and known npn transistor with the unknown with a resistance measurement, this can tell you which is the base and whether it is pnp or npn.

If a lot of voltage is dropped with a high current, then a lot of heat will be produced and a massive heat sink will be needed.

 

[MrClamperSir]

Look up the data sheet for the 317, it normally gives details of how to boost the output.
R2 is not necessary in your circuit. Q1 just works as an emitter follower.
If you use a pnp transistor, then R2 should come before the 317 and the collector should be connected to the 317 output.
You can compare a known pnp and known npn transistor with the unknown with a resistance measurement, this can tell you which is the base and whether it is pnp or npn.

If a lot of voltage is dropped with a high current, then a lot of heat will be produced and a massive heat sink will be needed.

Thanks, I will look up the data sheet right now.

In this circuit R2 is after the LM317 (which your saying isn't necessary) and the collector is on the input side of the regulator. Does that mean that it is a NPN?

 

[davenn]

R2 is not necessary in your circuit. Q1 just works as an emitter follower.

not quite, do that and you go back to a transistor doing nothing with it's base and emitter shorted

In this circuit R2 is after the LM317 (which your saying isn't necessary) and the collector is on the input side of the regulator. Does that mean that it is a NPN?

a resistor is needed but not in that config., and the base should be on the input side of the 317
I will find the correct circuit for you

OK here we go



This is for a 317 in a fixed output mode ... hence no trimpot
it's the placement of the transistor and it's resistors I want you to note


Dave