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. See the update below for more information on this.
U1 - RY0924S, Manufacturer: RECOM Power
Currently Mouser is only stocking the RY0924S/P version of the module. The difference is that the P model has continuous short-circuit protection and is slightly more expensive. It should work in this project without changes.
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.
If you decide to increase the output filter capacitor for greater reduction of noise, please be aware that the maximum capacitance on the output of the RY0924 chip should be limited to 220uF. Higher values may cause instability or prevent the internal oscillator from starting.
The Recom module and the wirewound inductors are expensive, and one way to lower cost is to use 2.2 ohm 1/4w resistors instead of the inductors. There should be little difference in performance for pedal applications.
Update: I have had a chance to experiment with adding a resistor at the J1 jumper position to regulate the output to a lower voltage. If the J1 jumper is shorted with a piece of wire, the output will be 0 volts, but if a resistor is installed on the J1 pads, a regulated output lower than 24v can be had. In my tests with a 9v input, a 50k resistor on the J1 pads reduced the output to a little over 20 volts. A 30k resistor gave me 18.3v out with a light load, and is probably a good starting value. This is much closer to the 18v target than the typical charge pump that will only yield 16.7v from a 9v doubler because of the diodes required in series with the power output.
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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©2019 Jack Orman