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9v to 24v Converter

DC-DC conversion with isolated output

©2019 By Jack Orman

This project is similar to the 9v isolating converter that was posted recently on this site, but instead of merely isolating the output from the input, it also steps up the voltage from 9v to 24v.

The chip that performs the conversion is also from Recom but it has a few differences from the 9v version. It has a different footprint for the pcb, can supply less current and has a CTRL pin for killing the output by grounding the pin.

The converter module is the Recom RY-0924S that can supply up to 42 ma of isolated 24 volts DC. Efficiency is only 60% to 68% so a minimum of 70ma is required from the 9v input supply.

The datasheet for the module claims 100mv pk-pk output ripple and noise, but I found it to be slightly higher than the spec. Even so, it is more quiet than a charge pump and the 139kHz clock frequency is far above the range of hearing, so it is easily filtered out. Note that the measured clock is higher that the frequency claimed on the datasheet, and is typical of the inconsistencies that I found with the chip data. CTRL pin is open to run and grounded to stop, though this is not specifically explained on the datasheet.

Since I did not have a footprint for this module to use with Eagle CAD, I had to create one for use in my designs. Once that was done, a few prototype pc boards were ordered so that it could be tested.

The schematic is straight out of the datasheet for the isolator module and was found to perform quite well. Since the plus and minus outputs are floating, the circuit can be wired up for negative ground, as used by most pedals, or connected with positive ground to power germanium PNP transistor circuits with a negative voltage. (Only one grounding method may be used for each module output.) The J1 jumper is left open so that the module is in RUN mode.

It is also possible to use a resistor instead of a jumper on the J1 pads to reduce the output voltage. This could be used to set the output to 18v instead of 24v, for example. I have not tried this yet, so I cannot recommend a value for the resistor.

Parts List

U1 - RY0924S, Manufacturer: RECOM Power
C1 - 10uF 25volts X7R +/-10%, Mouser Part #: 810-FK26X7R1E106K
C2 - 10uF 25volts X7R +/-10%, Mouser Part #: 810-FK26X7R1E106K
C3 - 10uF 25volts X7R +/-10%, Mouser Part #: 810-FK26X7R1E106K
L1, L2 - 470uH, Mouser Part #: 710-7447471471
D1 - 1N4001

Inductors and capacitors specified are not polarized so there are no plus or minus sides. The output of the module should have a load of several milliamps before trying to measure the output voltage. I normally use a red LED with a 3k to 4.7k resistor in series as the load.

I have a few extra pc boards of this project, so if you want one to use for experimentation, I am selling the spares for $5 each plus postage:

There are plenty of cheap DC-DC converters from China that sell in the $3 or $4 range listed on Ebay, but almost all of them have common grounds between the input and output, and therefore are not isolated. If you have one, an easy way to test this is to set your multimeter for resistance testing and put one probe on the input ground and the other probe on the output ground. A continuity beep or a reading close to zero ohms will mean that there is no isolation. An isolated converter should show infinite ohms (no continuity) between the input and output grounds.

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