Modified LM324 Based 150W 72V 10A Electronic Load [COMPLETED]

[enut11]

I have just completed an Electronic Load project based on a Chinese 150W/10A/72V LM324 four MOSFET kit:

https://www.ebay.com.au/itm/150W-Constant-Current-Electronic-Load-Discharge-Capacity-Tester-DIY-Kit/173078658449?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648

A 3.7v LiPo isolates and drives the LED panel meter. An LM317 is used as a manual fan speed control. Jiffy box measures 155 x 95 x 55mm.

I downgraded the wattage to <75 watts because of the CPU heatsink and to 33v max and 3A max based on the LED panel meter ratings.

The unit works well and is able to hold the load steady with <0.1% variation for currents over 100mA.

I now want to build the same kit with a wider range of constant current capability, up to 10A and  down to <10mA, perhaps down to 1mA.

The basic design uses an LM324 quad Op Amp to control four To220 MOSFETs through 0.22 ohm sensing resistors. The 4 current paths are paralleled at the input.

The new design is to have several input sockets and use different current sensing resistors for each range. I will aim for up to 100W max load for now.

The first MOSFET would have a 5 ohm current sensing resistor and control currents up to 100mA
The second MOSFET would have a 0.5 ohm current sensing resistor and control currents up to 1A
The 3rd and 4th MOSFETs would have 0.1 ohm current sensing resistors each, with current paths paralleled, and control up to 10A total (5A ea).

I expect to replace the 3rd and 4th To220 MOSFETs with higher rated units capable of handling about 50w continuous load each. Suggested devices?

Any fundamental reason why this approach would not work?

Is there a better Op Amp than the LM324 for this task?
enut11

EDIT: Some of the functionality above has been incorporated into the original CC Load as as result of my experiments.
Take #2 CC Load, which is discussed later in this thread (from Reply #8), will be aimed at the original advertised specs of 150W and 10A but limited to around 30v max.

[Kleinstein]

The electronic load might need an RC snubber across the load to avoid oscillation with a difficult (inductive) source. Something like 10 Ohms in series with 1 µF (maybe 10 µF as the LM324 is slow) should be OK.
It might be a good idea to have fuse in series, just in case, as MOSFETs tend to fail short if they fail.

A possible more powerful MOSFET would be an IRFP250( IRFP240.  Depending on the manufacturer (e.g. Fairchild) is might even be rated for linear use. It's kind of a compromise between low cost and suitability for high power loss - not guarantied to work, but usually will do. This types were used a lot in the 1980s when MOSFET based audio amps were popular.

[enut11]

Thanks @Kleinstein.
Would I need to add the RC snubber across all three loads, ie the 10A, 1A and 100mA inputs? If so, would the values be the same for R and C?

Is there an easy way to test the linearity of a MOSFET?

Just checked eBay and IRFP240 MOSFETs are available from $1.45 to over $10 each. Is it just pot luck or is there a way to pick the genuine ones?
enut11

[Kleinstein]

Ebay is about the second best source for fake transistors. So normally I would buy those at a special electronics company.
I Europe I found  1.20 EUR  (around $1.50) from a reasonably good source and EUR 2,20 from an official distributor. So I would consider the $10 some with high end audio voodoo, maybe a set of 4.

The problem with MOSFETs in this kind of linear application is that the forward bias safe operation area is not really good with many MOSFETs, especially modern ones made for fast switching. Very few and expensive MOSFETs are tested for a use like in a electronic load, but old types for relatively high voltage often work well. So with the IRFP240 it is not only the normal genuine vs fake but it can also be old original style and modern replacements that might be genuine from some manufacturers, but could fail the SOA testing.

Testing the MOSFETs can be kind of destructive: use it in load circuit and check that is does not brake down at something like 100 V and around 1 A. A high voltage is the more difficult case for a given power level.
Depending on the extra protection (e.g. classic fuse or extra electronic current limit), the MOSFET might literally blow up, fuse to a short or survive and work again after cool down. SOA testing is kind of tricky, especially if it needs to be non destructive (for the failing ones).

[enut11]

Thanks to @Kleinstein
1) I have now added a 5A fuse in series with the positive input. The fuse was inserted at the unused A+/A- terminals.
2) I added a snubber 10ohm/2uF across the input terminals to counteract possible oscillation with inductive leads.

Below are some more pix of the internals showing how everything fits in the small jiffy box. One of the pics shows fan speed control on one side and a 3.7v LiPo for the panel meter on the other side. At 15mA, the LiPo will power the panel meter for about 6 days. The small red plug out the back allows charging of the LiPo.
enut11

[enut11]

Details of where I connected the 3A/33v LED panel meter. There are 4 digits for the volts and amps. I have found these to be very accurate and the amps reads to 0.1mA when the load is under 1A.

I used a single cell LiPo (or 18650 cell) to power the meter. It turns out that the meter LEDs start to dim around 3.5v which is a reminder for me to charge the internal LiPo battery.

During long term tests, the panel meter can be switched off to save battery power.
enut11

[enut11]

To limit the maximum current to 3A I added a 39K resistor in series with R22 at the top of the 4.7K 10-turn pot

The 10-turn pot that sets the load current is good for fine adjustments but not very helpful in letting you know where the setting is within the range of 0-3A, especially at switch on.

I found it handy to calibrate the amps load for each turn of pot rotation. This now forms part of the specs label on top of the case.
enut11

[enut11]

Well, I found the limits of the ST P75NF75 TO220 MOSFET  . Shorted one while testing the CC load to 30v/3A. It appears that these devices are not very robust in linear mode so I would now down-rate this circuit to 50W and 25v/2A max. The heatsink did not even get warm so failure was rapid. I will try some better MOSFETs when the second kit arrives.

I also experimented with isolating one of the MOSFETs and using a 2.2ohm Source resistor in place of the 0.22ohm. Full range on the pot is a handy 0-100mA and will now become a permanent part of this project. A SPDT switch at the input is used to select either 100mA or 2.5A full range.

BTW, the MOSFET tab for the 100mA range has to be electrically isolated from the heatsink for this to work. The photo shows the replacement green 2.2ohm resistor where the 0.22ohm was. The red wire is soldered to the Drain pin which has been isolated from the PCB.
enut11

[enut11]

Take #2 for Electronic Load

After a few hiccups with the first kit, including blowing up one of the MOSFETs, it is now working well, albeit at reduced wattage and voltage, limited by the supplied 75NF75 devices.

