I cannot get an adjustable buck converter (DC-DC) to work - very frustrated!

[choochoo_baloo]

Following recommendations on my recent post; 

I bought three generic trim pot adjustable buck converters off of fleabay however none of them show a varying output voltage despite multiple rotations of the cheese head screw on the trim pot.

• I read online that occasionally these trimpots are faulty, so desoldered and replaced with hald decent spare pots and no change.
• Given the IC functioning I thought maybe it needs a load to function correctly. So I then hooked up a LED but the brightness is constant (photo of circuit below)

To save my sanity and smashing them up, please can someone advise why the ******* things are not adjusting their output voltage - what am I doing wrong? Surely all three can't be faulty?!

 

 

[Merlin66]

Why not just take the easy way and order a 12V to18v-5v convertor from JayCar????

I use two on my set-up - a 12V to 15V 120w to power the HEQ5 (or NEQ6) mount and another to drive my powered hub 12v to 6v. No issues no drama.

 

[johninderby]

I’ve used this type with no problem so don’t know what is up with yours.

If you just want a 5v usb output why not get one of these dedicated ones on eBay.

https://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=12v usb converter&rt=nc&LH_PrefLoc=1&_trksid=p2045573.m1684

[symmetal]

I've used several of these LM2596 buck converters with no problems. Clutching at straws here but as you've used two black wires as the input is there perhaps a chance you got the input polarity reversed. 

Alan

[upahill]

Do you have a meter you can hook up to measure the output?

A quick google suggests alot of people dont realise how much they have to be turned in order to adjust, 10+ revolutions.

[pete_l]

Be aware that when those multi-turn pots reach the end of their travel the screw can continue to be turned. Try turning it in the opposite direction.

I have used a lot of these. They don't need a load to run at the set voltage. I would suggest removing your LED for the time being and just hooking up your DMM.
Another thought: are you sure that the ones you have aren't a fixed output voltage and the pot is there simply to set a current limit, rather than to control the output voltage?

[zacrobmer]

The one I used in the past, I didn't hook it up to anything until I had the voltage set. Just used a multimeter and set it to the right voltage. 

 

[JimothyC]

I’ve used a few of these for various applications and they did take a large number of turns to see any voltage drop on my meter. Maybe 20 or so

if you replaced the pots, are they definitely the same value? I’d be tempted to put the old ones back on there. 

[Gina]

I've bought several batches of these and never had a problem but it is possible you have a bad batch.  I've also bought a few batches of Arduino Nanos and did get a bad batch of those once, so a bad batch is possible.

[Chriske]

Me too, again ordered 10 of these 2596, no problem at all.

[Chriske]

Seems to me that the potmeter is turned very deep in. Turn it out a bit please..

[johninderby]

Second the above. Looks like the pot needs winding out a lot. Here’s a pic of a similar one.

[Davey-T]

If you buy another one it's worth buying one with LCD voltage display, same size and no more expensive.

Dave

[choochoo_baloo]

Thanks all for the advice. Yes it was a matter of the trim pot needing a huge number of turns to get a sizeable voltage drop across the output - I actually used an electric drill to wind it out!

For reference: they are a worm+wheel design thus explaining the ~30 turns needed to get at least half way across the resistance track.

[johninderby]

Glad to hear you got it sorted out. ?

That can be the problem with new bits and not being familiar with them but once you’ve gotten familiar with it no problem.

       John

[choochoo_baloo]

2 hours ago, johninderby said:

Glad to hear you got it sorted out. ?

That can be the problem with new bits and not being familiar with them but once you’ve gotten familiar with it no problem.

       John

Exactly - I understand the electronics/physics of the device, so seeing all components well soldered etc and seeing the 0.7V dropout from teh timer chip made it even more frustrating why it would not adjust!

As you say, a relief when it's something basic eh John.

[Gina]

I think there are 50 turn trimpots.  Most I've used have been 20t though - a long time ago.

[johninderby]

Overthinking the problem is in itself a problem. ?

Reminds of the old television repairman story ( yes once upon a time they used to repair TVs in your home) who spent an hour at the customers house and couldn’t understand why it wasn’t powering up no matter what he did. Then someone pointed out that it wasn’t plugged in.  ?

[Gina]

I've had that problem at times! ?

[BrockDorf]

hi...you can remove your LED for the time being and just hooking up your DMM.Are you sure that the ones you have aren't a fixed output voltage and the pot is there simply to set a current limit, rather than to control the output voltage?