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Old 29th December 2007, 12:20 PM   #1
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Default PSU help please

I have just built my first unregulated PSU.

The toroid trafo is 60VA with two 18V secondaries whch are connected in series. The secondaries are connected to 4 Shottky diodes for full wave rectification. There are then two 2200uf/63V electrolytics across the DC output.

I read 55V DC at the output but also 10V AC on the output.

I just wondered whether someone with experience could tell me if these readings point to something being wrong: as they seem odd to me.

Aidan
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Old 29th December 2007, 01:55 PM   #2
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You have 10v of ripple on your DC voltage! The 2200uF caps are
rather small for an unregulated supply. Try increasing them to 10,000uF. If you have scope, then connect it to the supply to see the
ripple.
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Old 29th December 2007, 02:02 PM   #3
paulb is offline paulb  Canada
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The DC voltage sounds right. 18VAC * 2 windings *1.4 = 50.4V DC. The 1.4 is the ratio of peak to RMS value of the AC voltage, and with no load the transformer will put out higher than its rated voltage.
What load are you driving with the supply? There should be little or no ripple if there is no load. Actually, I wouldn't trust your multimeter to read the AC component correctly on a high DC voltage.
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Old 29th December 2007, 02:28 PM   #4
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Thanks for the help guys.

At the moment there is no load. I did wonder if I was doing right by just setting my DMM to VAC to measure ripple voltage across the DC output.

The PSU is intended to power a Preamp. In which will be two local regulators which will drop the voltage to about 30V. I cant see there being more than 250mA being required - but I don't know really.
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Old 29th December 2007, 04:01 PM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
some DMMs can give a good estimate of the average AC on a DC supply. But it looks like your one is affected by common mode error, which is being converted to an equivalent AC reading.

I have a very old DMM and a modern very cheap one and both give a reading that agrees with the RMS reading on the bench DMM and with the scope Vpp. Price and age do not seem to be an indicator of this AC ability in the presence of high DC bias.

BTW,
2200uF for 250mA is about right for ripple under maximum load.
You could add a second RC stage before the regulator if you want/need to suppress the hum and it's harmonics even further.
Are your supplies both +30Vdc?
A 36Vac transformer is just a bit high for that and will run the regulators hot.
Or
do you intend to make a +-30Vdc dual polarity supply?
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Old 29th December 2007, 04:24 PM   #6
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Hi Andrew,

My DMM beeps when I try and read AC on the DC supply. It doesn't seem too happy with the task.

The PSU will supply +30V only. There's a VBE in the reg that I'm going to use which will drop some voltage. The LM317 inside the reg will take the 55V supply and drop to 36V - a big drop but within spec.

At the moment there are 2 x 2200uf caps. I figured on 2200uf per amp so that 4400 would be fine. Would you recomment that I add another 2 x 2200uf?

I didn't expect the DC out from my supply to be as high as 55V. I knew about the factor of 1.414 but assumed that the final DC voltage would be less because 1.414 is supposed to give the peak. I have a spare 30VA trafo with two 12V secondaries. Perhaps I should use this instead.

many thanks

Aidan
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Old 29th December 2007, 04:51 PM   #7
AndrewT is online now AndrewT  Scotland
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regulated +30Vdc needs about 30Vac.
24Vac is too low and 36Vac is too high unless you drop it significantly.

An rCRC supply would be perfect for your 36Vac overvoltage situation.
take the first 2200uF to do rough smoothing. This cap takes a lot of ripple current and can overheat when used as the first C, so it needs to have an adequate ripple rating.

Now add a series power resistor of between 22r and 47r.
then follow this with a second 2200uF.
At low output current say 50mA, the 22r resistor drops just 1.1V and leaves about 54Vdc at the input to the regulator. The dissipation will be about 24V times 50mA ~= 1W, a small sink will do that.

Now increase the current to 250mA. The resistor voltage drop will now be 5.5V and the input to the regulator drops to about 49V. Dissipation increases to 19V * 0.25A ~=5W. The sink needs to be quite big to cope with this.

The reason your output voltage is high is down to two main reasons. Voltage regulation of the 60VA transformer and actual mains voltage delivered to your house. Your transformer may be 230:18+18 and 10% regulation.
On 240Vac it's output is 240/230*36*1.1~=41.3Vac.
The peak voltage will be about 58.4V. At very low current (leakage through the capacitors) the bridge diode drop will be lower than nominal, maybe just 400mV, leaving 58Vdc across your caps.

What would be the output if the mains voltage goes to the +6% maximum of 254Vac? What is the worst case dissipation in the regulator then?
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Old 29th December 2007, 05:09 PM   #8
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Quote:
Originally posted by AndrewT

What would be the output if the mains voltage goes to the +6% maximum of 254Vac? What is the worst case dissipation in the regulator then?


Looks like I need a 2 x 15V trafo then to get it right.

I'm getting quite a collection.
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Old 30th December 2007, 01:55 PM   #9
AndrewT is online now AndrewT  Scotland
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Plug the 12+12Vac into the mains.
Measure mains voltage and output voltage. You may find this is just high enough particularly with the schottky bridge.

Then calculate the minimum and maximum output voltages for supply voltages of 216Vac and 254Vac.
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Old 30th December 2007, 04:07 PM   #10
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Hi Andrew,

yes this is the way that I have decided to go.

My preamp works fine at 24V input. I use an SMPS laptop supply at the moment. However, it is supposed to sound better at 30V. It's the Air-guitariste preamp, which you will probably know about.

So I have changed the trafo for the 30VA, and 2 x 12V secondaries in series. I have kept the Shottkys and 2 x 2200uf electrolytics. I now measure 36V DC out. I set the DMM to read Vac and it goes wild, then beeps and gives up. A 10% change in this DC voltage won't upset the LM317 which is set to 30V out. There's a further drop because of the VBE which should give me 25V under load, hopefully.

Many thaks for the advice about how the mains voltage varies - a serious point that I had not taken into consideration.

The regs that I'm using are "Teddy regs" from PFM: low noise and low impedance. It wil be interesting to hear the difference between the laptop SMPS and the linear supplies.

Thanks again

Aidan
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