Heatsinks

[RobertW]

Does anyone have a worked example of calculating the size of a heatsink, from working out the power rating to the AREA of the heatsink, to allow the fabrication diagram to be worked out. I can calculate up to the thermal resistance, but am unsure how to calculate this into area.

 

Thanks for the help

 

Rob

[Crosland]

Does anyone have a worked example of calculating the size of a heatsink, from working out the power rating to the AREA of the heatsink, to allow the fabrication diagram to be worked out. I can calculate up to the thermal resistance, but am unsure how to calculate this into area.

 

Thanks for the help

 

Rob

You need the thermal conductivity of the material you propose to use (k). For a flat plate thickness t and area a, the thermal resistance is  t/(k*a). Adjust t and a until you meet your desired thermal resistance.

 

A finned heatsink will be much more efficient, but the calcs are a lot more complicated. Black paint or anodising will also help.

 

You need to consider the thermal resistance of the interface between the object being cooled and the heatsink.

 

What exactly are you trying to do? Why can you not just use an off-the-shelf heatsink?

 

Andrew

[RobertW]

I am building a model railway controller from the Diagram in chapter 3 of the Complete Book of Model Railway Electronics by Roger Amos. This is a Darlington Closed loop controller. the small transistor is a BC337 and the large is a 2N3055. I have substituted the BC337 with a BC547 and the 2N3055 for a TIP31A, although I am thinking of changing this for a BDX33C. I am planning on bolting the heat sink straight to the transistor using 0.5mm aluminium as the medium. How do I calculate the thermal conductivity of the metal. All of the legs of the transistor are on flying leads, but are heat shrinked. I have attached a 100cm²  heat sink to it (unpainted) in two fins.

 

Is this sufficient or should i redesign the heat sink from scratch.  

[Crosland]

I am building a model railway controller from the Diagram in chapter 3 of the Complete Book of Model Railway Electronics by Roger Amos. This is a Darlington Closed loop controller. the small transistor is a BC337 and the large is a 2N3055. I have substituted the BC337 with a BC547 and the 2N3055 for a TIP31A, although I am thinking of changing this for a BDX33C. I am planning on bolting the heat sink straight to the transistor using 0.5mm aluminium as the medium. How do I calculate the thermal conductivity of the metal.

 

You look it up, e.g. http://en.wikipedia.org/wiki/List_of_thermal_conductivities :-)

 

All of the legs of the transistor are on flying leads, but are heat shrinked. I have attached a 100cm²  heat sink to it (unpainted) in two fins.

 

Is this sufficient or should i redesign the heat sink from scratch.  

 

Maybe :-) Use some thermal compound or a thermal pad between the transistor and the heatsink. Monitor the temperature of the transistor. If you can keep a finger on it indefinitely then it's probably OK. Does Amos not give some guidance?

 

To do it properly you need to know the worst case power dissipation which will probably be when the output is shorted with the throttle turned up full. Hopefully there is some form of current limit.

 

Andrew

[Gordon H]

Not only does the area and shape affect this, but the environment where it resides does too.

How much airflow is there likely to be around the heatsink?

From the description, it does not sound like a hand held device (a 10cm x 10cm piece of warm/hot ally in the hand is not going to be particularly convenient). Or do you really mean 100cm x 100cm when you say 100cm² (as opposed to 100 sq cm) - a subtle but important distinction!

You might also consider using a single power darlington transistor to replace both the BC547 and 2N3055, such as the BDW93C. As a single hole TO-220 device it is much more convenient to mount on a heatsink than a TO-3 package.

I did this some years ago for some simple hand controllers, and used hand-sized diecast boxes which served for containing all the controller circuitry (a pot, a resistor, a darlington transistor and a reversing switch) - and as the heatsink. Also told you fairly readily when the current was becoming excessive!

[RobertW]

I am planning on mounting the circuit board and heat sink in a Maplin ABS type E box. The circuit board is 77 mm long and 40 mm wide  The heatsink will be painted black at the first opportunity  If the leads are heatshrinked to the flying leads then is it necessary to earth the heatsink?