A second kit is underway. Photo shows partially populated PCB. I have added a 7812 voltage regulator at the input stage in place of the 4 rectifiers. Power will come from a 15v plugpack.

Missing are the MOSFETs and Source resistors. I ordered some TO247 MOSFETs and will replace the kit 0.22ohm with 5W types (maybe 10W with parallel 0.47ohm 5W). The PCB has space for the larger transistors and Source resistors.

What I have learned so far is that using MOSFETs as linear controls is fraught with danger. It is easy to to be taken in by the numbers on the spec sheets, but these were intended for switching circuits. So it seems, for DC operation, the spec sheets may be of limited use. Practical advice seems to point to significant underrating of device wattage and current and to provide very effective cooling. I could use a very expensive but robust device or a number of lesser devices in parallel. I will also look at thick copper sheet as an initial heat spreader. This would be bolted to a larger fan-cooled heatsink.

As @Kleinstein pointed out, vintage high voltage MOSFETs are more suited to DC Loads but they are not easy to source.

So, I will now experiment further with four IRFP150N TO247 devices in place of the smaller 75NF75 TO220 devices.
enut11

[enut11]

I found this document from  Infineon to be easy to read and very useful. It is all about using MOSFETs in linear applications such as DC Electronic Loads. They explain the Safe Operating Area limits and how to choose the right MOSFET.
enut11

[enut11]

Another article on the linear application of power MOSFETs, this one a little easier to read.
enut11

First 2 pages...

[enut11]

Last 2 pages...

[CustomEngineerer]

Kerry Wong does a good job of explaining the benefits of a linear mosfet.

[enut11]

Thanks @CustomEngineer. The video is very informative and the IXTK90N25L2 MOSFET (960W, 250v, 90A) has very impressive performance in linear mode.
Also, the thick copper pad between MOSFET and heat-sink makes a big difference to transistor case temperature.

Just checked the price here in Australia. $32.70 AUD + delivery from Element 14. Hmmm
enut11

[sorin]

Check:

FCA35N60
SPW20N60C3

You need Mosfets that have the DC. operation specified on their "Safe Operating Area".

[enut11]

Thanks @sorin. Will also look at using the FCA35N60 MOSFET.

Take #2 Electronic DC Load
Progress with the second electronic load while I wait for the TO247 MOSFETs, heat-sink and fans to arrive. Plastic box is 200Wx70Hx174D mm.

The display is a 90x50 LCD that I modified to read down to 0v. Reads volts, amps and watts simultaneously.

https://www.eevblog.com/forum/projects/smart-panel-meter-can-it-be-modified-to-read-down-to-zero-volts/

With 4 large MOSFETs, I am hoping for the full 10A and 150 watts, up to 30v max. I have soldered copper wire to the Drain and Source tracks to reduce track resistance on the PCB.

The 4 power transistors will be mounted on a 3mm thick copper heat spreader which in turn will be bolted to a large aluminium heat sink cooled by 2 fans.

The fans controller is manual and based on a cheap eBay LM317 unit. I have ordered a heat sensing fan controller to automate the process.

I decided to go with 0.1ohm 5W Source resistors in place of the 0.22ohm kit items. This should lower the minimum voltage that the CC Load will work at with high currents.

Remote voltage sensing is only available on the positive line due to the relatively simple design for this CC Load.

The LCD display will be powered by 2S 400mAh LiPo pack. At <3mA current the battery will power the display for a very, very long time. I have incorporated a display switch.
enut11

[VEGETA]

I am doing a similar project and reached nearly the same result (check last page for v0.2): https://www.eevblog.com/forum/beginners/dc-dummy-load-circuit-calibration/

The only problem left for us is that current flows in potentiometer is very very small (12 uA or so) thus will be affected by EMI or so.

This is because we choose 0.2R to be shunt for each mosfet (you have 0.22 which is the same) so we need 100mV before it to achieve 2A total output current.

I want to ask: what is the voltage at each op-amp input? also, what is the current passing through it?

We concluded that we should ground the potentiometer metal body to ground in order for this project to work nicely. So kindly give me your information and if you suffer this problem or not. This has to do with zeroing the output too.

[enut11]

Hi @VEGETA.
In the original circuit for the kit that I purchased, the maximum op amp input was 4.7k/(22k+4.7k) x 2.5v=0.44v
When this voltage is present across the 0.22ohm Source resistor, the maximum current would be 0.44/0.22=2A per MOSFET.

The easiest way for me to limit the total current is to change R22. When I increase this resistor, the total current is reduced.

The pot I am using has a plastic body so no easy way to earth it but I do not appear to have a problem with noise pickup.

I can zero the output. In fact I need about a 1/2turn on the 10turnpot before any current flows.
enut11

[VEGETA]

Hi,

but he current flowing is gonna flow through 10k + 22K + 4.7K + 1K which is gonna be so low. Meaning 2.5v through 22K + 4.7k + 1k = around 90uA if my calculations are correct. My original design was around 12uA which is pretty bad.

I changed it a big with a proposed circuit and now it is around 0.2mA.

[enut11]

The 10k stabilises the TL431 (IC1) at 2.5V. The 22k + 4.7k pot form a variable voltage divider.
There is negligible current into the op amp.
So, the op amp positive input sees 0v-0.44v at the pot extremes. The current flowing through the pot makes little difference.
The op amp then drives the MOSFET so that the inverting input sees the same voltage as set on the pot.
This voltage is generated across the 0.22ohm Source resistor.
So, if the pot is set to the middle position, for example, the MOSFET will allow 0.22v/0.22ohm=1A load.

[VEGETA]

How about if you put the mosfet on to-220 heatsink?

[enut11]

If you expect a meaningful reply you need to be more specific with your question.
enut11

[VEGETA]

If you expect a meaningful reply you need to be more specific with your question.
enut11

I meant, instead of having one big heatsink for 4 mosfets... how about putting one per mosfet? One of those famous to-220 heatsinks. Will it work?

If your device is about 100 watts that means 25 watts per mosfet, so my question will such heatsink be able to tolerate these 15-25 watts? with or without fan cooling.

[enut11]

In this project all the MOSFETs will benefit by being on the same heat-sink, provided it is adequate to dissipate the total watts generated.
If all the MOSFETs are at the same temperature they tend to share the heat load.
It is beneficial to get the heat out of the MOSFET case and into the heat-sink as quickly as possible to reduce the junction temperature. For this I am going to use a 3mm thick copper plate between the transistors and the aluminium heat-sink. Of course you need good thermal transfer between the copper and aluminium otherwise you are simply creating another heat barrier. Both surfaces need to be flat with heat transfer paste in between.
enut11

[JS]

I meant, instead of having one big heatsink for 4 mosfets... how about putting one per mosfet? One of those famous to-220 heatsinks. Will it work?

