help needed with my DC/DC step down convertor.

[Andrew]

here's the convertor i'm using to wire my new bike lights to my 54v battery pack. it converts up to 60v down to just a few by the turn of a screw which I guess must be a potentiometer??







The small pins in each corner are of course for volt + and - and for in and out voltages.  The cover of the unit seems to be metal but as you can see the electronic parts are sunk in plastic/rubber. My embarressing question is........  when I solder on the wires can they not touch the outer metal case as this will be conductive?   It's just that there's only a couple of mm's to play with bettween height of pin and height of casing and the wires to handle the 54v battery are quite thick

[Bikemad]

Andrew, although you're using 54 Volts, the wires you are using for your lights will only be carrying around 2 Amps of current maximum for both the front and rear lights that you are using, so they do not need to be large heavy duty battery cables.
Some thinner wire can easily be soldered to the inside of the pins without them being too close to the casing.

I like to use the wires own insulation to cover the soldered joint by working the insulation back up the wire before soldering, and then sliding it back over the joint immediately after it has been soldered while the joint is still warm, but you can use pieces of heat shrink tubing to cover the joint instead of (or in addition to) the wire's own insulation.

I start by cutting the wire approx 5mm longer than required, and then I remove 5mm of insulation from the end of the wire being soldered. Then I grab the bared end with a pair of pliers so that I can gradually persuade the insulation to squeeze up the wire until about 15mm of the inner core is exposed. I then tin the wire and trim the surplus 5mm from the end of the wire so it's the correct length ready for soldering.

This technique will not work with very short pieces of wire as you will probably just slide the insulation right off the other end instead of compressing it.

Your guess is absolutely correct regarding the potentiometer, as it's a multi-turn preset potentiometer similar to this:



Alan
 

[Andrew]

Bloody electronics, it's all too small and miniature for me.

thanks for the prompt reply but i'm still bit worried about the soldering procedure...

 
quote:"I then tin the wire and trim the surplus 5mm from the end of the wire so it's the correct length ready for soldering".   Not sure what you mean by this.   

and.... As for wire thickness I need thick wires for the battery side (54v)of the convertor before the step down?? don't I ?? 

[DirtyGinge]

hey dude :0

(forum warning, gingers had 4 pints of 7% home brew ale before posting)

I use the exact same converter, but I chucked it in a screwfix chocbox

got a cable grip on each end, so no matter what cable you use, its not moving

http://www.screwfix.com/p/chocbox/46538

p.s. batteries will be done when I sober up hic hic not forgotten

[Bikemad]

"I then tin the wire and trim the surplus 5mm from the end of the wire so it's the correct length ready for soldering".  Not sure what you mean by this?

 
There's a good video on YouTube which explains the "tinning" technique very well.

I would also scrape the pins clean and tin them first  before soldering the tinned wire to to the pin as this tends to produce a better joint.

and.... As for wire thickness I need thick wires for the battery side (54v)of the convertor before the step down?? don't I ??  

No, you don't need thick wires on the battery side, the battery is only supplying around 0.5 Amp to the converter, even though your light will be drawing nearer 2 Amps from it.

Your combined front and rear lights will require approx 15 Watts of power on the full brightness setting.

As Volts x Amps = Watts,  7.5 Volts x 2 Amps = 15 Watts.  

If your converter is only 85-90% efficient, it will require ~11-18% more power than it will actually supply to allow for the inevitable losses caused by its 85-90% efficiency.

So, ~18 Watts will be required from the battery on the full brightness setting at 85% efficiency.

If we rearrange the above formula we end up with: Watts ÷ Volts = Amps

Now let's try that formula out with some real data:

• For a fully charged battery: 18 Watts ÷ 54 Volts = 0.334 Amps (334mA)
• For a fully depleted battery: 18 Watts ÷ 42 Volts = 0.429 Amps (429mA)

In either case, the maximum current on the battery side is still going to be less than half an Amp! Therefore, a thick wire is not required.

I hope this hasn't confused you too much

Alan
 

[SydneyCommuter]

If you don't want any 'dry joints' the way to solder is to use the solder to create a heat bridge between the iron and the wire (and later the terminal) feed the solder onto the heated wire, let it ( the hot wire) melt the solder and run the solder wire over the wire, that way the flux inside the solder flows to all the wire and propperly 'tins' it chemically cleaning it so the solder fully bonds to all the surface, apply the final ammount of solder to fill the joint, no big blobs, just enough preferably with a slightly convex shape in the solder between the 2 things connected. Hey in the old days they dragged all us techies thru high reliability soldering, one of the few useful things I haven't forgotten, that and P=iV & V=IR. After that what else is there to know about this stuff?
Cheers mark

[Andrew]

   Alan, I feel like i've joined the open university     Everything you've said makes perfect sense. It's one year since I started all this stuff and joined the forum and I still can't do a respectable solder.
I've actually watched Ginge doing a bit of tinning i'm just not a very good apprentice.

Ginge! you drunkud! saying that, you lasted longer than me last night . I fell asleep with a glass of Gin in my hand shortly after posting on here.   but funnily enough I ordered an isolator box yesterday afternoon

Mark, cheers fro the pointers, sounds quite advanced, i'm happy just when I manage to bond two wires together full stop, never mind tinning, bridging and coating

[MonkeyMagic]

Mr. Drew please tell me you paid only a few bob for that converter...
The same style, minus the casing are $4 incl. postage from eBay.... I recognized the PCB under the epoxy seal underneath.

A really easy way to wire these up, would be to use RCA or even 3.5mm mono audio jack for a neat connection. Should be able to source extension cables pretty easy that way too.

Anyway the Pcb versions on eBay, some have screw connectors lol that would have saved you!

[Andrew]

Mr. Drew please tell me you paid only a few bob for that converter...
The same style, minus the casing are $4 incl. postage from eBay.... I recognized the PCB under the epoxy seal underneath.

A really easy way to wire these up, would be to use RCA or even 3.5mm mono audio jack for a neat connection. Should be able to source extension cables pretty easy that way too.

Anyway the Pcb versions on eBay, some have screw connectors lol that would have saved you!

I paid a bit more than $4.00 but willing to pay a bit more due to knowing it would do the job properly as I had no other ideas of what to buy.

Screw Connectors! DOOH!!

[Andrew]

well.....  thanks for all the help. Both lights ar now up and running.  I 'Tinned' the converters pins and then made a hook/loop on the wire, I hooked around the pin underneath the solder blob and just melted the solder over the top of the loop.  

One good thing about the magic shine front light is that is has a charge indicator light that changes colour. I dialed the converter to 7.5v and then hooked up the front light which only showed a 75% power supply by the colour indication shown, so all I had to do was to slowly turn the screw on the converter until the maxium charge 'green' light appeared.

'The jobs a good un!!'