Help with power supply/heatsink bits n' bobs..

[Cam]

Hi all,

I had a couple of questions about an off the shelf power supply I'm using - hopefully someone can help me out..

I picked https://quasarelectronics.co.uk/Item/velleman-k8042-symmetric-1a-power-supply-kit up to get some op amps happening in a little project I'm working on.

Off it I'm powering the following: 10x ne5532 (which from the datasheet can draw a maximum of 10mA per rail?), 6x SSM2019 (8.5mA each) 1x THAT4301 (18mA), 2x AD825 (I think only 6.5mA per rail, although I'm unclear on this), and 1x TPA6120, which I'm using as a headphone amp - I don't entirely understand how much current this draws either!

Anyways, I figured the lot had to be under 200mA, so I bought the above +-1A supply. And if I measure this circuit connected up but with no audio running through, it draws around 150mA per rail (assuming I can read a multi-meter, which isn't a given!) at +/-15V.

The supply, however, gets a bit of a smell on! After a minute or so of the supply being on I can squeeze a heatsink for no more than a second or so - I've no idea if this is normal or not? I haven't run it for long enough to see how hot it gets - I'd rather know if this is right! So I looked up a few things, and I'd love to know if what I think I've worked out is correct.

The output of my toroid is 21VAC on each of the secondary windings (that's another question - it should be 18VAC - why is it high?), so can I calculate 21V(input voltage), minus 15V(supply output voltage) multiplied by 0.15W(circuit current draw) to get 0.9W of heat to be dissipated by the heatsink? I'm using two of these little guys https://quasarelectronics.co.uk/Item/to220-heatsink-pre-drilled-19mm-black which I see have "21°C/W thermal resistance" - does this mean its temperature rises by 21°C for every Watt it dissipates? And would you say it's a big enough sink for this project? I'd have imagined for anything under a Watt, anything remotely heatsink shaped would have done... Which makes me question the heat and smell, and whether the amount of power the regulators are dissipating is far more than I realise. Or is it just new component smell!?

Finally, and a separate question, can I run two separate power supplies off one secondary winding? ie, if for example I bought a second identical supply, could I run the toroid into both of them? Or is there a reason I see every power supply having a single diode bridge before splitting off to multiple supplies (other than cost, obviously!)?

Many thanks for any help you can provide!

C

 

[ruffrecords]

Anyways, I figured the lot had to be under 200mA, so I bought the above +-1A supply. And if I measure this circuit connected up but with no audio running through, it draws around 150mA per rail (assuming I can read a multi-meter, which isn't a given!) at +/-15V.

MNore or less OK up to this point.

The supply, however, gets a bit of a smell on! After a minute or so of the supply being on I can squeeze a heatsink for no more than a second or so - I've no idea if this is normal or not? I haven't run it for long enough to see how hot it gets - I'd rather know if this is right! So I looked up a few things, and I'd love to know if what I think I've worked out is correct.

The output of my toroid is 21VAC on each of the secondary windings (that's another question - it should be 18VAC - why is it high?),

Its rated output at full load is 18VAC. Like everything else it has an internal resistance so if it is lightly loaded its output will rise. What is its rated output current?

so can I calculate 21V(input voltage), minus 15V(supply output voltage) multiplied by 0.15W(circuit current draw) to get 0.9W of heat to be dissipated by the heatsink?

No. The 21VAC is recitified so the dc produced is closen to the peak value which is 21 x 1.414 or about 29V. So the drop across the rectifier is 29 - 15 = 14V and the disippation is 14V x 0.15 watts = 2.1 watts - more than twice what you expected.

I'm using two of these little guys https://quasarelectronics.co.uk/Item/to220-heatsink-pre-drilled-19mm-black which I see have "21°C/W thermal resistance" - does this mean its temperature rises by 21°C for every Watt it dissipates?

More or less but there are other thermal resistance in the thermal circuit. The main one you need to know is the thermal resistance junction to case of the regulator ship. This is in series with the heatsink. So you start with your ambient temperature, say 20 degrees Celsius. The 2 watts of heat flowing through the heatsink raise its temperature by 42 degrees to about 62 degrees. The same heat flows from the junction to the case (because it is the junction that is actually creating the heat). Lets say its thermal resistance is 5 degrees/Watt; this means the junction will be another 20 degrees higher at about 84 degrees. This is well within its maximum rating of 150 degrees so the circuit should not fail but the heatsink will reach overh is definitely too hot to touch. I definitiely recommend you find a meatier heatsink. If you can find one rated at 10 degrees/watt that should drop the heatsink temperature by 20 degrees to a comfy 40 or so degrees.

Rememeber, all the op amps you have connected are class B devices. This means the quiescent current is the minimum they draw. As soon as a signal passes though them into a load, the supply current increases. How muh depends on the signal level and the load.


Cheers

Ian

 

[Cam]

That makes a lot of sense, thank you for writing all that up!

It's a 50VA transformer. But that's fine I suppose?  The supplies I'm using are happy up to 25VAC I think, so if it's up at 21V it's not going to do any harm to them.. And there may end up being a few other things hanging off the transformer - I might do some voltage tripling circuit to try and find 48V for a few microphones, and I'll be running a small 5V supply off it too to power a microprocessor - so perhaps it will drop a little.

And that's perfect regarding the heatsink - it's what I suspected, even though I didn't have the numbers to back it up! The trick is now finding heatsinks that will fit in my 1u case... It's getting tight!

Any chance you/someone could answer this little question?

Finally, and a separate question, can I run two separate power supplies off one secondary winding? ie, if for example I bought a second identical supply, could I run the toroid into both of them? Or is there a reason I see every power supply having a single diode bridge before splitting off to multiple supplies (other than cost, obviously!)?

C

 

[ruffrecords]

It's a 50VA transformer. But that's fine I suppose?  The supplies I'm using are happy up to 25VAC I think, so if it's up at 21V it's not going to do any harm to them..

That is 25VA per winding so if the rated voltage is 18VAC then the rated current must be 25/18 = 1.39 amps. So drawing 150mA from it is about 10% of rated load which explains why the voltage is high. There is no harm running a transofmrer at less or a lot less than it rated load. You just need to be aware the output voltage will be higher.

And there may end up being a few other things hanging off the transformer - I might do some voltage tripling circuit to try and find 48V for a few microphones, and I'll be running a small 5V supply off it too to power a microprocessor - so perhaps it will drop a little.

The more you load it the more it will drop.

And that's perfect regarding the heatsink - it's what I suspected, even though I didn't have the numbers to back it up! The trick is now finding heatsinks that will fit in my 1u case... It's getting tight!

If you use the right insulation kit there is no reason not to use the case as the heatsink.

Any chance you/someone could answer this little question?

Finally, and a separate question, can I run two separate power supplies off one secondary winding? ie, if for example I bought a second identical supply, could I run the toroid into both of them? Or is there a reason I see every power supply having a single diode bridge before splitting off to multiple supplies (other than cost, obviously!)?

C

Not really is the answer. If the two dc supplies are completely separate and there is never any connection between them then it may work. The problem is you usually want them to have a 0V in common for example.

However, there is a trick sometimes used which couples the second bridge rectifier to the transformer via some large value capacitors. It is OK for generating a relatively low current supply. I have seen circuits for creating a 48V phantom supply  from 2 x 25V windings this way. The circuit has been posted here at least once. A search may find it.

Cheers

Ian