Upgraded LM324 Based 300W 72V 20A Electronic Load
Before the kit arrived, I realized I wanted to go by a different path as modern MOSFETs are generally not designed to work in DC region and all designs seem to use them past their specifications.
I built it using as many parts from my stocks as possible, which explains the choice of cabinet, heat sink, passive cooling, and complexity of the power supply. I adapted a 7107 based panel meter to provide direct current read out.
Fitting of the transistors on the heat sink is critical. Silicone isolators don’t work (I tried). 0.47R emitter resistors are on the high side, but eliminate the tendency to thermal run-away provided mica-shims and good heat conductive paste is used. Only one TIP3055 died during my testing... RIP
I saw significant differences in running temperature under heavy load when measured directly at each transistor, but this was down to mounting problems (screws needing tightening).
Up to 30 volts, MOSFETs will usually meet their full power specification, or at least come close.
Is that why you used the TIP41? I likely would have used another TIP3055 for the driver unless I had some random but suitable transistor to use up.
I wonder how good those ST TIP3055s really are. The ST datasheet is bare compared to the OnSemi datasheet.
How did you tune the frequency compensation? Or did you?
If I have extra transistors, I usually match them by Vbe to encourage better current sharing even when ballast resistors are present.
In a production environment a torque driver should be used, and this is not a bad idea for prototyping as well. Belleville washers will help to get consistent mounting pressure. Alternatively a stack of wave washers can work. Do not bother with a split ring washer; they always compress fully without applying enough force.
What is a bigger BJT transistor that you can use for a load?
MJ11028G ?
Good to know! The data sheets I checked seems to have quite limited SOA area. The 75F75's delivered with the Xiaolin kit are only specified to 10ms pulses.
Nicely done! Finally a use for all those TO3055's I have laying around! I built a 20A FET-based load years ago which still sort-of works, maybe it is time for a refresh. Now I need a good job to put my several LM12CLK's to work. Love those cans.
Before the kit arrived, I realized I wanted to go by a different path as modern MOSFETs are generally not designed to work in DC region and all designs seem to use them past their specifications.Up to 30 volts, MOSFETs will usually meet their full power specification, or at least come close.
There are 3 problems with using BJTs...
There are 3 problems with using BJTs...
Thanks for clarifiying!
Mine can't get lower than 60mA due to offset of the 723 I'm using. Also, it does not work well below 2V, so the load is obviously NOT useful for testing low voltage batteries. I have not tested its transient response, which could be interesting, but I'm not sure how to do it in practice without a an expensive and fast high-power source. I made it to test a power supply so I don't have an issue there with the drawbacks of the triple darlington configuration.
/Anders
With MOSFETs it really depends on the type how good they can stand linear operation. Modern low voltage MOSFETs may start to see a power reducion even as low as 10 V. A problem here is that there are also data sheets around that show a wrong SOA curve that is pretty use-less. With a modern low voltage ( < 200 V) part and no faster drop shown from a certain voltage on I would be suspicious. Some manufacturers (e.g. Infineon) seem to be OK with the data-sheets, but not all.
There are 3 problems with using BJTs: one is the base current that is needed - this slightly reduces the precision. The transistors can also get quite slow a lower current with no BE resistor in a darlington. Finally with a darlington on similar configuration to reduce the base current problem the minimum voltage drop can be an issue, e.g. when testing single NiCd or NiMH cells or other low voltage sources.
The circuit as show with a tripple darlington (2 external and 1 x inside the LM723) there is a minimum drop of some 1.2 to 1.4 V.
If one really wants BJTs, one could use a smaller MOSFET to drive the output stages and this way reduce the drop to some 0.6 V and include BE resistors (would not be an issue with the precision here).
oh its a 3055 I thought it was a 3053 so I actually looked at TO-3 parts.
Looks like a nice project, I need to make an electronic load again some day too. So for your under 2V and low current testing, if you think you need it sometimes, wjy not just make another little circuit/PCB, and with another switch or 2, to disconnect it, form the output, or even the power too, when not needed ?
If you went to all this trouble, 1 more little PCB shouldn't matter!