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| Fool for the 8656A Sig Gen |
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| Swainster:
Hi Tony, No problem - I am just worried that JRH's latest observations dont seem consistent with the rail drooping under heavy load but being fine at light load. Having the case floating while under test might explain the discrepancy. Regarding the 461A I am thinking about starting a separate thread, but first waiting for the postman to deliver some RF transistors as possible candidates for the missing output driver. Hi JRH, as far as electronics load go, I find that I rarely use them, but on that rare occasion then they are indispensible. I actually have 2 (or maybe 2 and 1/2) electronic loads. The one I'm calling "1/2" is actually a 10k pot cobbled together with an op-amp and a random power mosfet with heatsink from my parts collection - this was improvised to give me a controllable few 10s of mA load for a project, once I had got fed up with playing with combining resistors is various combinations (I also didn't want to risk my decent resistor decade boxes on prototype circuits). The first "proper" -ish load was an aliexpress 80W load like the one discussed on a popular thread here on eevblog. Finally my most recent load is a huge 19" Chroma mainframe with a single 300W module installed (and 3 empty slots for future expansion). I bought this one when I needed a bit more oomph and it was available locally and cheap - I think it was less than USD150. The type that Tony has actually looks really convenient, but like you say, that form factor at least always seems quite expensive on the 'Bay. I just recalled that I have another home made load which is just a bunch of power resistors mounted on perf board with a fan strapped to the front. Its got a pretty limited input voltage as the fan is driven directly from the load voltage - it was a another project specific 'special' to stress test a 12V 2A supply for a couple of weeks. Anyway, the point is that you can work around not having a fancy electronic load (but they are nice to have available). For a another example of a self built load, Kerry Wong has a nice video. I have a feeling that he touched on MOSFET SOA and the difference between linear and switching power MOSFETs |
| Tony_G:
Oh yeah, I forgot that Kerry did the video on those - He has some great videos on loads & power supplies - Well worth the sub and watching. TonyG |
| jrharley:
Thanks swainster, you guys know my weak spot for any new project! I've got a beefy old case from an Astron power supply that I parted out a few years back, that would be a great home for a DIY electronic load. Although they are only the basic guts of a working unit, what do you guys think of those $20 bag of parts kits you can see on the bay and other places, bangood, Aliexpress, Amazon? (Not the USB loads). Just curious since I have enough salvaged parts packed away to probably create a power supply, display, etc. to put something like that together. Plenty of old fans and heat sinks too. At any rate, new old stock LAS1905 regulators should arrive this weekend and I'll stick one in to the 8656A for the next phase of the journey. Should be interesting no matter what happens. Seem to come from a reputable source in the States, "warranted", and came in at about $12 each, shipped. JRH |
| Tony_G:
JRH - The journey is as important as the destination - So if grabbing an electronic load kit for $20 from a website gets you enough bits to start building your own load then go for it - I just wouldn't put any stock in the numbers they rate and in the actual working of the unit - But, and this is the journey part, getting something that doesn't work, learning why and then correcting it is worth the time as well. Just make sure that you characterize the device before you depend on it. TonyG |
| Swainster:
Hi JRH, The other day I tried to list all the projects and repair candidates on my to-do list - I realised that I probably need a spreadsheet to keep them all organised :-DD I'm not sure which kit you mean, but if it's the simple constant current source based on LM324 (i.e. no microcontrollers involved) then that looks like a fun project - for that price (seems to be a low as USD10) then I'd be tempted to add it to my next aliepress buy just for fun (actually you could build it on prototype board but the kit PCB would save a lot of donkey work). Anyway, if it is that one, then be aware that the kit mosfets (110N8F6 in the listing that I was looking at) are not specified for DC loads i.e. no DC Safe area of operation. You can't expect reliable operation when operating outside the datasheet specs so treat it as a bit of fun and maybe see how far you can push it before the supplied FETs pop. At that point you can investigate SOAs and linear power mosfets etc, and maybe identify some more suitable parts (though proper linear power mosfets are not cheap). Basically I'm just paraphrasing what Tony says. |
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