If your device is about 100 watts that means 25 watts per mosfet, so my question will such heatsink be able to tolerate these 15-25 watts? with or without fan cooling.

You need to specify which heatsink you are thinking, a pict would do.

Usually they are kind of small and good for LM137 when you need a bit more umpf out of it without them.

JS

[VEGETA]

I meant, instead of having one big heatsink for 4 mosfets... how about putting one per mosfet? One of those famous to-220 heatsinks. Will it work?

If your device is about 100 watts that means 25 watts per mosfet, so my question will such heatsink be able to tolerate these 15-25 watts? with or without fan cooling.

You need to specify which heatsink you are thinking, a pict would do.

Usually they are kind of small and good for LM137 when you need a bit more umpf out of it without them.

JS

I meant one of these:

https://www.aliexpress.com/item/5pcs-lot-38x34x12-8mm-TO220-TO-220-heatsink-heat-sink-radiator-for-IC-triode-7805/32622932747.html

and these are more quality ones (branded): https://eu.mouser.com/ProductDetail/Ohmite/FA-T220-64E?qs=sGAEpiMZZMttgyDkZ5WiumlCfl50RTwzVA%252bY4U4BtvA%3d

And in the end of the case (inside) one of these small fans: https://www.aliexpress.com/item/High-quality-3010s-30MM-30-x-30-x-10MM-12V-2Pin-DC-Cooler-Small-Cooling/32603431500.html

so 4 heatsinks close to each other and one fan sucking air from them and toss it outside.

[JS]

I meant, instead of having one big heatsink for 4 mosfets... how about putting one per mosfet? One of those famous to-220 heatsinks. Will it work?

If your device is about 100 watts that means 25 watts per mosfet, so my question will such heatsink be able to tolerate these 15-25 watts? with or without fan cooling.

You need to specify which heatsink you are thinking, a pict would do.

Usually they are kind of small and good for LM137 when you need a bit more umpf out of it without them.

JS

I meant one of these:

https://www.aliexpress.com/item/5pcs-lot-38x34x12-8mm-TO220-TO-220-heatsink-heat-sink-radiator-for-IC-triode-7805/32622932747.html

and these are more quality ones (branded): https://eu.mouser.com/ProductDetail/Ohmite/FA-T220-64E?qs=sGAEpiMZZMttgyDkZ5WiumlCfl50RTwzVA%252bY4U4BtvA%3d

And in the end of the case (inside) one of these small fans: https://www.aliexpress.com/item/High-quality-3010s-30MM-30-x-30-x-10MM-12V-2Pin-DC-Cooler-Small-Cooling/32603431500.html

so 4 heatsinks close to each other and one fan sucking air from them and toss it outside.

The first ones are definetly too small, the second ones go down to 3°C/W to air the biggest. That, plus 2°C/W from junction to heatsink, with 25W is 125°C rise over ambient, way too much for it to work. With a fan might get to more reasonable numbers.

Having them in different supplies makes no difference in theory, thermal circuit is equivalent given the 4 HtoA resistance are equivalent to the big one for the 4. Now, the one dissipating more will get warmer, but the heatsink also and more heat will be dissipating making difference smalls. The major difference might be mechanical or electrical if using the heatsink as the conductor to tie the tabs.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[VEGETA]

I tried searching for one big heatsink but couldn't find a standard one. So this project, what heatsink comes with it? or should you put a heatsink of your own?

Say I want to make such a project which will be sold as a commercial open source one... okay? now how can I source heatsink if there are no standard ones? especially a cheap one with a fan to make it affordable.

So you think one heatsink is better than 4 smaller ones? perhaps you are correct since it could dissipate all the heat better but I don't really know.

[JS]

I tried searching for one big heatsink but couldn't find a standard one. So this project, what heatsink comes with it? or should you put a heatsink of your own?

Say I want to make such a project which will be sold as a commercial open source one... okay? now how can I source heatsink if there are no standard ones? especially a cheap one with a fan to make it affordable.

So you think one heatsink is better than 4 smaller ones? perhaps you are correct since it could dissipate all the heat better but I don't really know.

Using an Al enclosure could be a solution, you might want to electrically isolate the transistors fromr it for safety, but would end in a cheaper product as you get two for one. You still need an enclosure. Then if you want to add a fan it can just circulate air from outside to the inside and the inside of the case would have the extra airflow.

The bigger ones from the second link are probably fine, you will be depending on airflow and having well distributed airflow can be tricky.

I've seen catalogs for big heat sinks, you need sub °C/W here, 3/4 would be equivalent to the 4 of the link. I can't mention one brand or a supplier as have plenty of big heatsinks from a friend who took it from broken phone transmision lines powe supplies, so I didn't come in need for them.

You would be amazed how the ones I have match the PCB I've done for the μSupply without planning for it.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

The heat-sink, fans and power MOSFETs have arrived.
I purchased ten IRFP4668 130A 200v MOSFETs on eBay with a view to selecting 4 matched ones.
I will publish how I tested and matched them shortly.
The heat-sink measures 150x69x36 also purchased on eBay. I do not have any thermal specs for it.
The fans are 70x15mm 12v units from eBay
enut11

[JS]

  Sorry I join a bit late, wouldn't be possible to have all the mosfets for the higher range with 4x0.1Ω resistors and switch a series resistor in with all of them for the lower ranges? Then you just turn one mosfet in or leave them do whatever as it doesent matter if they are unbalances. Possibly using a different output low connector so you dont have to switch. Then you just need a switch to switch in or out the second connector but it  just has to handle 1A.

  I'm building a dummy load right now, small, just one IRF540 but I added CV control up to the CC limit and thermal shutdown when needed till it cools down a bit. After it I'd probably scale up things to get more umf, but I don't usually need to deal with high power stuff so who knows when.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

Hi @JS. The first CC Load that I built (see above) had a switched range with 100mA or 2.5A available and it works well. I switched the Drain input to one of the MOSFETs using a 2.2ohm Source resistor for the lower range. Switching the Source resistor for different ranges may also work.

For this version (Take #2 Electronic Load) I am aiming for the full 150W/10A advertised for the kit.
Choice of MOSFETs is often dictated by economics. I decided that 4 'robust' switching MOSFETs would do the job. Will know soon.