 

Thank you for the replies.

[Suzie]

The mounting hole of the transistor will be connected to the central collector pin so if you want to earth the heatsink you will have to use an insulated mounting kit to mount the transistor on to the heatsink. When you take in to account the smaller mounting area of the TO220 transistor and the extra insulation of the insulating washer (which in practice is probably not that much) you might find yourself needing a bigger heatsink than if you had used a TO3 transistor like the 2N3055 mounted directly to the heatsink and isolated the heatsink using nylon pillars.

 

I don't have access to the book, but I guess it is not this one:-

 

http://www.rogeramos.co.uk/pwam.htm

[RobertW]

No. it is a simple darlington throttle. There is a 20K linear Pot (connected across the supply), which i have changed for a 10K pot, attached through a 1K resistor the base of a BC 337 (which i have substituted for a BC547). The collector of this transistor is connected to the base of a 2n3055, however i will substitute this for a BDX33C. There is a 1n4001 in reverse bias across the output, and then a DPDT switch for reversing. I am Using a 240V; 0-12 0-12 5A transformer. The Bridge rectifier is made from  1n5401 diodes.

 

Hope this helps

 

Rob 

[Gordon H]

I am planning on mounting the circuit board and heat sink in a Maplin ABS type E box. The circuit board is 77 mm long and 40 mm wide  The heatsink will be painted black at the first opportunity  If the leads are heatshrinked to the flying leads then is it necessary to earth the heatsink?

 

Thank you for the replies.

 

You are rather defeating the object of the heatsink if you keep it in a plastic box. There will be nowhere for the generated heat to go no matter whatever colour you paint it.

Better to use a diecast or similar metal box as your enclosure and heatsink combined which will have better dissipative qualities due to the airflow across it during normal use. As Suzie said, an insulating kit for the transistor would be required too.

[RobertW]

For the insulating kit could I use a Nylon bolt and nut or would I need the correct Mica and washer.

 

Rob

[Suzie]

In the olden days a mica washer with heatsink compound was used with a bush to insulate the hole so a metal screw could be used, ideally screwed into a tapped hole on the heatsink, normally 6BA.

 

Nowadays newer rubber washers are used (not just any rubber!) that do not require heat sink compound, with a bush and an M3 screw.

 

If the heatsink is inside the case you will need to allow some ventilation with a grille or louvers since you will have up to 15W of heat to dissipate when running slowly into an overload (less when running flat out). Maximum dissipation with a linear current source or constant voltage controller such as you have described tends to occur at around half throttle in normal operation.

 

It is possible to do this, I have made one in a louvered metal case salvaged from an old varaible resistance controller but it is not really best practice because the internal temperature can get rather high without a cooling fan if the layout of components is not very carefully considered. You might be able to make a some louvers from a vent grille from a DIY shop to attach to your plastic case but they need to be carefully positioned to ensure they pass the foreign object penetration test if mains voltage is present inside the controller. Wall wart 12V AC PSUs (like Maplin L54BR) are very handy for keeping mains out of the case, but a double insulated DC switch mode one (Maplin L49BL) might be more useful in your application if you do not need it for an auxilliary AC supply since it will save you having to fit the rectifier and be more efficient.

[Crosland]

I am planning on mounting the circuit board and heat sink in a Maplin ABS type E box. The circuit board is 77 mm long and 40 mm wide  The heatsink will be painted black at the first opportunity  If the leads are heatshrinked to the flying leads then is it necessary to earth the heatsink?

 

Thank you for the replies.

It's your choice. If you don't earth the heatsink then you do not need any insulation as you have only a single device om the heatsink.

 

Insulation kits are only need where the heatsink is at a different potential to some or all of the component bodies mounted on it, e.g. if it is earthed or if you mix NPN and PNP transistors on the same heatsink.

 

Andrew

[RobertW]

Having asked the questions. I have designed a inertia controller using the BDX33C. Can somebody just check this diagram and make sure there are no stupid mistakes in it. It is designed to function on two buttons. the first controls the speed and the second the direction. The diagram is self explanatory, but if you ahve any questions then just as.

