Need some clues

[xtrmi]

I recently built a linear power supply and I expected 1.25 - 28v from it @1.5amps. I'm using a Triad F7-28 transformer I get about 34 vac from it. I'm using a LM317T regulator and at the output I've seen up to 43-44 volts DC. I've been able to draw down the voltage to 37volts when drawing .5 amps. I assume once I draw 1.5 amps I'd see around 28v or so. I'm not really happy with this so I'm needing some clues on how to bring this down to 1.25 - 28v range and not see such a big voltage drop based on the load.

 

[marcbarker]

I recently built a linear power supply and I expected 1.25 - 28v from it @1.5amps. I'm using a Triad F7-28 transformer I get about 34 vac from it. I'm using a LM317T regulator and at the output I've seen up to 43-44 volts DC. I've been able to draw down the voltage to 37volts when drawing .5 amps. I assume once I draw 1.5 amps I'd see around 28v or so. I'm not really happy with this so I'm needing some clues on how to bring this down to 1.25 - 28v range and not see such a big voltage drop based on the load.

really, you need to post the schematic annotated with voltage readings, or this is just guess-work!

probably something missing between ground and the 317's com pin.

 

[marcbarker]

The most important voltage annotation is completely missing!

What's the voltage on the 317's COMMON pin, (with respect to ground)?

 

[k7elp60]

An interesting thing. I have a power supply that I built that has 4 of the F7-28 transformers connected in parallel. The thing that I found interesting was that in trying to use 3 terminal regulators I could not achieve the range of voltages and currents I desired, because the internal circuitry of the regulator would shut it down even thou it was on a adaquate heatsink. What I am saying is that the internal circuitry monitors the load current and the input/output difference and shuts it down. I tried LM338K and LT1083CP regulators. I finally went to LM723 with an external pass transistor. The power supply has two ranges. 6-14V @ 8A and 12-28V@ 4A. On the low range if the voltage is set @ 14V I cannot get 8A but at 12V I can get 8A.
I have found that the rule of thumb for brute force filtering for linear power supplies the filter capacitor needs to be 3000uF or greater per amp of load current. For continous duty the maximum load current from the transformer is .56 x RMS rated current. This is assuming brute force filtering. The reason for this is that the charging current for the filter capacitor really exceeds the current rating of the transformer and the extra current will cause transformer excessive heating.

I don't believe you will be able to get 28V @ 1A regulated from your supply because there is voltage drop across the bridge rectifier, drop across the 317 reglator, and the internal resistance of the secondary will cause a voltage loss.
If you change the pot to 2.5KΩ and change the 100Ω to 120Ω, you will have near full range of the pot.

 

[xtrmi]

Ok so I updated the graphic with the new values. I also hooked two 25w 39ohm resistors in parallel and got up to 1.56 amps at 30v between the output of the lm317 and ground. Under the same load the voltage between +- of the bridge was 32.7v.

 

[marcbarker]

I saw the annotation "0-44V" but it's ambiguous. This is the most important clue.

Are you saying it's

1. 0.44V? (& you'd made a typo) ,
2. 0 to 44V? (Volts varies up and down, according the pot setting)
3. 44V? (the 0- was a typo)

If it's 3, you'd probably blown the 317. Check for 1.25 V between Vreg-Out and Common.

And check there is actually any resistance reading at all between the Common pin and the Ground. You should find the Pot resistance here.

 

[xtrmi]

Hey thanks for all the clues. Ok, I just checked and there is resistance equal to the pot resistance between the common pin and ground. Also the annotation means I saw a voltage that varied from 0 to 44v between those points. I really just want to get a more regulated voltage that's all. I want to set it to 5v and get close to that once the load is applied. I don't want to start at 10 and have it go to 5 or start at 2 and have to move it up to 5. Thats really my biggest issue with this psu.

 

[marcbarker]

I saw a voltage that varied from 0 to 44v between those points.

Isn't that what supposed to happen for a 5K pot? Have you done the sums to calculate what the resistors (including the pot) should be? Maybe 5K is a bit too high?

I really just want to get a more regulated voltage that's all. I want to set it to 5v and get close to that once the load is applied. I don't want to start at 10 and have it go to 5 or start at 2 and have to move it up to 5. Thats really my biggest issue with this psu.

Are saying the output voltage varies too much with output load?

 

[xtrmi]

I ran some more test on it and I set it to 24v under no load between outputs. Then I put 19.5ohms resistance between the output terminals and it drug the voltage way down to like 10v then it slowly crept up to about 18v. I'm thinking of trying a different transformer and providing a lower input to the regulator. I probably get a new lm317 so I can have a spare on hand.

 

[marcbarker]

Have you done all the sums right?

It's normally 240 Ohm, not 100 Ohm. << Edit: Bum steer! Sorry. It's the 117 that's 240 Ohm, the 317 is 'officially' 120 Ohm. 100 was correct

LM317H is only 0.5 A rated.

I bet the heatsink isn't big enough!

 

[xtrmi]

I'm using a lm317t attached to a heat sink from my ocz power stream. I do feel some heat but not much. I didn't see that in the simulation. I changed it and now the simulation is a bit closer to reality.

 

[xtrmi]

The schematic I posted was all wrong. Obviously the ac cannot be connected to dc ground. Not sure how the simulation work this way. I will redo this and post it.


EDit: I haven't thought about this enough. I will post a schematic when I know its correct.

 

[xtrmi]

Ok, I changed the resistor in the divider from 100ohms to 298ohms. With the 5k pot this will allow me to get up to 21v, but I still get a huge drop once the load is applied. I can't figure out why there is such a big drop. Here is the data I took with a dmm.
**broken link removed**
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Would this help?

**broken link removed**

 

[marcbarker]

0.9 V across R3 smells of 317 not able to regulate.

When the load's connected, what's the voltage on the 317's Line Voltage pin?

If that collapses under load there's something going on before the 317. Is your transformer man enough for the amps? Have you added some more uF (like 4700uF) and seen if that made difference?

 

[xtrmi]

I thought I got all the details. I posted the new schematics above. ^^^

 

[marcbarker]

What's the voltages like for say, output set to 15 V?

I'd say in the switch open figure, the voltage control is set too high. The 317 does need some volts across it to work, at least 2V

See the 'switch closed' fig.?, the same thing, the voltage control is still set too high.

See the 317 input voltage drop down a few V under load? That's normal.

**broken link removed**

 

[xtrmi]

I will take these measurements in a bit. What do you think of using the transistor to get more amps. Also will this help lessen the voltage drop under load? The transformer says secondary @ 28vac rated for 56va.

 

[xtrmi]

Ok here it is. And thanks again for looking at this with me. I chose to increase the load for this one from .45amps to .61amps by decreasing the RL from 39ohms to 19.5ohms.
**broken link removed**
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[Hero999]

Have you done all the sums right?

It's normally 240 Ohm, not 100 Ohm.

When there's a possibility the regulator will be required to run unloaded, I use 120Ω because 240Ω might not provide enough of a minimum load current.

For 15V use 91R and 1k, 180R and 2k or 200R and 2.2k for R1 and R2 respectively.

 

[xtrmi]

I'm actually using about 298ohms for r1 and 5030ohm for r2. I was thinking of using lower resistance to increase the load across the divider, but honestly I thought that it might make matters worse since this circuit seems so sensitive to the load. I'm just shooting in the dark though because I'm not familiar on the workings of the regulator other than it maintains 1.25v between out and adj. Based on that I can't figure out why it would drop since I've kept everything within limits based on what I know.