Bench Supply from a PC PSU

[MikalE]

I have recently earned my Technician ham Radio operator licence and have purchased the Kenwood TH-D74A handheld radio. It is capable of using a 11-16 VDC external power supply when used as a base radio but Kenwood wants $150 US for a simple regulated power supply.

I have a few old towers sitting around with perfectly good PSU's so I decided I could use one of these for my new radio.

I looked up a few videos and internet papers on making these and had one made in a couple of hours last night. I used a cap across the 12V output for ripple and switching noise control. A resistor was also suggested for a small load to keep the PSU on, but so far with no load, on this particular PSU (Dell) it doesn't appear to be necessary. The PSU never shut off while running.

I haven't used it yet as I have to get a pair of real banana jacks to mount to the cabinet. My efforts at using a pair of banana jacks from an old Radio Shack LX5 speaker was not going to work. They are all metal and attempts to insulate these jacks from the rest of the cabinet were not successful. I may instead use a plastic plate to mount these to, and then mount the plate to the PSU housing.

I'll post a photo when I finish it.

[Thinkpad4by3]

I was debating doing this for a while but haven't had a need to. I would need something with a fully variable voltage control. Interested to see your take on it.

[MikalE]

For DC, couldn't you put a 100K pot on the 12 volt rail? Or maybe a rheostat if a lot of current is generated?

This PS for my hand-held is rated at 16 A but I haven't checked. I'm not too worried about the current output as long as it's at or above 1.5 amps when the transmitter power is set to high.

Since the time I wrote that initial post, I fabricated a plastic plate to hold the metal banana jacks and screwed it to the cabinet after enlarging the holes that the teminal ends of the banana jacks pass through. I also used the grey wire from the PSU to drive an LED to indicate correct voltage is being provided. It has no on/off switch on the AC side as I don't have a rated switch for it.

I hope these are allowable at this size.

[thinkpadcollection]

Find the PSU feedback chip that monitors the outputs and make a new voltage resistor divider to raise the 12V to 15V. Perfectly within the rectifier output voltages. I did this to power a toshiba notebook years ago.

Cheers, thinkpadcollection

[MikalE]

I would not know what to look for without a schematic. I'm not an engineer, only a technician.

[Thinkpad4by3]

Find the PSU feedback chip that monitors the outputs and make a new voltage resistor divider to raise the 12V to 15V. Perfectly within the rectifier output voltages. I did this to power a toshiba notebook years ago.

Cheers, thinkpadcollection

I could place a pull up resistor on my left mouse button on Amazon. I'm not going to screw with modding a PSU, I'm going to buy a premade unit, oops my resisor shorted against the case and sent 120V right into a device.

[thinkpadcollection]

The MikalE's post not yours.

There is datasheet on some PSUs online but it takes some tracing to build a schematic of this said PSU around the area where feedback circuit are which monitors the voltages and there is one voltage being monitored against 2 resistors (voltage divider resistors network being compared to internal IC's voltage). Altering the resistor divider for 15V but output divided voltage being monitored remains the same will raise the voltage to 15V on formerly 12V output.

Cheers, thinkpadcollection

[MikalE]

Find the PSU feedback chip that monitors the outputs and make a new voltage resistor divider to raise the 12V to 15V. Perfectly within the rectifier output voltages. I did this to power a toshiba notebook years ago.

Cheers, thinkpadcollection

I could place a pull up resistor on my left mouse button on Amazon. I'm not going to screw with modding a PSU, I'm going to buy a premade unit, oops my resisor shorted against the case and sent 120V right into a device.

And why would you put a resistor on the AC side of the mains? Put down the screwdriver and back slowly away from the e-Lab bench...

[Thinkpad4by3]

If a wire pops loose from the mod and shorts out on the AC side of the PSU, the power supply and my devive are both equally screwed.

[thinkpadcollection]

No no,, Not the loose wires. Just replace both resistors in the power supply. There are voltage divider formulas online.

There are 2 resistors that forms voltage divider from a TL494's output regulated supply voltage to ground with a 2.5V reference voltage to a TL494 PWM voltage monitoring pin controller or TL341 (looks like a small transistor package.) Just change this two resistors while maintaining 2.5V reference with slightly higher voltage supply to this different resistors will result in 15V to 16V instead. This is in cold side of PSU where PWM controller is.

May have to also change yet another two voltage divider in the overvoltage trip circuit for TL494 as well.

If PWM controller is different part number, a quick google will give you answer on how to set up new voltage output. My preference is 1% tolerance 1/4W resistors for these. Typical tolerance is 5% to 10%.

Voltage divider circuits are rather common on many schematics for voltage reference and in different applications such as trip circuits to shut down for safety even I had to swap two resistors in deflection circuit board near flyback tranformer for late models of RCA rear projection televisions because the resistors is tiny 1/8 watt through hole resistors and had tendency to drift with age because they were specified carbon film not wire wound blue body 1% precision resistor tolerances and should be 1/4W but space is too small in that location for 1/4W size.

And I had carbon flim 1/8W 1Megohm resistor in startup circuit often go open in RCA CTC177 (1995-2000 era) switching power supply results in no automatic start up function to get switching supply running for both standby and main run modes. 1/4W 1Megaohm fixed that easily.

I guess better go back to on topic.

Cheers, thinkpadcollection