Would you share how you implemented thermal shutdown?
enut11

[VEGETA]

The heat-sink, fans and power MOSFETs have arrived.
I purchased ten IRFP4668 130A 200v MOSFETs on eBay with a view to selecting 4 matched ones.
I will publish how I tested and matched them shortly.
The heat-sink measures 150x69x36 also purchased on eBay. I do not have any thermal specs for it.
The fans are 70x15mm 12v units from eBay
enut11

Can you please provide the links and prices? shipping?

Also, I would be interested to see what is the performance of heatsink alone at say 5A compared to with fan. I hope you can do that, you would benefit us.

One way you can determine heatsink temperature coefficient, it is by recording its temperature at no current (= ambient) then put a known power like 1W and measure its temperature, then repeat with 10W... finally you will get to know the coefficient.

[enut11]

Proposed MOSFETs location on heat-sink. The cardboard represents a template for a copper heat spreader sheet between MOSFETs and heat-sink. The idea comes from Kerry Wong who demonstrated a useful temperature reduction in his Youtube video (see Reply #12 above).

Setup I used to test and match the MOSFETs. Flying leads for Gate, Drain ad Source from the PCB go to screw terminals allowing quick change of MOSFET.

I monitored the MOSFET Vgs at 1mA, 10mA, 100mA, 1A and 2A Drain currents while keeping an eye on the device temperature.
enut11

[JS]

  Here is the full scheme in mine, it simulated ok, I should solder the components tonight and tomorrow pick up my new scope and test it.

  Nothing fancy, LM34 (the BJT in the schematic as I didn't had the part) comparator to a diode with some hysteresis, it should cut down at about 70ºC at the sensor and go back up at 50ºC. I could probably add a hand switched fan if I see the need, I have an old CPU heatsink to start the tests, the PCB allows direct mounting to the heatsink of the mosfet, LM34 and sense resistor if needed, 0.22Ω 2W, would be 1.4W at 2.5A I'm thinking as absolute max so it should be fine, I don't think to use this puppy up there, I'd probably use a log pot for the current so the control in the lower side is more smooth, there are a few tweaks to be made to the circuit, but the current PCB can accept external control (or set sensing) and output sensing with the available pin headers.

  The CV and CC can be switched by the pin header between the opamps and the mosfet, I think CC should be left always on, I'm running some simulations now for doing so, replacing the D2 with a resistor and eliminating R14.

  The circuit has a few tweaks to be made, I think I can do better for the step response but it already sim better than the basic config, as the voltage control makes it smoother but much slower, I'd like faster response with less overshoot (don't we all). I did tried the snubber on the output, step was much better seen from the dummy load but not from the DUT side, it could be wired externally and switched in and out.

  Switching mosfets are optimized for a different task, be aware, they are usually good for high short peaks of power in them but not optimized for long sustained power applications. Have an eye on them and tell us how it went.

JS

[enut11]

The heat-sink, fans and power MOSFETs have arrived.
I purchased ten IRFP4668 130A 200v MOSFETs on eBay with a view to selecting 4 matched ones.
I will publish how I tested and matched them shortly.
The heat-sink measures 150x69x36 also purchased on eBay. I do not have any thermal specs for it.
The fans are 70x15mm 12v units from eBay
enut11

Can you please provide the links and prices? shipping?

Also, I would be interested to see what is the performance of heatsink alone at say 5A compared to with fan. I hope you can do that, you would benefit us.

One way you can determine heatsink temperature coefficient, it is by recording its temperature at no current (= ambient) then put a known power like 1W and measure its temperature, then repeat with 10W... finally you will get to know the coefficient.

MOSFETs here:
https://www.ebay.com.au/itm/IRFP4668PBF-TRANSISTOR-MOSFET-N-CH-200V-130A-TO-247AC/123131585110?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Heat-sink here:
https://www.ebay.com.au/itm/Aluminum-Heat-Radiator-Heatsink-Cooling-Fin-150x69x37mm-Silver-Tone-F6/282280422071?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Fans here:
https://www.ebay.com.au/itm/Hot-Quiet-7cm-70mm-70x70x15mm-12V-Computer-PC-CPU-Silent-Cooling-Case-Fan/112935110852?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Fan covers:
https://www.ebay.com.au/itm/70mm-Iron-Net-Fan-Cover-CPU-Fan-Grill-Computer-Cooling-Fan-70x70mm-For-AC-DC/282789089373?ssPageName=STRK%3AMEBIDX%3AIT&var=582088181424&_trksid=p2057872.m2749.l2649

Heat-sink needs drilling and tapping for MOSFET bolts.
 
"Also, I would be interested to see what is the performance of heatsink alone at say 5A compared to with fan"
This is not enough info. A heat-sink is an energy dissipator. Tell me how many 'watts' you need tested and I will see if it is possible.
enut11

[enut11]

Hi @JS
Interesting circuit. Have you Posted it separately on the Forum?  Should raise some useful discussion.

Temp control on mine is manual. Awaiting for these controllers to arrive for testing.
https://www.ebay.com.au/itm/Temperature-Speed-Controler-DC-12V-Denoised-Speed-Controller-for-PC-Fan-Alarm/132595582255?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have yet to think about step response. That will come later.

I like the ability to switch CV/CC.
enut11

[VEGETA]

Try to test it at say 30v/5A = 150 Watts. Or make a spreadsheet with 1W, 5W, 10W, 50W, 100W, 150W, and so on. Record the mosfet's temperature and heatsink temperature. This could be a nice experiment as well as a reference for such heatsinks since they don't have actual data. You could build the accurate data yourself by recording it, then deduce the temperature coefficient from it. Next, we can all use that coefficient with such heatsinks as an approximate reference value.

I found some nice heatsinks as well as cheap like 5$ delivered! this is one example:

https://www.ebay.com/itm/Large-Big-Aluminum-Heatsink-Heat-sink-radiator-for-Led-High-Power-Amplifier-CN3/253326048354?epid=1974571145&hash=item3afb68bc62:g:xIcAAOSwiHpaPdH9

it would be so nice if these can operate properly at 150 watts with or without a fan.

Your experiment can provide reliable data about when do we need a fan too.

[JS]

Hi @JS
Interesting circuit. Have you Posted it separately on the Forum?  Should raise some useful discussion.

