How is the "load" in load testers put through the mosfets? - Page 1
Forum section: electronics / testgear

author: Beamin
author_rank: Super Contributor
timestamp: April 01, 2018, 09:38:33 pm
content: I was watching the eevblog bk load tester tear down and was wondering how they put the load through the mosfets instead of having huge resistor banks . Wouldn't the resistors in it take the power and not the mosfets? How do they do that without shorting them out. I understand it's built to take the heat through the mosfets but when I tried this the mosfets over heated or the resistor took the watts out.Do they some how make them inefficient and that's why there are no huge coils /resistors?Also what do the squiggly shunts do? Do they get hot?

author: ataradov
author_rank: Super Contributor
timestamp: April 01, 2018, 09:50:09 pm
content: MOSFETs are used as variable resistors, they are not driven on and off, but linearly. Switching banks in addition to MOSFETs may be possible, but will result in discontinuities in the load at the moments of switching, which may not be desirable, especially for lab equipment.

author: BNElecEng
author_rank: Regular Contributor
timestamp: April 01, 2018, 10:02:53 pm
content: The MOSFETs are kept in their linear region, meaning that they effectively become controllable resistors. The concept of an electronic load is to use MOSFETs or BJTs as the load resistors and all of the power is dissipated across them. They usually parallel a bunch of them in order to share the power and keep the transistors within their operating temperature. The circuitry and processor is used to monitor the load, adjust it as needed and protect the transistors. The resistors you can see in the bk load do not dissipate much power. They are marked as 0.5 OHM each,so I'd say they are used to measure the current going through each individual transistor. Since the transistors are paralleled, these resistors share the total load current between them and don't need high power ratings. From their size, they are probably only rated for 5 Watts each.Some larger electronics loads do use large resistors and the transistors are then used to switch them in and out of circuit, as well as provide a limited amount of variable control. This is for big loads though, I'm talking 15kW and higher. This is just my experience, so I'd guess it depends heavily on manufacturer and cost.The squiggly shunts are used to measure the current going through the entire load. They probably will get warm but they are designed to handle the current they are rated to measure. The bk load has 2 shunts in parallel by the look of it, which is probably done to share the current and make sure each individual shunt doesn't exceed its rated current.When you did your test with the MOSFETs and they overheated, did you just turn them on completely? Or did you slow ramp the gate voltage?

author: nctnico
author_rank: Super Contributor
timestamp: April 01, 2018, 10:34:59 pm
content: Quote from: BNElecEng on April 01, 2018, 10:02:53 pmOne of my special products is DC loads for discharging batteries. These do several kW in a relatively small casing. Using resistors makes them rugged and since you can run resistors at much higher temperatures the cooling solution can be much smaller. Count on several $ hundred for the resistors alone though!

author: Beamin
author_rank: Super Contributor
timestamp: April 04, 2018, 08:58:12 pm
content: Quote from: BNElecEng on April 01, 2018, 10:02:53 pmjust random voltages. Was more a play around with heat sinks and what size could handle what.

author: Fungus
author_rank: Super Contributor
timestamp: April 04, 2018, 09:27:24 pm
content: Quote from: Beamin on April 01, 2018, 09:38:33 pmCarefully.

author: SWR
author_rank: Regular Contributor
timestamp: April 05, 2018, 01:16:13 pm
content: The main approach seems to be taking a MOSFET in a big housing mounted with low thermal resistance to an actively fan cooled heatsink.Monitor the current with a low ohm shunt and the temperature of the transistor for thermal protection.Turn off the transistor to protect the load if the current or temperature becomes too high.

author: Candid
author_rank: Regular Contributor
timestamp: April 05, 2018, 07:26:58 pm
content: I played with a simple 150W load I got from aliexpress. With the original cooling it did not even work stable at 100W. After modification and killing some MOSFETs in the end this little beast was able to consumpt stable 185W.

author: mc172
author_rank: Frequent Contributor
timestamp: April 05, 2018, 08:42:20 pm
content: Those loads have a nasty habit of not disconnecting the load and instead shorting it when the lower voltage threshold has been reached.

author: Candid
author_rank: Regular Contributor
timestamp: April 06, 2018, 12:02:20 am
content: What lower voltage threshold do you mean?

author: Fungus
author_rank: Super Contributor
timestamp: April 06, 2018, 07:30:07 am
content: Quote from: Candid on April 05, 2018, 07:26:58 pmLook for small print. Those listings often say things like "additional cooling may be required above 100W".(ie.willbe required, unless you're using it outdoors in an Antarctic winter)

author: Candid
author_rank: Regular Contributor
timestamp: April 06, 2018, 08:24:27 am
content: You are right. I did not expect any other. I think it's always the same with the technical data on this cheapy chinese products. You just need to know how to read and handel them.

author: Fungus
author_rank: Super Contributor
timestamp: April 06, 2018, 08:35:24 am
content: Quote from: Candid on April 06, 2018, 08:24:27 amIt's not a Chinese thing, it's the same with technical data everywhere.All datasheets put "absolute maximums" at the top and hide a temperature derating chart somewhere in the middle (if you're lucky).What temperature will your device actually run at? Suck it and see.

author: mc172
author_rank: Frequent Contributor
timestamp: April 08, 2018, 01:50:30 pm
content: Quote from: Candid on April 06, 2018, 12:02:20 amYou can program it disconnect the load at a certain voltage, which is useful for testing the capacity of batteries. For example, if you're testing a 12V lead acid battery you might want it to disconnect the load at 10.6V, completing the test, but what happens with some counterfeit versions of that particular design is that the load becomes short - the FET gets turned hard on and tens of Amps then flow, damaging your battery.

author: Candid
author_rank: Regular Contributor
timestamp: April 08, 2018, 02:22:46 pm
content: Ok I know what you mean. I have two of these ELs. The first was a very bad copy of the good one. With the first exactly what you describe happens and other things too. Only with the bad version I could go above 150W. The good version limits the power to 150W (exactly 155W). So you cannot overload it and disconnecting when the voltage goes under a set value works fine with the good version. The hardware looks very similar but the software of the fake version is very bad. But only with the fake version you can experiment and go to the limits because it does not turn the load of at any situation.I had a longer discussion with the seller and finaly aliexpress stepped in and I got a full refund. Then I ordered another one and asked the seller before my order and I got the good one. I attach a picture with which you can easily distinguish between the good and bad version of this particular EL.