I have a KBD101 (a brushless motor controller from Thorlabs) that, in the manual, specifies a regulated 15V/2A supply. I need to power it from a 14.4V/10A Li-ion battery. The other electronics are rated at 12V and are protected by LDO regulators but my understanding is that I can't do that for the KBD101 because the voltage from the battery is going to vary between 16-14V as it discharges.

How likely is it that the device will work reliably with this varying voltage? I've connected it to a bench supply at 15V and it draws 300mA during the experiment.

I've asked Thorlabs but, understandably, they can't recommend such a use-case.

Thanks in advance!

  • 3
    \$\begingroup\$ There is no such thing as exactly 15V. Thor labs should be able to tell you the maximum allowable voltage. Your battery sounds like a 4s lithium ion battery pack. If so, the max voltage will be 4.2V * 4 = 16.8V. If the maximum allowable voltage is less than 16.8V, then you are pushing your luck if you run it that way. You said 16V, not 16.8. I don't know if that is accurate. But if it is, it seems very unlikely that 16V would be too much for a 15V controller. \$\endgroup\$
    – user57037
    Commented Jan 4, 2018 at 9:18
  • \$\begingroup\$ The bigger problem may be that as the battery discharges, the controller doesn't work correctly or is unable to maintain target speed (due to Kv issues). \$\endgroup\$
    – user57037
    Commented Jan 4, 2018 at 9:20
  • \$\begingroup\$ Thanks - it is a 4s battery but I have spare power for the experiment and was simply going to only charge it to ~80% capacity. \$\endgroup\$ Commented Jan 4, 2018 at 9:23
  • \$\begingroup\$ @TobyJackson, that makes sense. I mean, before MrGerber's answer that made sense. Obviously, now that we see the tight range from the manual, it is pretty clear that a battery is not going to cut it. You will need a regulated supply. They do make power supplies, it looks like, but they are AC/DC type. So may not work for you. I mean, I assume you need to be battery powered, not mains powered. \$\endgroup\$
    – user57037
    Commented Jan 4, 2018 at 9:31
  • \$\begingroup\$ Use a buck-boost battery regulator to get your desired 15V out over the battery lifetime. \$\endgroup\$
    – Jon Custer
    Commented Jan 4, 2018 at 17:48

2 Answers 2


The voltage input tolerance is specified in the manual for the Thorlabs KBD101, pp 63. See screengrab below.
It clearly states the voltage tolerance to be +/- 3% - 14.5 V to 15.5 V.

It's impossible for anyone of us to tell you any different without knowing what goes on inside the unit. It might handle a wider tolerance, but I'd not recommend you trying, that's an expensive bit of gear ($731 at time of answer) to experiment with. enter image description here

  • 2
    \$\begingroup\$ Oh wow. Now I feel stupid - I'd gone through the manual several times looking for a voltage range. Thank you for not flaming... \$\endgroup\$ Commented Jan 4, 2018 at 9:21
  • \$\begingroup\$ The link to the manual seems to be broken. \$\endgroup\$
    – Tejas Kale
    Commented Jan 4, 2018 at 15:16
  • \$\begingroup\$ Thanks for the heads up, @TejasKale. Should be working now \$\endgroup\$
    – MrGerber
    Commented Jan 4, 2018 at 15:18

A specified value cannot be infinitely precise and has some inaccuracy, be it a voltage, current, resistance, capacitance, inductance, temperature or anything else. The inaccuracy on a value, the specification of its deviation from the intended value, is usually specified as a tolerance and may be given in absolute units (+/-1 V, +2 V/-3 V etc.) or as a percentage (+/-2%, +10%/-6% etc.).

Looking for tolerances on every value and using them is the normal view in electronics engineering and many mistakes are made when they're disregarded or ignored. For example, the UK mains voltage is currently specified at 230 Vac +10%/-6%, so a load plugged into it must be designed to operate continuously and successfully from any voltage between 216 Vac and 253 Vac. (I still think to 240 Vac +10% = 264 Vac but that's just me being old!) Designing equipment around just 230 Vac could lead to nasty, smouldering results. Another is designing an RC oscillator, where the oscillator frequency must be calculated for highR and highC and also for lowR and lowC (R and C at their highest and lowest values from their tolerances), to obtain the max/min frequency.

If it sounds like ongoing aggravation then just see it as part of engineering life :-) It's really that the view of every value as precise ("a 10K resistor is 10,000 ohms") is fake and leads to mistakes, problems and unreliable equipment.

Returning to your specific case, you can obtain a supply voltage tolerance from your supply load device's manufacturer. If the supply you have, your battery, can't meet this, you could consider using a +15 V to +15 V DC-DC converter. Look at the specifications for these, you should find one with a wide input tolerance for your battery and a tighter output tolerance for your load. They're not particularly cheap, though, and they introduce switching noise that you must design for or design out.

  • \$\begingroup\$ Do you mean highR·highC and lowR·lowC? \$\endgroup\$
    – supercat
    Commented Jan 4, 2018 at 16:27
  • \$\begingroup\$ No, you don't want to feed a brushless motor driver with a DC-to-DC converter, instead source a motor driver compatible with the available input voltage. \$\endgroup\$ Commented Jan 4, 2018 at 16:41
  • \$\begingroup\$ Yes I do @ChrisStratton, otherwise I wouldn't have written it. Please re-read the OPs question, it describes the constraints they have. No use prescribing a wish-list, I'm afraid. \$\endgroup\$
    – TonyM
    Commented Jan 4, 2018 at 17:52
  • \$\begingroup\$ @TonyM: I used dots rather than slashes, since the time or period will be proportional to the product of R and C, but slashes give the appearance of a ratio. \$\endgroup\$
    – supercat
    Commented Jan 4, 2018 at 17:59

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