For Sale: Heatsinks for Mk2 PV Router

When building a PV Router, one of the most awkward items to find (and probably the most expensive) is the heatsink.  The one from Discount Devices on Rich's parts list for my Mk2 Router is now £14.98.  Despite this high price, its thermal performance is unknown (but you do get a free craft knife, or similar!)

During periods of good generation, the heatsink becomes a vital part of the system.  When a 3kW immersion is fully on, the triac needs to dissipate around 11 Watts.  The effectiveness of a heatsink is given by its thermal resistance, in units of the temperature rise per Watt. To minimise the rise in temperature, its thermal resistance value needs to be as low as possible.

So how much temperature rise can be tolerated?  When passing a continuous current of 13 Amps (3 kW), the case temperature of the BTA41 triac must never exceed 115 degC.  With an ambient temperature of, say, 40 degC, that sets a thermal resistance value of no greater than 7.5 degC/Watt.  In practice, to allow for various de-rating factors, we should be looking for a value somewhat lower than this.

A more practical limitation for the max temperature of the heatsink is that we wouldn't want anyone to be branded if they happen to lean on it.  Section 422.3.2 of the 17th Edition Wiring Regs specifies that "Measures shall be taken to prevent an enclosure of electrical equipment such as a heater or resistor from exceeding ... 90 degC under normal conditions".   For the same conditions as above (40 deg C ambient, and 3 kW load), the max thermal resistance value has been reduced to 4.5 degC/Watt (i.e. 90-40 degC / 11 Watts).

ABL Components do an excellent range of heatsinks and also provide a handy calculator to show what the likely performance of their various profiles would be.  Their 146AB profile has a particularly good performance and looks to be well suited to the physical dimensions of a PV Router box.  I therefore ordered a number of 110 mm lengths of this profile.  According to their calculator, the thermal resistance for this profile and length is just under 1.5 degC/Watt with passive cooling.  Impressive.

These items arrived yesterday, and have been very nicely fabricated with a black anodised finish.  The vertical fins are very fine which no doubt accounts for the excellent performance.   As I have many more of these items than I am likely to need for a good while, I am happy to sell them on to anyone who may be interested. 

To mount these items of the side of an enclosure, I have drilled two 3mm holes between the central vanes, 15 mm from either end.  A third hole, also on the central line but 10 mm from the mid-point allows the triac to be mounted with its body at the central point of the heatsink. 

By rebating the central hole slightly, a plastic mounting bush can be used along with an insulated mica or silicone washer/gasket.   For securing the heatsink to the box, and also the triac to the heatsink, I'm using 20 mm stainless M3 pan-head screws with their heads filed so they will fit neatly down between the central vanes and won't not turn.

I'm happy to sell these items either as supplied by ABL, or pre-drilled and with a set of 3 mounting screws and locknuts to suit. Prices as below, all including 2nd class postage with Royal Mail (it's the same postal cost for either one or two):

One heatsink, plain - £8

 or with holes and fittings - £10

Two heatsinks, plain - £14

 or with holes and fittings - £18

I've attached various photos to show these items.  Any questions, please ask.  If you'd like to buy any of these, please send me a PM.  All of the heatsink's dimensions, apart from the finished length of 110 mm, can be found on the ABL website. 

FWIW, the ABS enclosure that I've mounted the first of these heatsinks on is from Schneider Electric's  "Thalassa"range.  Its dimensions, according to the catalogue, are 175 x 150 x 80.  These must be internal values; the external dimensions are more like 195 x 165 x 90.  It's a nice enclosure but rather pricey: £11.37 + VAT from a local supplier.


calypso_rae's picture

Re: For Sale: Heatsinks for Mk2 PV Router

For securing the triac to the heatsink, 20 mm screws are rather long so I've bought some 16 mm ones.  This leaves just a few threads sticking out beyond the lock-nut after fitting the triac with a mica washer & insulating bush (photo).

The outer pair of screws need to pass through the wall of the enclosure, so their optimal length will be different for each project.  Some enclosures have serrated inner faces for securing vertical circuit boards, so their walls can be quite thick.

The "complete" version of my heatsink includes three M3 screws and ny-loc nuts.  Unless otherwise requested, I will supply one screw at 16mm and 2 screws at 20 mm, all with their heads filed to fit between the heatsink's fins. 

If you would prefer any different lengths, please let me know and I'll get some in at no extra charge.  Many different lengths of this M3 screw are available.  Being stainless, they are easy to cut to length with a small hacksaw and there's no plating to damage.

I normally ship next day, but please allow an extra few days if requesting a different length of screw that has to be ordered specially.


calypso_rae's picture

Re: For Sale: Heatsinks for Mk2 PV Router

To date, I've soldered wires directly to my triacs which is a time-consuming process.  With five wires extending in line with the pins, I've always routed them parallel to the rear face of the enclosure.  For a wall-mounted box, this means they are in-line with the heatsink's fins. 

If using a PCB, it may be better for the triac to be mounted in "transverse" fashion, as per Rich's photo at the start of the online article.  To keep the source of heat close to the centre of the heatsink, the mounting hole for the triac needs to be moved slightly.

If you would like this arrangement (as per attached photos), please let me know.  Otherwise, I'll continue to supply drilled heatsinks with all of the holes in line for "longitudinal" use.  

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