So here is today's instalment of my investigation into my Maynuo M9712B electronic load, and a tiny bit of reverse engineering!
I've been impressed with the way the twin 60mm fans are driven - then only come on when they need to, and then only to the degree required. Being mid-mounted and some way from either the air intake (either side) and the exhaust (rear end) ports, they are reasonably muted, too.
A fellow poster uploaded some pictures of the M97xx series loads, so I refer you to those. Bolted to the centre of one of the heat sinks is a mystery device - an 8 Amp bridge rectifier. Now, this component alone isn't enough to manage the potential full 15 Amp load of the device, and is some way from the Maynuo's own power supply, to have been likely connected with that. What's more, I can find nothing connected to the two AC pins of the bridge rectifier, and the "+" lead is connected to ground -
wtf... Very odd... Wait a minute!
Imagine a bridge rectifier symbol, but turn through 90 degrees, cathodes all pointing downwards. "-" at the top, two "AC" connections in the middle, "+" at the bottom (and connected to 0V). You now have two pairs of silicon diodes in series, all in a handy, insulated case, convenient for bolting onto a heat sink. Now, as all electronic scholars will know (do they still teach this stuff?), a forward biased PN diode will conduct at about 0.6V, but that this drops at around 2.2mv per degree C. So two diodes in series will drop about 1.2V, reducing at about 4.4mV per degree rise in temperature.
And that's just what this rather over-rated component is doing! I can see 1.12V across it at 21C, dropping to 0.9V at 54C (which is as hot as it gets). That goes into a small circuit based on a couple of op-amps, and thence to a TIP41C TO220 NPN power transistor. This controls the current reaching the fan motors from about a 14V power supply.
And there you have it - a simple,
analogue controller for the fans. They sit idle at room temperature, but just max out at full RPM after the Maynuo has been sinking 300W for six minutes. I imagine the same circuitry also sends the signal to the processor to disconnect the load, should it reach 80C, but I'm not about to try and force that...
Some more tests at FULL load, and to correct an earlier comment of mine about temperatures. Given the full 300W to sink, the MOSFETS nearest the hot end of the heat sink do get hotter than those nearest the fan, the former each reaching a case temperature of 70C after six minutes from cold, with the fans just at max rpm. Given that the MOSFET specs talk about a maximum temperature at their maximum (in this application) current of about 130C, I think they are well within thermal limits. Indeed, the whole thermal management system seems to have no difficulty keeping pace with the dissipated heat.
Back to that TIP41C series pass element transistor which controls the fans, for a moment. The whole of the electronics are isolated from the grounded case, so in theory (and practice), neither "end" of the load + or - terminals needs to be referenced (i.e. connected) to earth. The negative lead could be at 100V with respect to earth, and the positive lead at 110V, for example. There must be a limit, here; at some point, something is going to spark over to earth. Earlier comments suggested that a true earth connection on the PCB was too close to the positive terminal (it is certainly more than close enough). But I do now also have minor concerns that the TIP41C transistor is electrically very close to the load's negative connection, but only separated from true earth (the heat sink) by a standard mica washer and nylon bush. I have not been able to find any "official" figures for the insulation breakdown of standard T0220 insulators, but I don't think I'll be conducting any tests with the M9712B's negative lead more than 200V away from earth.
This isn't really a criticism of the Maynuo - any instrument is going to have limits to its isolation performance. But no claims either way are made for this, in the specifications - you are rather on your own. Contrast this to my TTI power supply, which clearly states on the front panel "Outputs +/- 300V max to Gnd" So there you have it.
More on components - I notice that the capacitors in the power supply section are all G-Luxon brand. Now, I know that some readers will now be making the sign of the cross! These are not top-drawer, Panasonics or whatever. Does it matter? But then, neither is anyone's life going to depend on this instrument running without fail for 30 years. The power supply section is linear and quite low-powered, so no nasty SMPS AC currents swishing about that might tax poorer quality caps. At least they are 105C rated. I am not going to lose sleep over it, and anyway they will be a piece of cake to replace, should the need ever arise.
One remaining calibration issue to sort, which is very easy to replicate and could well be a software bug. Well, this is Version 3.2, so there must have been 3.1 previous variants... I don't think the software is going to be field-upgradable, alas
I am in communication with Maynuo on this issue, however.
CALLING ANY MAYNUO M97xx series electronic load owners - can you get in touch? I would be interested to know if you can replicate my problem...