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I have a 28VDC, high current supply (constant voltage) that is required to charge an emergency power supply.

These EPS internals are rated at 15 amps max input current, however, it’s stipulated that I must regulate at 10 amps.

Issue is my power supply can supply 80+ amps nominal (as it charges multiple units). I’m thinking of a 10 amp limiting circuit using a mosfet and op amp for feedback?

Please advise me on anything - I’m still a student.

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  • \$\begingroup\$ What is that EPS, technically? Just a stack of batteries? Batteries of which type, chemically? Usually, you'd charge with some kind of charge controller, not just with a straight (and dangerously powerful, 80 A 28 V is 2.24 kW) power supply. \$\endgroup\$ – Marcus Müller Aug 13 '19 at 8:07
  • \$\begingroup\$ This isn't an answer, but you should know that BJTs are safer for varying resistance to limit amperage than MOSFETs. MOSFETs are almost all designed for switching, and they have a failure mode that's usually not described in the datasheets. If you want to learn more, tag me and I'll find the past questions I've asked on this topic. Use a Darlington or dedicated regulator to regulate current, not a MOSFET. \$\endgroup\$ – piojo Aug 13 '19 at 8:10
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    \$\begingroup\$ Also not an answer: have you calculated how much power you would dissipate as your regulator steps 28 V down to a little bit above the battery's level while running 10 amps? It's probably enough to fry most small chips without a great heat sink. So I suspect the answer will involve a switching voltage regulator followed by a current regulator, or a system that does both. \$\endgroup\$ – piojo Aug 13 '19 at 8:23
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    \$\begingroup\$ @piojo actually mosfets are becoming the industry leading technology for charging and discharging batteries now. Take the irfp4229 for example. They can regulate high current and allow easy integration with micro controllers. I haven’t done much work with bjt’s in all honesty, so I have little sway to working with them... \$\endgroup\$ – Tom Marshall Aug 13 '19 at 8:32
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    \$\begingroup\$ @piojo yeah they’re exactly that, being used as a linear regulator. The issue with this new technology is actually the voltages. Current sense is made using current sense resistors and a feedback circuit. But the voltage sense is done by an ADC, which as you may know are notoriously inaccurate (unless extremely precise components are used) and even more so at low voltages which batteries can be. If you can come up with an ADC that’s precise with a large bandwidth then you’re laughing. The guy that makes them says they’re fine but my flukes tell me otherwise ;) sorry bit off tangent 😆 \$\endgroup\$ – Tom Marshall Aug 15 '19 at 8:27
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Well, here’s a simple way: use a 10A PTC resettable fuse. Like these: https://m.littelfuse.com/~/media/electronics/datasheets/resettable_ptcs/littelfuse_ptc_radial_leaded_ruef_datasheet.pdf.pdf

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  • \$\begingroup\$ Great! I’ve used these before on ac circuitry, I’ll have a look at some. Didn’t even think about them. Thank you \$\endgroup\$ – Tom Marshall Aug 13 '19 at 8:41

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