Need help in stablizing a power supply

Guessing that the converter's oscillator is being affected by feedback from the motor. I'd try

1) Adequately rated diode in series with motor 2) The high value capacitor you suggested, plus a 10nF ceramic 3) Fresh 12V battery (existing may have highish internal resistance) 4) Choke-based filters on input and/or output of converter 5) Combinations of the above

This assumes it really is impossible to get a second 12V battery to give you the requisite 24V, or a 12V motor!

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Terry Pinnell
Hobbyist, West Sussex, UK

Hi, Everett. I'm assuming you're using one of those automotive 12V-to-24V DC-to-DC converters here.

First, you should look carefully at the difference between peak power rating and steady-state power rating. Your second DC-DC converter limited out, and that sounds like something one of those converters would do. But even if your first supply is only rated for 700 watts peak, it would probably be able to handle 3A.

You might want to put an 0.22 ohm power resistor in series with the +, and then put your "mongo cap" across the output (I'd go with at least 4700uF, 35V for the cap rating). That will help with charge storage and help to even things out. Automotive doublers are made to handle a capacitive surge at turn-on, but the 0.22 ohm resistor should help. Then put a voltmeter across the 0.22 ohm resistor and measure voltage drop when your motor is on. By Ohm's Law, you should get 0.22V per amp of current. You can then measure the steady state current and see if it's really 3A (V = I * R = 3A * 0.22 ohm, somewhere around

0.66V across the resistor). If it's too high, there may be something wrong with the motor or the driver circuit.

.---------------. 0.22 ohm | 12V-to-24V | ___ .------o+ +o----|___|--o----- | | | | +|Batt | | +| --- |12V 24V| 4700uF --- - | | --- | | | | '------o- -o-----------o----- | | '---------------' created by Andy´s ASCII-Circuit v1.24.140803 Beta

Good luck Chris

Thanks for the suggestion. I got a 38,000 uF cap. But voltage is rated 25V . Should it be OK for 24V application? I don't think I will go over 25V.

Thanks.

Everett

I wouldn't do it, but it's up to you. I would guess your peak voltage will probably be above 25V, which will probably smoke the cap, and might smite your converter in rage on its way to electronics heaven.

You can get a 4700µF 35V 20% Axial-Lead Electrolytic Capacitors for $5.29 USD at RadioShack (Catalog #: 272-1022).

Also, you can get a couple of 0.47 Ohm/5W 5% Wirewound Resistors for $1.59 USD each at RadioShack (Catalog #: 271-130 ). Put them in parallel to get about

0.23 ohms.

For a less money/more time/probably equal or better quality tradeoff, try Mouser.com.

Good luck Chris

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Might consider using an inrush current limiter it limits the current at startup to a value depending on it's startup resistance, and (after it heats) acts *almost* as a wire I'd consider using a cold value of say 12Ohm limiting startup to 2amps, and with an operational value of say 0.5Ohms Just search the net, lots of them out there Only thing to keep in mind is it's operational current so the operational resistance stays low enough