EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: jkrbasu on November 18, 2017, 08:02:38 pm
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Hi EEVBloggers!
I'm building an ATE for cycling Lithium-Ion coin cells, but I've just realized that the HP 6632B power supply I have is limited on the low end of current that I can sink/source. I can only get to about -5 mA sink, and though I can get to lower source currents, the accuracy is pretty poor (programming 1 mA gives me about 0.5 mA). I think this all makes sense given HPs specifications for the tool- I just didn't realize how low the capacity would be for the coin cells I'm cycling- and hence I never realized I'd need to source/sink such low currents!
Anybody have a suggestion for cheap power supplies that can accurately, programmably source/sink low currents (<1 mA)? I'm hoping to not break the bank and get Keithley sourcemeters or similar.
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If you want to source and sink currents, and maybe measure them, then that sounds exactly like a source-measure unit (SMU) to me. Some instruments, like the HP 6632B, can act like poor man's SMU, but with limitations. Since you mention HP-branded equipment that has to be close to two decades old, some of the older Keithley SMUs, like the Keithley 236/237, are occasionally available on eBay for well under $1k. Their low current specifications are actually quite close to current model Keithley SMUs. Other old instruments that might be suitable and should be much cheaper than a new SMU are the HP 3245A (https://xdevs.com/fix/hp3245a/), and the Keithley 228A (https://www.testequipmentconnection.com/specs/KEITHLEY_228A.PDF) which might just barely be sufficient (a little over an order of magnitude better than a HP 6632B for low current).
You might be able to find some 'toy' SMUs like the AD ADALM1000 (http://www.analog.com/media/en/technical-documentation/evaluation-documentation/Eval_ADALM1000_Documentation_Active_Learning_Module.pdf) that perform sufficiently well, but obviously those are not professional tools you'd expect to find in an ATE system.
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Thanks for the suggestions! I'm not quite at the point where I need "professional tools"- I'm the founder of a bootstrapped startup with zero cash- so cheap options are good for now, at least until we get to a proof-of-concept. I'm also a chemist/materials scientist, so my EE background is limited...forgive me!
The reason why I'm using 6632B supplies is because I came across a ton of them for only $50 each, so it would be great to enable their functionality. What do you think about setting up a voltage divider to effectively trim a larger constant current that is within the range of the 6632? I've already got a 34970A with reed relay cards that is accurately and rapidly sensing current going into the battery by measuring voltage drop through a sense resistor in series with the battery (since I didn't want to rely on the current sense of the PSU itself). Maybe I could use this value to feedback into another resistor- a DVR that is also in series with the battery that helps trim the current? I suppose the other leg of the voltage divider would just be a precise fixed resistor?
Thanks again for your help!
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Yeah, this is a SMU job normally, but there are ways around it. For these low currents you can actually use a run of the mill opamp as current source. The trick is to put your current meter inside the feedback loop, that way the effect of the burden voltage is eliminated in your circuit. Another option is to use the REF200 chips from Burr Brown/TI: http://www.ti.com/lit/an/sboa046/sboa046.pdf (http://www.ti.com/lit/an/sboa046/sboa046.pdf)
P.S.: A cheap 3245A popped up on eBay: https://www.benl.ebay.be/itm/Hewlett-Packard-3245A-Function-Generator-Universal-Source-FREE-USA-SHIPPING/263215404079 (https://www.benl.ebay.be/itm/Hewlett-Packard-3245A-Function-Generator-Universal-Source-FREE-USA-SHIPPING/263215404079)
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Yeah, this is a SMU job normally, but there are ways around it. For these low currents you can actually use a run of the mill opamp as current source. The trick is to put your current meter inside the feedback loop, that way the effect of the burden voltage is eliminated in your circuit.
True, assuming your load is sufficiently well-behaved so the op-amp is happy driving it (e.g. mostly resistive).
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How about the good old HP 59501A? Intented use is generating programming voltages controlled by HPIB to feed power supplies. Can source a few mA, you might have a look at the schematic to see what it can sink.
Hendrik