30 volt P S U

[Mike Brett]

Help please
I needed a 30 volt power supply for my project which I have built using a 18v x 18v transformer to give 36 volt dc via a rectifier.This is regulated by a LM317 to give me the 30 volts I need. However on switch on the LM317 died , I checked the output of the transformer, a new one from Rapid Electronics, and there was an output of 54 volts on it, this is what killed the LM317. Assuming I reconnect the transformer to output just 18 v I will not get the voltage I need to power the LM317. Is there anyway of reducing the transformer to the 36 v I need.
Mike

[julie_m]

36 volts AC, rectified and smoothed, will give you about 50 volts. If it's a decent transformer, that will be 36 volts under load; so it's not surprising that you are seeing more than that.

This seems to be a lot of excess voltage to drop. You really need to be using a lower-voltage transformer (0-15, 0-15V maybe; 0-12, 0-12V might work, but would be pushing it).

[cmjones01]

Where were you measuring the 54 volts? Directly on the transformer windings, or after rectification and smoothing?

I would expect a rectified, smoothed, unloaded 36V transformer-based power supply to give about that many volts. Starting with 36 volts, add 10% because the transformer is unloaded, which gets us 39.6 volts. Then multiply by sqrt(2) to get the peak voltage: 39.6 * 1.414=56 volts. Lose a couple of volts in the rectifier, and there's your 54 volts.

To keep your LM317 happy, you'll either have to replace it with a higher-voltage variant or adapt the circuit. There's a National Semiconductor application note on it here:

http://www.national.com/ms/LB/LB-47.pdf

Your simplest answer might just be to get hold of an LM317HV, which is rated up to 60 volts.

Chris

[Mike Brett]

Many thanks for your advice. I have decided to take the easy option and replace regulator with a LM317HV, hopefully this will work ok. Had to buy minimum of five though from R.S. but only £10.68 so not too bad.
Cheers, Mike.

[Ed_Dinning]

HI Mike, another caution.
If you have any significant caps after the reg, then add a regulator "backwards" across the reg.
This allows the output cap to discharge at the same rate as the input caps and no current is forced backwards through the reg, destroying it.

Ed

[G8HQP Dave]

You mean add a diode backwards across the reg?

[Mike Brett]

Yes got that one, I always fit a diode for safety, thanks for the prompt anyway.
Cheers Mike

[Herald1360]

Don't forget about dissipation inthe 317 either. With 54V input and 30V output, every amp of output current will dump 24W into the regulator.

In free air a TO220 317 is only good for about a watt, a pretty good heatsink will be required for 20W plus!

You could spread the load a bit by using a few regulators in series... spread them out on a heatsink too.

Start by regulating the 54V down to about 46V, follow that with 38V and finally 30V. That way each regulator will only get about 1/3 of the grief.

[Refugee]

There is a circuit in one of the data sheets for the 317 that shows a couple of big TO3s strapped across it to increase the power.

[Mike Brett]

Regarding the power output of my P S U, the output needed from the 30 volt section is just 1 ma. so I did not think heat would be a problem, although I have put the LM317 on a small heat sink. Just for the record the other two voltages needed for my project are 12v at 150ma and 5v at 30ma and for these I have used a 7812 and 7805 fixed regulators.
Cheers Mike

[Herald1360]

The absolute max input voltage for 78XX regulators is 35V- I hope you're feeding them from that 30V supply!

[Amraduk]

Originally Posted by Herald1360

The absolute max input voltage for 78XX regulators is 35V- I hope you're feeding them from that 30V supply!

Don't you mean that you hope he isn't feeding them from the 30V PSU?

Regards,

Dave.

[Mike Brett]

I said I measured 54 volts ac on the transformer which is correct but when I measured the DC output from the rectifier I turns out to be 72 v, I thought I should have lost a couple of volts. I hope that LM317HV can handle it.
Mike

[Amraduk]

Hello Mike,

Originally Posted by Mike Brett

I said I measured 54 volts ac on the transformer which is correct

That cannot possibly be correct, if you're referring to the transformer you mentioned in your first post. It has two 18V windings which, when connected in series would give 36V on load, off load, it would be about 10% higher, about 40V. In a previous post, you said that you measured the 54v after the rectifier, which would be about right.

but when I measured the DC output from the rectifier I turns out to be 72 v, I thought I should have lost a couple of volts.

The transformer has two 18V windings, which, when connected in series, would give 36V. This would be higher off load but shouldn't exceed about 10%, which would give approx. 40V.

The rectified voltage (assuming there is a reservoir capacitor) would, therefore, be:

40 x sqrt(2) = 56.5V - 2V lost in the rectifier = 54.5 DC Approx. maximum.

That is nowhere near the 72V you say you are getting.

I'm puzzled by those figures, I don't see how that is possible, unless the mains is connected to a lower voltage tap (if it has any) on the transformer.

Regards,

Dave.

[Nickthedentist]

What VA rating is the transformer? Some of the very titchy ones can deliver very high off-load voltages ISTR.

N.

[Herald1360]

Originally Posted by Amraduk

Don't you mean that you hope he isn't feeding them from the 30V PSU?

Regards,

Dave.

No, I meant I hope he isnt feeding them from the unregulated 54V, rather from the regulated 30V.

[Amraduk]

Originally Posted by Herald1360

No, I meant I hope he isnt feeding them from the unregulated 54V, rather from the regulated 30V.

Ah, I see what you mean. I misunderstood what you said.

Regards,

Dave.

[Mike Brett]

I have just taken some more readings using another meter. My mains is delivering 240 volts to the transformer, the ac output is 53.6 volts then after the rectifier it is 72.6 volts DC .
I to am very puzzled by the high voltage, I believe the LM317HT has a maximum input of 60 volts so I assume I am stuffed. The transformer I am using is a 230 volt 6va 18v x 18v PCB mounted. Could the problem be it has 230 volt marked on it and my mains is delivering 240 volts, not much I can do about that is there.I did try wiring the transformer to give 18 volts at a higher amperage but the output then dropped to 33 v to 31v so just to low for the 30 volts I need.
Mike

[Mike Brett]

Regarding the transformer input it does have four pins on it, I was puzzled by this so I contacted the company that makes them, Vigortronix and they said to use the two outside pins only as the two middle pins are for stability only. Not sure if this is relevant .
Mike

[Nickthedentist]

Mike, can you provide a link to the exact one you have, from their website: http://www.vigortronix.com/subcategory.aspx?catid=11