SMPS seem to be an option but I am not familiar enough with them to know if they are properly isolated.

 

SMPS seem to be an option but I am not familiar enough with them to know if they are properly isolated.

1) Most of them are.
2) In case I'm wrong, read the label.

 

It would be extremely rare and unusual these days for an off-line SMPS to not be isolated.

I think every mains SMPS I have ever seen was fully isolated, apart from some special cases where the SMPS was inside a television in the old days.

If it is a wallwart type SMPS or something like a cellphone charger SMPS then it will be fully isolated.

But like #12 says, you should always read the label and use some common safety sense.

 

I have a Quantaray SMPS which I got for a digital camera I don't use now. I opened it out of curiosity and found it had pads and switch contacts for multiple resistors/voltage settings. As provided it was 6v@1500ma. with the resistor in the second to last of 6 positions. I wonder how adjustable this might be. If it would do 5-6-9-12-15-24v for instance maybe it could be adjusted to 28? I only want 300ma though not 1500.

 

I only want 300ma though not 1500.

You don't need to worry about that. It will put out whatever current your circuit needs.

 

I was using the LM317 in constant current configuration because I thought LEDs were like Neon in that they have a negative resistance coefficient. As I know what voltage they use for the current supplied I suppose I could change the current until they self limit and hope they don't burn out. Then I could precisely adjust a Voltage based supply.

 

I have a Quantaray SMPS which I got for a digital camera I don't use now. I opened it out of curiosity and found it had pads and switch contacts for multiple resistors/voltage settings. As provided it was 6v@1500ma. with the resistor in the second to last of 6 positions. I wonder how adjustable this might be. If it would do 5-6-9-12-15-24v for instance maybe it could be adjusted to 28? I only want 300ma though not 1500.

What is it's peak voltage setting? Unless it's already capable of 24 VDC output getting it to 28 VDC is highly unlikely without doing a bit of transformer rewind and voltage feedback loop rework.

If you need 28 VDC the easiest rework to do that is to get a old 32 - 36 VDC SMPS power pack from a printer and tweak its voltage down a bit.

 

I can't say. The internals obviously had a selector switch and Multiple fixed resistors. Based on the number of positions I was just speculating on the values. I could take a pot and measure the range.

 

Well, interesting development, I changed to a different transformer and dropped the measured load to 17w. I measured the unloaded power in the new transformer at 4w. I noticed the F-61U was running about 132deg F in the core. I will see how the new one compares after a day. The previous one was rated at 3a while this one is 4a.