2
\$\begingroup\$

I want to do some experiments developing brushless DC motor control hardware, and maybe wiring my own motors for my own edification. LiPo batteries aren't convenient enough, and I want current-limiting protection against shorts or failure in my electronics or motors.

The intention is to use commercial 'Hobby' RC BLDC motors, and my own hardware to drive BLDC motors. (I will try to use commercial RC ESCs as well, for testing)

Almost all of the BLDC motors I am interested in are 3 LiPo, i.e. a nominal 11.1V, or less.

I would like a bench power supply which lets me set the current limit, in order to reduce the chance of damaging my electronics or motors. The aim is to emulate LiPo batteries, but with a user-defined current limit. For the sort of things I expect to do, 40A should be plenty.

Unfortunately, the sort of equipment sold by electronics distributors is unacceptably expensive. 40A Bench PSU's seem to start at 500GBP, which is too much cost for a 'hobby' project when motors and electronics are under 50GBP. e.g. Farnell

So I am looking for a cheaper solution which would provide over-curret tripping (like an electronic fuse).

There are lots of switch mode power supplies which claim 12V at 40A or more, for around 50GBP. 12V is close enough to 3LiPo's that it is an acceptable compromise.

A real bench power supply limits the current, and that would be ideal. However, I don't see a way to achieve that at sub-100GBP. Something which rapidly turns off power when it detects over-current, would be an acceptable compromise (unless I am missing something).

I could make an over-current sensor using an off-the-shelf current shunt measuring chip, and a microcontroller. I would 'dial in' my required current limit using a suitably tasteful human interface. It would measure the actual current, and signal an over current measurement, which needs to disable the power supply.

I am not sure how fast switch-off should be. I expect, say, 20ms (1 cycle of UK mains) would be good enough (500W for 50th second sounds survivable). I would like faster, but I am assuming I might need to switch the mains input power off. I would much prefer a solution which does not require me to control the mains input.

I am happy to put fuses in the line too, but I do not believe they are flexible enough, or convenient enough to be the primary current limit

I had imagined some switched power supplies would have an "enable" input, but I haven't found any so far.

Edit: Q1: is no longer relevant
The proposal is to 'disconnect' the PSU power from the BLDC driver after the PSU, using a suitable implementation of an electronic fuse.

Q1a. Will a rapid (couple of ms) disconnect of the PSU from the reactive load create a problem that might damage a switched PSU?

Q2: There is going to be a lot of quite high frequency modulation from the motor control PWM, as it switches power to the motors (I am guessing in 100's of kHz). Is it likely that this will significantly effect a switched power supply, and maybe even make the whole idea unworkable? Might a simple fix, be several mF of capacitors, or is it more complex than that (i.e. it'd need a tuned circuit, Edit: or adjust motor PWM to work around the power supply)?

Edit Q3: was "I assume the current shunt should be on the high-side. Is that correct?"
Response:
Commercial RC ESC's don't care because they are designed to work off batteries.

However, using a high-side shunt should make experiments and test-rig measurement a bit easier, by maintaining a ground reference.

\$\endgroup\$
  • \$\begingroup\$ Have you considered using lead-acid batteries with an adjustable current limit or a current-activated disconnect? \$\endgroup\$ – EM Fields Aug 10 '14 at 20:59
  • \$\begingroup\$ @EM Fields - No, I hadn't thought of using that. I think I am describing a DIY "current-activated disconnect". However, if you can point me at a ready made commercial device that I can 'dial in' my current limit then I think I may be a happy man! \$\endgroup\$ – gbulmer Aug 10 '14 at 21:11
  • \$\begingroup\$ I just Googled "electronic fuse" and got a huge number of hits... Sounds like a fun design project, though, so I'll see if I can work up something cheap (inexpensive ;) to build and post the details it in a day or two. \$\endgroup\$ – EM Fields Aug 10 '14 at 21:28
  • \$\begingroup\$ @EMFields - That is a very kind offer, and more than I could hope for. My initial searches for 'electronic fuse' are turning up devices which are under 5A (consumer electronics), so even if you only point at a plausible device, that would help. I am going to pursue 'high-side current shunts', which was my initial idea for sensing. I guess I need either a high-side driver for an N-MOSFET, or a pretty beefy P-MSOFET \$\endgroup\$ – gbulmer Aug 10 '14 at 22:10
  • \$\begingroup\$ That sounds about right. :-) \$\endgroup\$ – EM Fields Aug 10 '14 at 22:54
3
\$\begingroup\$

You want a power supply that reasonably regulates voltage and can put out 1/2 killowatt at 12 V. That's going to cost some money one way or another. If you add to that current limiting (current regulation) and adjustable voltage, it will cost even more money. These things take some engineering to do well, safely, with regulatory approval, and the volume won't be that high. That all means a commercial product that does all that will cost real money.

If you are going to draw 40 A, then maybe you should be using a higher voltage, like 24 or 48 V. 40 A is going to require thick cable and will otherwise create hassles. You can probably deliver and use 10 A at 48 V more efficiently than 40 A at 12 V. Note that the issue around efficiency isn't so much wasting the power, but dealing with the heat the wasted power causes. A 90% efficient 480 W supply will cause about 50 W of heat.

