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UPDATE #2: 2020-03-28 - I ditched the MC34063 solution and just built an 80 KHz oscillator with a 90% duty cycle with a LM339 comparitor, 4427A nfet driver, IR630 fet, and a very larger ferrite core 1mH toroid. Using a 5V/2A wall-wart supply, I can now deliver 17V at ~200mA which is enough to turn the motor. Not sure why my MC34063 design stunk so bad, probably should have used an external NFET driver instead of the internal, or probably due to the Rsc value, I dunno. But it's fixed! The biggest impact to current deliver was, as explained in the app note: frequency of the oscillator. The next limiter was the coil.

UPDATED 2020-03-27 based on feedback. Closing this since it is basically a problem delivering current with a boost converter, and there are many other issues similar.

Problem: I cannot get a 24V motor to turn fast enough using a boost converter to step up 5V.

I have a simple 24VDC motor with a no-load speed of 6600 rpm hooked up to a gearbox that reduces it to 145 rpm (specs on the motor are here). Max torque current is ~180mA. I'm using it to stir a viscous liquid.

I have three power supplies:

Supply A: 120VAC 45W power supply with 24V out rated at 2A max

Supply B: A Jabinco boost converter set to convert 5V to 25V, 2A max... specs are here: https://www.amazon.com/gp/product/B081DKJVBK (site won't let me link to amazon for some reason)

Supply C: A home-made MC34063-based boost circuit I built using a TI datasheet design-guide to deliver a max of 24Vout at 200mA.

Both Supply B & C are powered with 5V 4A wall-wart (20W max).

Experiment 1. Test motor with beefy supply

I connect an ammeter in series with Supply A and power the motor. Motor spins fine, DC current is 180mA continuous.

Experiment 2. Test motor with off-the-shelf boost converter

Same setup as Exp 1 but I use the Jabinco boost converter. Motor moves very slowly, makes a weird sound. Update: voltage drops to ~2.5V.

Experiment 3. Test motor with my boost converter

Same setup as Exp 2, but I use my own boost convert which is supposed to deliver a max 200mA. Same result as experiment #2. Update: voltage drops to ~3V.

Experiment 4. Verify boost converters can deliver enough current

Place a 5 ohm (5W) resister in series instead of the motor and check the max current I get out both boost converts. #2 gives a max 280mA, #3 gives a max of 400mA (clearly my design parameters were wrong, but that's still double the required current).

Summary

UPDATE: The boost converters cannot maintain 24V and are dropping down to ~3V. I need to get more current out of them. So even though the previous summary has been corrected, the question is still valid. How do I get enough current out of a boost converter? The beefy supply provides 24VDC at ~200mA which is 4.8W.

The guide I am using from Motorola indicates Lmin is inversely proportional to Ipeak(siwtching), and Ipeak is proportional to the timing frequency. I will try a smaller inductor and a higher frequency...

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    \$\begingroup\$ learn more about stall current. \$\endgroup\$
    – Jasen
    Mar 27 '20 at 1:00
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    \$\begingroup\$ Measure input voltage during max load, is it dropping? \$\endgroup\$
    – winny
    Mar 27 '20 at 7:22
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    \$\begingroup\$ On supply A, measure the stall current. Or if you have a scope, the starting current. \$\endgroup\$ Mar 27 '20 at 13:22
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    \$\begingroup\$ ~12 ohms explains why the Jabinco boost converter didn't work. 12 ohms at 24V = 2A, which is 48W. But 48W at 5V is 9.6A. The boost converter is rated for 2A max, so the motor startup current is much too high for it. \$\endgroup\$ Mar 28 '20 at 17:32
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    \$\begingroup\$ @PeterT, you deleted your LM317 question after someone had taken the time to write an answer for you. I was also in the process of writing one. That's not respectful of those who take of their time to answer your questions. \$\endgroup\$
    – Transistor
    Mar 29 '20 at 17:56
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A 5 ohm resistor will try to draw a current of 4.8 amperes from a 24 volt power supply. You say you measure currents of 280 mA and 400 mA with 2 different supplies. Clearly neither supply is providing 24 volts. If you want to test your supplies at the same current level as your motor, you need a much larger resistor (24/0.18 = 133 ohms). Also, have your measured the voltage provided by your power supplies when driving your motor? It is clear that neither one is putting out 24 volts. If your supplies pass the test with a 133 ohm resistor, then I suspect that the problem is the inrush current of the motor which can be many times higher than its running value. Your beefy supply can handle it since it can provide 2 A which is more than 10 times the running current. Your other supplies cannot do this and are shutting down in some fashion depending on their design.

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  • \$\begingroup\$ /facepalm/ ... I'm embarrassed to admit i didn't bother checking the voltage during all of this. The boost converter voltage(s) drop to ~3V in steady-state. Next question: how do I get more current out of a boost converter, higher frequency? bigger coil? \$\endgroup\$
    – PeterT
    Mar 27 '20 at 15:29

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