Temp control on mine is manual. Awaiting for these controllers to arrive for testing.
https://www.ebay.com.au/itm/Temperature-Speed-Controler-DC-12V-Denoised-Speed-Controller-for-PC-Fan-Alarm/132595582255?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have yet to think about step response. That will come later.

I like the ability to switch CV/CC.
enut11

  I haven't yet, I have this and a µSupply on the bench right now, the µSupply is already populated, only missing the LT3080, a 7805 for supplying the µC and a few caps I should get tomorrow. The dummy only has 3 unsoldered resistors but I plan to start soldering right now.
  I want to build them and be able to give some real world data on the circuits before posting, I guess two posts are on the go right now, maybe a single one, I have a few projects as I'm rigging up the lab and maybe would be good to have them in one place. I built a mΩmeter (measures consistently down to a single mΩ and has 2 digits to spare) and planning a differential probe a bit different to the projects I've seen out there. The idea for this would for measuring small signals floating highish (measure components floating inside circuits and things like that) not too much bandwidth, say audio and bit else, lowish noise and easy to get components.

JS

PS: 1h later PCB fully populated, well, it's missing one cap (voltage control compensation) would go in tomorrow.

[JS]

  That value looks useful for transient tolerance but not much for a DC dummy load as it's going to be used in a steady state a lot. No doubt a higher number will translate in higher reliability, tolerance to transient, but junction-heatsink thermal resistance is also quite important. Both will go up together but power switching transistors might very well have higher P2t relative to the thermal resistance. Big transient response should be managed as it not only can be bad for the transistors but for the DUT as well and we don't want to harm the DUT.

Where do you get the values from?

JS

[enut11]

Using the setup from Reply #33 I tested the 10 IRFP4668 MOSFETs for Vgs at various Drain currents at Vds = 5v. Based on this data I have selected #3, #4, #8 and #9 for my CC Load.
enut11

[JS]

Hey, here is my tiny one! Builded, tested* and posted 

I do like the constant voltage mode to be able to test constant current sources...

https://www.eevblog.com/forum/projects/cccv-dummy-load/

JS

*barely tested, just low power under a 9V battery.

[enut11]

Having set aside 4 reasonably matched MOSFETs from the 10 IRFP4668 purchased it was time to see how far I could push one of the remaining ones.

Aim for this CC Load project is 150W/10A/30v with 4 power devices, ie 37.5w/2.5A per transistor if they all share the current load evenly.

1) My power supply is good for 28v/1A/28W so I tested with that first using the setup in Reply #33. MOSFET case temperature taken at the transistor notch was around 30C (ambient 18C).

2) Next I tested using an IBM Server 12v/69A PS. At ~12v/4.4A (CC Load pot limit) the MOSFET was now creating ~50w heat and the case temperature was around 49C (ambient 19C).

So, with one device able to handle up to 50W this project looks to be on track for 150W total with 4 devices.

In these tests the MOSFET was bolted to a fan cooled Al heat-sink using thermal paste. Even with direct mounting, ie no insulating washer, I noticed that the heat-sink was much cooler than the transistor in all tests. This tells me that the real challenge will be the rate of heat transfer through the transistor case-to-heatsink junction.

Next step is the heat-sink and this will be the biggest effort with lots of drilling and tapping.

[JS]

Yes, case-to-heatsink thermal resistance is a bitch, with 4 transistors they are kind of in parallel, so you expect to see the same thermal difference in that juncture if using the same mounting technique. Anodized heatsinks from a respectable vendor might have enough anadization to be considered enough insulation, if you need them to be isolated.

Other improvement that could be made is to use a better thermal paste, any decent metal based PC grade is an improvement over regular silicon based ones.

As mentioned cooper plates might be helpful, as it's softer than aluminum might get better contact for the same pressure and then you have a greater area from the cooper to the aluminum and the cooper softness helping here again.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[VEGETA]

How about this panel meter that you use?

I read that it doesn't go to lower voltages, is that correct? can it be mounted high-side and low-side?

[sorin]

According to SOA on the datasheet IRFP4668 can handle:

500W @  5V (not a realistic value, limited by Thermal Resistance)
30W   @10V
10W   @20V

So I don't think that is suitable for your application.

[VEGETA]

According to SOA on the datasheet IRFP4668 can handle:

500W @  5V (not a realistic value, limited by Thermal Resistance)
30W   @10V
10W   @20V

So I don't think that is suitable for your application.

Where did you get such info?

I used IRILZ44N for my design for 30v/2A specs, it could go much more. It is in an isolated package which is a plus.

[sorin]

IRILZ44N don't exist maybe you mean IRFZ44N or IRLIZ44N? 

IRFZ44N is not specified for DC Load, just for pulse Load.

Its not about how large the transistor is but who the transistor is constructed internally.
High current, low voltage transistors usually are composed from many (10-100 or 1000) small current transistors inside the die.
If you use this transistor in linear mode then the current is not shared equally through them.
On the forum are several discussions explaining this topic.

Where did you get such info?

On the IRFP4668PbF Datasheet Fig 8, page 4.

I used IRILZ44N for my design for 30v/2A specs, it could go much more. It is in an isolated package which is a plus.

And what about the junction temperature???
Have you measured that?
Isolation is not a good thing on out application because  it have a large thermal resistance Junction-to-Case 3.3 °C/W

Its all about long term reliability...

[enut11]

Yes, case-to-heatsink thermal resistance is a bitch, with 4 transistors they are kind of in parallel, so you expect to see the same thermal difference in that juncture if using the same mounting technique. Anodized heatsinks from a respectable vendor might have enough anadization to be considered enough insulation, if you need them to be isolated.

Other improvement that could be made is to use a better thermal paste, any decent metal based PC grade is an improvement over regular silicon based ones.

As mentioned cooper plates might be helpful, as it's softer than aluminum might get better contact for the same pressure and then you have a greater area from the cooper to the aluminum and the cooper softness helping here again.

JS

Enviado desde mi LG-M250 mediante Tapatalk

As for CC Load #1 above, I have decided to mount the MOSFETs directly to the heat-sink, ie no mica washers. This means that the heat-sink will be 'live' but we are only dealing with low voltages <30v. I will be using thermal paste from Jaycar (Australia) intended for CPUs.

I have finished drilling and tapping the heat-sink and will post pix soon.
enut11

[enut11]

How about this panel meter that you use?

I read that it doesn't go to lower voltages, is that correct? can it be mounted high-side and low-side?

