use AC/DC converter for DC/DC?

[izeman]

hi

i'm looking to feed my led lamp from my 24s life pack. this is 88v hot. i can't find a suitable dc/dc converter.
so i thought if it would be possible to use a standard 110/220v ac/dc converter to do that? they are available with any voltage/current you need and super cheap.

thanks!

 

[Lebowski]

if it's a switch mode power supply then yes, you can use it (as the first thing a switched mode
power supply does with the AC input is turn it into DC). A modern power supply is a DC/DC
converter with at the input an AC->DC converter (bridge rectifier). I've used a 110AC -> 20V DC
laptop supply with 66V DC input and it worked fine...

 

[dnmun]

bypass the rectifier diodes on the input and attach your 24S directly to the terminals of the input capacitor. if you use the AC input for DC then only part of the power goes through half the diodes.

you can disconnect the AC stuff in front of the diodes so it does not short to anything, all you need is to have the 24S DC on the input caps.

 

[Lebowski]

bypass the rectifier diodes on the input and attach your 24S directly to the terminals of the input capacitor. if you use the AC input for DC then only part of the power goes through half the diodes.

you can disconnect the AC stuff in front of the diodes so it does not short to anything, all you need is to have the 24S DC on the input caps.

if you leave the rectifier diodes you're not sensitive to the input polarity, so you can connect your DC supply whichever way you want...

 

[richmpdx]

There's a substantial amount of info about this topic in a thread in the technical reference area of ES:

http://endless-sphere.com/forums/viewtopic.php?f=16&t=2264

Rich

 

[NeilP]

I run my bike lights off a ac-dc unit..no issues running at 20s.
I have not bothered to bypass the bridge rectifier, i just went straight in the usual AC input.

It did take about 4 different supplies before I found one that worked happily at the 80 odd volts of the 20s.

When powered up, it often takes between 5 and 10 seconds before power comes on at the output and the lights come on

 

[izeman]

Thanks for so many answers. So it's no problem to try one out and all that may happen is that it just ain't work? It will not burn or something?
Unfortunately I don't have the knowledge to remove any parts from the power supply so I have to use what I have.

 

[NeilP]

Power it with a fuse in line go be safe, id for no other reaon than protecting your battery pack from a dead short should something short out inside

 

[rkosiorek]

not normally, but hey, you are asking too broad a question. there may be that one oddball power adapter with some weird input wiring where something could go wrong.

but normal chargers/power supplies for laptops. first convert the AC to DC straight at the plug. so just hook up your battery to the power plug and measure the output voltage. polarity is not important if you are connecting to the AC power plug.

it will either work or it won't. unlikely that anything will smoke. you are only using 88V DC and the input of a universal adapter is designed for 340V peak after the 240V AC is rectified. so 88V DC should not be a problem. all you need is a pair of test leads and a voltmeter to check. a fuse may or may not be much protection. damage to semiconductors will occur long before the fuse will blow. but the fuse might prevent a fire in the event of a catastrophic failure.

if it does not work it would only be that that specific adapter is unable to cope with voltages as low as 88V. most will some won't. all you can do is try.

for example the stock supply for my ASUS laptop will work down to about 40V. the Dell one i have will only work down to about 60V.

rick

 

[NeilP]

My reasoning behind a fuse was not to protect the PSU, metely to protect battety and wiring feeding it

 

[icecube57]

Ive used HTC phone chargers as low as about 24-25v up to 100v DC on the AC input side have it put out regulated 5v output. Just a thought.

 

[izeman]

i have a drawer full of small power supplies with all different output voltages and currents (i guess everyone involved in IT business has )
i just will try a 5v ps and feed it to my led lamp, and see what will happen, and will report back.
thanks!

 

[Farfle]

Go to amazon and search "12v 8A " there will be a laptop power supply looking thing. I have used several of them between 18s and 24s and they work great. Cool as a cucumber powering two 27w led headlights, a brake light and various turn signals.