 

Thankyou

 

Rob

[Suzie]

Rob

 

The direction LEDs D3 and D6 could be simplified by sharing the resistor and doing away with the protection diodes. You could use a single 2-legged bi-colour LED in that scenario if you wanted.

 

You have shown the heatsink as uninsulated from the transistor, which means that the earth connection is connected to the collector of the transistor (pin 2) making the controller and stock liable to damage if the track should be shorted to earth at any time. This can very easily occur if you were to solder the track with an earthed soldering iron without disconnecting the controller, and simply powering off the controller would not eliminate the problem if there was charge stored in the reservoir capacitor C1.

 

Since your track feed passes through a 680R resistor R2 and an LED, you will not be getting sufficient current to power your train, I suspect that there is an error in that part of the diagram. Are the LEDs supposed to show current or applied voltage?

 

You say it is an inertia controller, but there does not appear to be a control to set the inertia - it appears to be fixed at either zero or infinite by switch S2. When the switch is open there is no brake adjustment, just the non-linear effect of LED D8 draining the capacitor down to about 1.5V where it will stay.

 

VR1 at 10K is a very high value that will only operate at the top of its range because of the load present from the LED D8 in series with the 680R resistor. You will get approximately half voltage at the output when VR1 is about 5% from maximum, so very little useful adjustment from it.

 

Do we have the right diagram? there are two switches and a knob in it which does not match your description of a two button controller.

[RobertW]

S1 is a locking push button that controls the direction of the train through a DPDT relay. S2 provides the voltage set by the preset potentiometer to transistor, charging up the capacitor in the process. When the button is released the capacitor is drained through the resistor and base. This slows the train gently the potentiometer (VR1) sets the maximum speed attained by the train. VR2 sets the capacitor charging current, while C2 provides the slowing function, to stop the train in around 10 seconds. R3 is the transistor protection resistor. T1 is the buffer between the high current and the low current in the circuit. D4 is the inductive overshoot diode. R4 and D5 provide a indication of output voltage.

 

Hope this helps

 

Rob

[Suzie]

That looks better, I think it might work, but operationally it really could do with a brake to give you all the useful controls that you have on a loco.

 

VR2 the inertia control sets the weight of the train, so should only need adjusting when coupling up or uncoupling when the train weight changes, VR1 is the throttle. You just need to add a VR3 in series with a fixed resistor across the capacitor to discharge the capacitor faster when you want to slow down. Select the fixed value resistor for the maximum braking you want, and select VR3 to give a useful range of braking. Ideally get a potentiometer with a switch so that braking can be switched off for normal running and the brake is only invoked when you want to slow down by turning it.

[RobertW]

I have updated the diagram to put in all the changes that you have suggested. Could you please just check it to make sure that I have included all the suggested changes.

 

Thank you for all your help

 

Rob

[Suzie]

OK now I think. A switch (ideally integrated into VR3) will be useful to switch off the brake, at the moment minimum brake and maximum inertia will only allow half speed (which might not be a problem).

[jamesrj]

Robertw

             I'am a bit late in reading this topic, but a word of warning regarding your diagram   .we do not put fuses (if I read your dia correctly) in the neutral of  the supply to the transformer this is bad electrical practice and was phased out many years ago ,

 

             If the neutral fuse were to rupture you would have mains voltage on the transformer terminals,,,,

james

[RobertW]

Sorry James

 

Should have picked up on that sooner. It stemmed from me misreading some advice on the internet which said to fuse both the primary and secondary and I mistook it for Live and Neutral. Have updated the diagram and am reposting it. If I can I will remove all the other circuit diagrams. I haven't built this circuit and am glad of your help in this.

 

Thankyou

 

 

Robert

Inertia Controller Twin Button.pdf

[jamesrj]

RobertW

 

              Thank you for updating your circuit  Sorry to you and Mod6 that I went the wrong way by opening another post but didnt

realise that was what the report was for..

 

 

james