@VEGETA
See this posting

https://www.eevblog.com/forum/projects/smart-panel-meter-can-it-be-modified-to-read-down-to-zero-volts/

for info on how to modify it. It can only be mounted low-side due to the common ground.
enut11

[JS]

Yes, case-to-heatsink thermal resistance is a bitch, with 4 transistors they are kind of in parallel, so you expect to see the same thermal difference in that juncture if using the same mounting technique. Anodized heatsinks from a respectable vendor might have enough anadization to be considered enough insulation, if you need them to be isolated.

Other improvement that could be made is to use a better thermal paste, any decent metal based PC grade is an improvement over regular silicon based ones.

As mentioned cooper plates might be helpful, as it's softer than aluminum might get better contact for the same pressure and then you have a greater area from the cooper to the aluminum and the cooper softness helping here again.

JS

Enviado desde mi LG-M250 mediante Tapatalk

As for CC Load #1 above, I have decided to mount the MOSFETs directly to the heat-sink, ie no mica washers. This means that the heat-sink will be 'live' but we are only dealing with low voltages <30v. I will be using thermal paste from Jaycar (Australia) intended for CPUs.

I have finished drilling and tapping the heat-sink and will post pix soon.
enut11

You can do that, even with riskier stuff as long that's hidden well within the case and marked.
If you intend to leave it outside, I wouldn't recommend it, as it might very well end shorting with simmering in your desk.

Regards!

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

According to SOA on the datasheet IRFP4668 can handle:

500W @  5V (not a realistic value, limited by Thermal Resistance)
30W   @10V
10W   @20V

So I don't think that is suitable for your application.

Hmm, interesting data. I ran one again tonight at ~50 watts (4.4A at 12v) for about 10 min. This time I used in IR thermometer and measure 60C on top of the MOSFET case. Junction temperature will be higher but no way to measure it. Heat sink was about 10C cooler. Ambient here is 18C.

Four MOSFETs pumping out 150W into the new heatsink should be interesting!
enut11

[enut11]

Yes, case-to-heatsink thermal resistance is a bitch, with 4 transistors they are kind of in parallel, so you expect to see the same thermal difference in that juncture if using the same mounting technique. Anodized heatsinks from a respectable vendor might have enough anadization to be considered enough insulation, if you need them to be isolated.

Other improvement that could be made is to use a better thermal paste, any decent metal based PC grade is an improvement over regular silicon based ones.

As mentioned cooper plates might be helpful, as it's softer than aluminum might get better contact for the same pressure and then you have a greater area from the cooper to the aluminum and the cooper softness helping here again.

JS

Enviado desde mi LG-M250 mediante Tapatalk

As for CC Load #1 above, I have decided to mount the MOSFETs directly to the heat-sink, ie no mica washers. This means that the heat-sink will be 'live' but we are only dealing with low voltages <30v. I will be using thermal paste from Jaycar (Australia) intended for CPUs.

I have finished drilling and tapping the heat-sink and will post pix soon.
enut11

You can do that, even with riskier stuff as long that's hidden well within the case and marked.
If you intend to leave it outside, I wouldn't recommend it, as it might very well end shorting with simmering in your desk.

Regards!

Enviado desde mi LG-M250 mediante Tapatalk

No room inside. The heat-sink and fans will be outside the box with a big warning sticker

[JS]

According to SOA on the datasheet IRFP4668 can handle:

500W @  5V (not a realistic value, limited by Thermal Resistance)
30W   @10V
10W   @20V

So I don't think that is suitable for your application.

Hmm, interesting data. I ran one again tonight at ~50 watts (4.4A at 12v) for about 10 min. This time I used in IR thermometer and measure 60C on top of the case. Junction temperature will be higher but no way to measure it. Heat sink was about 10C cooler. Ambient here is 18C.

Four MOSFETs pumping out 150W into the new heatsink should be interesting!
enut11

You can estimate the junction temperature from the case temp, power and junction to case thermal resistance, usually to the back of the case, where the heatsink goes. Often a hole is drilled to measure there with a thermocouple.
50W
60°C at the case
50°C at the heatsink (0.2 °C/W case to heatsink thermal resistance)
20°C ambient (0.6 °C/W heatsink to ambient thermal resistance)
 Look datasheet to find juncture temps.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[JS]

No room inside. The heat-sink and fans will be outside the box with a big warning sticker

[emoji85]

The thing is that it isn't just at 30V, it is at whatever your DUT is at, and that can be much more.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

True @JS. That is why I have programmed the LED display to flash on and off at over 33v to remind me that is the limit for this CC Load.
If the heat-sink is up to the task of dissipating 150W and the transistors are at a reasonable temperature I could experiment with mica washers at a later date.
enut11

[enut11]

Trial fitting of the 3mm copper plate and power MOSFETs to the aluminium heat-sink for the CC Load. Hoping to sink 150W here . Heat-sink measures 150x69x36 and will have twin fans.

Note to myself: 'check component clearances before drilling holes!' The left MOSFET Gate and right MOSFET Source are very close to the bolt heads. There is just enough clearance to use heat-shrink on the MOSFETs and bolts.
enut11

[JS]

Are you sure that plate is making good contact with the heatsink, I'd use a few screws more! [emoji3]

Mechanicals are a bitch, they are my project stoppers ans I deal with that sort of thing at work every day, just heavier stuff. When I'm with electronics I want to be soldering, not tapping holes!

JS

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

@JS. The reason there are not more screws is that I ran out of them! Agreed, the mechanicals, not the electrics, form the bulk of a project. That is why some of my projects end up being used without a box.

Pictures showing CC Load twin fans mounted and the heat-sink screwed to the box using L-brackets. Next is the final wiring and testing.
enut11

[JS]

That's nice dude!

For the insulation, you could do it between the cooper and aluminum, such surface has very little thermal resistance, insulating it you will be adding much lower resistance than doing so in the transistor cases. You need a big piece of insulator, but maybe a thin sheet of maylar would do, I don't know the thermal characteristics of it, but a few micron would do for electrical insulation, can take the heat no problem and it's cheap.

JS

Enviado desde mi LG-M250 mediante Tapatalk

[enut11]

Wiring all done. Added labels to front panel. Decided to leave some open space at the back to let the MOSFETs breath. I added a heat shield to isolate the PCB, etc from the heat generating components. It is important to keep the TL431 voltage reference as stable as possible.

Had time for a quick test to see that it works, and it does. Final testing up to 150W soon
enut11

[JS]

Nicely done! good idea about the heat shield, it looks siny, I hope is not metal, but... you forgot the resistors inside! They are working pretty cool but still 2.5W will warm a bit.