 

[dnmun]

i gotta agree with the lebowski, if you don't know how to get the case open and cannot tell which terminal is which on the input caps then you have to go through just one side of the rectifier diode bridge by connecting through the AC terminals.

but for those interested, if the AC adapter, or charger brick as they are generally known, will not work at the lower voltage on the input, and it will not start oscillating even if you do put the voltage right on the input caps, then there are mods you can still do in order to try to make it oscillate.

the pwm IC current controller has a pin that is used to kick off the oscillations as soon as the power is turned on. there is one leg that comes off the transformer and goes through a 5meg-10meg resistor directly to one of the pins on the pwm IC current controller. you may be able to make it start oscillating if you can reduce the value of that resistor by putting another in parallel, by soldering another surface mount directly on top of the other surface mount resistor, or soldering a leaded resistor across it. the idea is to get more current to the pwm IC in order to get it to start oscillating.

you can also reduce the value of the gate resistor if there is a separate mosfet for switching the DC current in the front end through the transformer. but most of these tiny charger bricks have the mosfet integrated into the pwm IC and you cannot change the gate resistor. but if you can get to the gate resistor, and changing the kick start resistor did not work, then you can add another resistor in parallel with the gate resistor to see if the mosfet will then turn on and off fast enuff. maybe even change the oscillator frequency on top of that, all to make it work at low input voltages.

but if you are down below 60V, then the 2596hv regulator will work and you should be able to find cheap 60V max DC-DC converters without using a charger brick and you would then be able to adjust the final output voltage too.

 

[amberwolf]

http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=14193

added
http://www.endless-sphere.com/w/index.php/DC-DC_converters

 

[izeman]

ok. i'm one step further. i now got the led lamp. it's a singfire lm-522. not from dx, but it's the same lamp:
http://dx.com/p/singfire-sf-522-cree-xm-l-t6-1000lm-3-mode-white-light-headlamp-black-silver-2-x-18650-170615
it's fed by 2x parallel 18650 batteries. so this equals to 3.7v. i measured current draw from the battery. it was some 2a straight. output to the led (behind the circuit in the battery case) was 3.5v.
i read that xm-l 6t leds take 3.7v and a max of 3a.
if i now want to directly feed the led: can this be done with a power 3.7v power supply directly connected? problem is: i can't find any in my drawer that does more then 350ma (from an old nokia cell phone).
but i have several ac/dc converter than can do 5v and 2a. that would be ok (power wise). how should i reduce the voltage? with a pre-resistor of 3ohm and 2w?

 

[Lebowski]

simple you open the 5V 2A one, look at the chip inside, find its datasheet, read how the output voltage is determined and change it.

just your basic reverse engineering

 

[izeman]

ok. i connected 4x 6s lipo in series, and got 90v. connected that to the power supply, and it got 5v output. so at least this question is solved - not that i doubted that
now for the "reverse engineering". i surely can't solve that.
i opened the ps's case, and this is what it looks like - who can help me? where to start?

 

[izeman]

i could easily desolder some parts, but i need to know which of them. then i can tell what type they are, and give values of caps and resistors. if this helps. my knowledge is more on the assembly side, then really understand what's going on. ohm's law e.g is where my knowledge ends

 

[docnjoj]

When I used to run 54 volts on the trike I did use an Averatec power supply to drop the voltage to around 20 volts and then a Lyen D/C DC converter for my Magicshine and rear view TV camera. It may not be the most efficient way but the Averatec was free and the Lyen converter (there are others) was about 20 bucks. Worked great until I went to lower voltage. Now I just use the Lyen converters off of 36 volts. That 54 volt trike was faster than me. I'm sure there are other Computer power supplies that would work. Toshiba seems to also.
otherDoc

 

[Lebowski]

can you read the type numbers on the to220 devices ?

 

[dnmun]

i cannot see the pwm IC controller that controls the mosfet in the front end. but the resistors R9-R13 are in the bridge across the output voltage. you have to find the junction of the resistors between the output+ and - (or ground) that connects to the cathode of the opto transistor that is connected to the voltage reference (3 legged TO92 with a cap in parallel) and the other end of the opto does the feedback to the pwm IC current controller.

 

[izeman]

i desoldered the two to220 devices, and took a picture. they are not very clear, but i attached a picture, and from what i can identify it says:

LT6237 - SBLI040CT
http://www.linear.com/product/LT6237

04N70BP - A - 63027-1
http://pdf1.alldatasheet.com/datasheet-pdf/view/146808/A-POWER/AP04N70BP-A.html

looks funny, but the first one doesn't seem to be connected to anything but only one leg.

 

[izeman]

man those pictures are crap. i'm gonna take new ones. sorry.
edit: updated 2nd picture. i won't get much better w/o a macro lens

 

[dnmun]

you did not have to remove the schottky diode and the mosfet.