As for your current limiting spec, it seems you don't really want current limiting at all, but rather overcurrent tripping. That is also sometimes called a electronics fuse. Fortunately, unlike with current limiting, overcurrent tripping can be added separately after a canned supply. For sake of keeping a common ground for measurements and the like, I'd probably use a high side current sense resistor, with something like a INA169 to bring the signal down to the ground reference.

I'm am doing a project right now that includes two electronic fuses. A microcontroller is watching the current sense signal every 10-20 µs in a periodic interrupt. If the current is above the trip point, a counter is incremented. If below, the counter is decremented unless it is already zero. If the counter reaches a particular level, which means the current has been high for some pre-determined amount of time, then the output is shut off for two seconds.

You need to set the trip time long enough to allow for inrush as power on. Or, you apply the algorithm differently at power on. Right now I am using a flat 2 ms for one supply and 750 µs on the other, but that one does a soft start during which the fuse is handled differently.

20 ms seems like a long time, but is still faster than most real fuses take to blow. I'd look at the current profile at power on, and set the fuse a little longer than that takes.

You don't need extra fast switches after the current sense resistor. At most, they will switch once every two seconds, or whatever you set the fuse recovery time to. You don't want to make the switching time so slow that significant heat is dissipated in one transition, but a few µs as apposed to the more normal few 10s of ns should be fine.

\$\endgroup\$
  • \$\begingroup\$ I'll try to improve my question. The BLDC's work at (roughly) 11.1V, i.e. 3x3.7V LiPo (nominal); there is little wiggle room. I would much prefer current limiting, however for what I am prepared to invest, overcurrent tripping is acceptable, and much better than nothing. There are lots of devices, and I have seen marketing for the INA169, so I'll take your answer as a more informed recommendation :-) Thanks for sharing your design, that covers a lot of useful thinking. 20ms was just a worst case; I thought it might need zero crossing and 'mains' end too. Happily you say not! \$\endgroup\$ – gbulmer Aug 10 '14 at 22:03
  • \$\begingroup\$ Oops, and further thank you for 'electronics fuse'. I had Assumed they were either a few amps for consumer electronics, or specialised and very expensive. I will go search. \$\endgroup\$ – gbulmer Aug 10 '14 at 22:05
  • \$\begingroup\$ @gbul: I didn't mean to imply that a electronic fuse was something you can go out and buy. I meant it as something you design and create yourself that can be added on to a existing supply. \$\endgroup\$ – Olin Lathrop Aug 14 '14 at 3:09
  • \$\begingroup\$ Thank you very much for following up. That is very helpful and thoughtful. I appreciate it. I had made that mistake initially. However, after some searching, it became clear that it is something that needs designing and building. I am happy to investigate that now that I understand. EM Fields has kindly provided a design for a low-side design, and pointed me at the LT6108 for a high-side design. Thanks again for your help. \$\endgroup\$ – gbulmer Aug 14 '14 at 10:10
3
\$\begingroup\$

Here's a low-side fast electronic fuse with a trip point adjustable from zero to over 40 amperes.

If you build it, it's IMPERATIVE that the cold side of the [50 ampere 50 millivolt] shunt be connected to the battery either directly or with a short length of large diameter wire, and that the cold side of the load be connected to the hot side of the shunt either directly or with a short length of large diameter wire.

The LTspice circuit list is here in case you want to play with the circuit.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Thank you very, very much. I'm a bit speechless! Until this instant, I was thinking of a MCU based solution using a commercial part (INA169 as identified by Olin Lathrop). You have given me some very tasty food for thought. Thanks again. \$\endgroup\$ – gbulmer Aug 13 '14 at 21:31
  • \$\begingroup\$ My pleasure. :-) If you want to go the high side route, Linear has a chip you might be interested in at: linear.com/solutions/1660 \$\endgroup\$ – EM Fields Aug 13 '14 at 23:51
2
\$\begingroup\$

Here's a 3-15V 40A switching power supply for £129.00

Maas sps 8400 bench power supply 3-15v dc 40a

A PSU with adjustable output voltage is very useful for simulating different battery voltages (eg. 11.1V, 7.4V) and to compensate for voltage drop under load.

1/ You can build an adjustable external current limiter, and then you don't need an enable for the PSU itself. An external limiter may operate faster and provide better protection for sensitive circuits.

2/ Adding several mF across the output does help to soak up ripple (I used Sanyo OS-CON capacitors for lowest ESR). When running the motor at part throttle it may feed current back into the PSU, which will raise the voltage slightly. You can compensate for this manually if the PSU has adjustable output voltage.

3/ RC ESC's are normally powered by a battery and don't need to be earthed, so the shunt can be on either side. A low-side shunt may be preferred as the sensing circuit can then be referenced to the negative rail.

\$\endgroup\$
  • \$\begingroup\$ Thank you for the link to that PSU. That looks good, and I may consider it if I get past my first stage. For now I think I'll be happy with a fixed 12V. I believe I can make an external current limiter. I think I have seen power supplies with an external enable, but I do not know how fast they switch off, and so that is part of my question. 2/ Ah! I see why voltage control might be needed. That is a helpful comment. 3/ Thank you. Yes, RC ESCs are powered from batteries. Doh!-) Thank you! \$\endgroup\$ – gbulmer Aug 10 '14 at 21:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.