You kind of cheated with the front panel display I guess 

JS

[enut11]

Thanks @JS. Yes, the Source resistors will create some heat at high amps. I will run a long term test sometime and measure the box internal temperature. I will monitor the 2.5v Ref and if it proves to be a problem I could relocate the heat shield further towards the front. Btw, the shield is hard cardboard.
enut11

[enut11]

150W-10A-33V Electronic Load test results with open case.

Blue temp sensor monitors MOSFET (at the notch) and was in contact with the metal tab.
Green temp sensor monitors heat-sink copper.
Black temp sensor monitors 0.1ohm Source resistor.

I also used an IR Thermometer on the MOSFETs and it proved to be more reliable. Even so, it too may have given a slightly low reading given the small surface area of a MOSFET.

Temperatures were taken for currents up to 10A/118W. Max MOSFET temperature was less than 50C. The copper heat spreader was 5C lower. I did not measure the Al heat-sink.

Temperature difference between the 4 MOSFETs was less than 2C. This seems reasonable to me. I took the trouble to match the MOSFETs for Vgs vs Load and also matched the 4 Source resistors within 1%.

I also took the load up to 150W/13A for about 10sec but did not take any temperatures.

@JS, thanks for your tip on Source resistor heat. The results show a maximum of 35C at 10A. I will now look at shielding the TL431 Ref from this potential heat source.

Thanks to all who contributed to this project. Collective knowledge helps to get to the best solution faster.

That's about it for this project. I will test temps with a closed case and report any significant results.
enut11

[JS]

30ºC rise at 118W, math says 0.25ºC/W from case to ambient, doesn't look so real to me, 0.21ºC/W from heatsink to ambient. That's at maximum load, with lower loads even higher.

This means 0.04ºC/W from mosfet to heatsink, or for each one 0.16ºC/W.

Datasheet for your mosfets says 0.24ºC/W from case to heatsink, flat surface grease, did you use insulation at the end? I will guess not for the next calcs.
https://www.infineon.com/dgdl/irfp4668pbf.pdf?fileId=5546d462533600a40153562c8528201d

You did said the case probe was touching the heatsink, so I'll say it's half way and you acctually has 0.24ºC/W the datasheet suggests from case to heatsink. This means the case is actually at 54ºC, just add 2ºC to the cases, we are still at a safe 50ºC. Now, from case to junction, that's 58ºC at the junction, or 41ºC raise at 118W, 0.35ºC/W J-A. For your rated 150W you could say you'd only have 52 ºC temp rise, at a more confty room for my girl (I'm fine at 17º) that's 75ºC at the junction. This things are rated for 175ºC at the junction, running them at 100ºC at J is safe enough for me, you should cover the 150W on your sticker with a 220W even if your measurements weren't soooo reliable  you still get a good margin for junctions to get to 175ºC.

How long did you run the tests? this is a big thermal mass you need to heat up before taking reliable measurements. You could even find the thermal capacity of your heatsink in the process, if you measure the heatsink temp over time you should see an exponential grow in temps, and is just a RC circuit. Even if you don't let it settle to the final temp you could get a better approx on your final values. Few samples, set 100W to do the math easy, read 15s for the first 3 intervals, then 30s intervals, take maybe 10 samples, quit early if you see the temps are converging. Taking the heatsink temp as the cooper plate seems fine, then you just take from there to ambient as a single number (settling time would be a bit longer considering the full scheme)

I can't wait for my 121GW to do the work for me, I just would need to check my phone that temps aren't going outside safe ranges and come back to process data with the PC  so comeonnnnn Dave, ship fast!!!

JS

[enut11]

@JS, to reduce thermal resistance I did not use any insulation between MOSFETs and copper or between copper and heat-sink. For the time being I can live with a live heatsink.

I will have to think about your thermal analysis. I take it that you are satisfied the 150W label is warranted?

I am now happy with the performance of this CC Load. When I purchased the original kit I thought that it was going to be an easy project! In the end I am glad it was a challenge. I got to learn about the behaviour of MOSFETs, a device designed for rapid switching, and how to force them to behave in a linear fashion. Also, the headline numbers on the spec sheet, Rds, Ids max and Vds max are relatively useless in linear mode. For DC operation I found you have to rely almost entirely on the SOA diagram and even downrate that! An interesting journey.
enut11

[JS]

@JS, to reduce thermal resistance I did not use any insulation between MOSFETs and copper or between copper and heat-sink. For the time being I can live with a live heatsink.

I will have to think about your thermal analysis. I take it that you are satisfied the 150W label is warranted?

I am now happy with the performance of this CC Load. When I purchased the original kit I thought that it was going to be an easy project! In the end I am glad it was a challenge. I got to learn about the behaviour of MOSFETs, a device designed for rapid switching, and how to force them to behave in a linear fashion. Also, the headline numbers on the spec sheet, Rds, Ids max and Vds max are relatively useless in linear mode. For DC operation I found you have to rely almost entirely on the SOA diagram and even downrate that! An interesting journey.
enut11

Yes, seems fine for me, I think you do have some margin to insulate the heatsink if you wanted to do so. At least for the thermal analysis.

I only watched the thermal specs on the datasheet, I guess you are well within SOA for VA rates.

JS

[enut11]

To answer your question on how long I let the CC Load run before recording temps, it was generally  several minutes, until the temp change was insignificant. I started with low amps and finished on the highest setting.
enut11

[JS]

In that case, anf if you are limited by SOA, you can ease up on thermals and add the insulators.

I'm not saying you are going yo die because of the live heatsink, but a falling screwdriver can ruin your day... I'd also add a grill around so nothin can fall between the transistors. It seems pretty open space between project case and heatsink.

Be aware, cases always stall my projects, I think your solution is great! Just some comments to be on the reliability side.

JS

[enut11]

Thanks JS. Another solution would be an isolated guard over the heat sink. I do believe that direct mounting the MOSFETs (ie no insulating washers) keeps them cooler at max amps and that can only be good for component life.

In one test I forgot to switch on the fan and the heat-sink got hot very quickly. However, temps came down fast with a bit of air moving through the fins so another reason to be happy with the thermal performance of this heat-sink.

In the final setup I found that the pot allowed the amps to be set well over the 10A limit that I needed. I inserted a 15K resistor after R22 and in series with the pot. I have produced a label that now sits on top of the plastic enclosure and it shows the expected amps for each rotation of the 10-turn-pot.
enut11

[enut11]

A final reply (I think  ) on this project. After several more tests I am happy to report that this 'Take #2' DC Electronic Load does work reliably all the way up to 10A and 150 Watts max. I have limited my version to 33v (by programming this into the panel meter) but the MOSFETs are rated to 200v so it would probably work up to 100v.

Bear in mind that my heat-sink is 'live' to improve thermal performance, hence my self imposed voltage limit. Also, whether the copper heat-spreader I used was necessary, I really cannot say except that it works and I am happy with that.

Thanks again for your suggestions and constructive feedback Forum members.

Here is a pic of the CC Load display taken during testing of a 32v/5A (160W) Gophert CPS-3205 power supply.

[tekguy]

Can you not do a quick summary of the parts that differ from the original load?,ie resistors changed and fets etc with the values?.

[enut11]

Hi @tekguy. Built that 4 years ago! A very handy instrument.

LM324 150W 10A Constant Current Electronic Load
Changes from standard kit:
x4 FETS changed to higher rated IRFP4468 130A 200v (preferably matched - see Reply #33. Other FETs are suggested by Forum members in the thread.
x4 FET Source resistors from 0.22R to 0.10R (5watt).
Added 15A automotive fuse and holder to input.
Added V/A digital panel meter with its own 7.4v 2S Li supply. Reply #15.
Added LM317 manual fan speed controller. A better design would be a heat sensing circuit to automatically switch on fan at a particular temperature.
Added 8.2R + 10uF snubber across input to suppress unwanted oscillation.
Added 7812 voltage regulator to stabilise TL431 and IC supply.
Added SPDT switch for local/remote input voltage sensing.
Added an additional 15K resistor in series with the helipot. The actual value depends on the final maximum current required.

Most of the parts are available on eBay and easily searched. Direct references are not given as they go out of date quickly as sellers change their listings.
Listing in Reply #35 may or may not work.
enut11

PS
Parts/kits seem to have changed since I built my electronic load 4 years ago. There are other designs, with fully built circuit boards and heatsink/fan already attached, available on eBay.

I did see the original kit that I built available from this supplier:
https://tinyurl.com/42ejr993

The photo below shows the general layout including two heat-shields, made of thick cardboard, to isolate the voltage reference.

[tekguy]

where did the 7812 go in the schematic you talk of,can you draw it?.

[enut11]

@tekguy
If you look at the circuit in Reply #7, the 7812 12v regulator can be inserted between C7 and R7.
You will need 12vAC or 15vDC at the power input jack for proper regulation.
The 7812 is really a belt and braces solution. A well regulated DC input will also work.
enut11

[tekguy]

so what pins on the 12v reg go to what points on the schematic then,just to clarifie ok,thanks.

[enut11]

@tekguy, I suggest you forget about the 7812 mod and use a regulated 15vDC/1A plugpack into the '12v AC input' socket on the PCB. Much easier to implement.
enut11

[Vovk_Z]

I have produced a label that now sits on top of the plastic enclosure and it shows the expected amps for each rotation of the 10-turn-pot.

I used an additional "0-I-II" switch and a multiturn potentiometer in my electronic load. Additional switch gives two ranges and '0' position ('No load'). That is much more convenient to have different ranges and an ability to turn off a load completely. Ranges in my load are '2 Amperes' and '15 Amperes'. So I have ability to work with high voltage (up to 250 VDC) low-current powers supplies, and with low-voltage high current power supplies. Load current is distributed across 4 TO-247 MOSFETs (IRFP450).

[enut11]

@Vovk_Z
I like the idea of a "0-I-II" switch and the ability to quickly disconnect/reconnect the load. By switching resistors above the helipot you still retain the 10-turn resolution.
enut11

[tekguy]

is there a mod to have this cut off automaticaly say at 3.5v for 18650 lion cells?

[enut11]

Not that I know of. Only if the cells have a BMS board. Most 18650 BMS cut power at about 3v. This one is rated at 5A.

[misa2]

This thread was very good source of information for my own build of DC load based on this kit. I’ll post my experience, maybe someone will find it useful.

I used enut11 modifications (R1 shunts, extra trim pot to limit current), and Kleinstein’s suggestion about snubber circit(4,7uF and 10R across terminals). Thank you guys.

I omitted bridge rectifier and got 12V 500mA wall mounted power supply. As it is non switching, I expected a bit higher voltage without load, but this one had 17V!. Under load it produces 15V, which this kit happily uses. Even 12V rated fan works without overheating. I considered using 7812, but opted not since everything works pretty stable as it is. When i find less powerful power supply I will ditch this one.

MOSFETs are STP9NK50Z, TO-220 devices which are specified for DC operation. They cost 1,5EUR a piece here in Serbia.

The box is locally produced plastic one. Dimensions are 138X120X55mm. Because of mounting points I had to drill the PCB. It looks ugly but it works.

The cooler is for Intel socket 478. I drilled 2,5mm blind holes into aluminium heatsink and used M3 self threading screws to fasten MOSFETs. Drill broke on the last one, so I had to mount it a bit lower. Fan is working nonstop, and this is not an issue for me.

I used M430 panel ammeter/voltmeter, which has interesting feature to display power instead of current for 1sec every 4secs. I tried old version that doesn’t display power, but the new ones was more convenient to use. There is a big gotcha with these panel meters: thin black wire must not be connected to GND terminal, as it is not the same ground as shunt. This leads to erroneous current display. Ultimately, I didn’t connect that wire at all.

In the works, device was tested up to 60V and up to 10A, which are limits of my lab power supply. In this MOSFET/heatsink/cooler configuration it can sink up to 100W continuously – I tested it for 30minutes. MOSFETS didn’t go to thermal runaway, but at this power level, rightmost MOSFET had 140 deg celsius case temperature. I wasn’t comfortable pushing them further. For brief periods of time (1 minute) I tested it up to 150W, which confirms that this kit has potential for more, if better cooling is provided.

For my use case (testing power of unknown transformers, stress test of PSU’s) this is very good kit.

[misa2]

Just relevant SOA for STP9K50Z.

[gevv]

Hello, single layer pcb DIY version.






Attention: If you make a reverse connection, the MOSFETs may malfunction. As a precaution, you can connect a high-power diode in series to the positive line.

Parameters of the original kit;

Input voltage: 0-15v/0-72V (according to mosfet type)

Load current: 0-10A/0-2A (according to mosfet type)

75NF75 75V 150W

Interchangeable with R18 16K-18K 10A, 20K 8A C9 22uF