Voltage Dividers, High Power resistors

I have searched a lot for simple ways to tranform high voltage to lower voltage, and I was settled on using transformers, but then I thought maybe a voltage divider could be ok.

Now I found these 25-35watt/250-350volt resistors that mounts to heatsink's, is it ok to use them for 230 > 110, 230 > 48v division etc, or would you see it as a hazard or issue to use resistors when dealing with higher voltage conversion? My power requirement is about 6watts on the current solution.

Thank you for your time Kris

• Voltage dividers are not any good at being a source. They are very, very inefficient, and unless your load impedance is very high compared to the impedance of the divider, innacurate and unpredicatble. Their main use is for measurement and dividing voltages, not providing lower source voltages. – Joren Vaes Oct 31 '17 at 9:12
• A transformer also has the additional benefit of providing mains isolation meaning that you can safely connect other circuits to the (low) output voltage of the transformer. When using a voltage divider you will be making anything you connect to it live meaning you get an electric shock if you touch it. – Bimpelrekkie Oct 31 '17 at 9:19
• Here a voltage divider will be dissipating a lot of heat. You may consider a buck converter for higher efficiency. – dirac16 Oct 31 '17 at 9:25
• If you are talking transformers I assume the 250~350V is AC. So almost 600V peaks. Rarely any single resistor is spec'd up to this voltage, so you'd have to use multiple in series. I'm not saying you should do it with resistors, just that if you do it, be careful. – Wesley Lee Oct 31 '17 at 9:25
• Ok. So I should only use them between 5-3v logic etc? Why then have the rating of 350v +? is that mainly for limiting current to ac lamps etc? Ok thanks for pointing that out, I would seal my solution to provide some safety. Problem is, there are almost no suppliers I can find of transformers that allow a variable input/output. Only thing that pops up is led drivers. – Kristian Martinsen Oct 31 '17 at 9:25

Resistor ladder dividers are no good as a voltage source for all but very specific cases. The reason for this is twofold: they are very inneficient, and they depend on the load connected to them.

To give you an example: Say we want to deliver 6W at 6V. Working from 48V DC, this would mean that we need 1 Amp to flow through the voltage divider (at least!). On of the resistors will have 48V-6V across it, which means it will be dissipating at least 32W. Your powersupply has a best-case efficiency of 12.5%!

In addition, the output voltage is sensitive to changes in load. If your load increases, the output voltage of the divider will decrease. How much depends on the values of the divider resistors. The higher the stability, the lower the efficiency (and it can already be no more than 12.5% efficient in our example!)

• I guess I should not do this then. Can't get how I need a transformer the size of a apple and weight of close to a kg when my phone is charged with this 20gram 3by5cm ac/dc transformer/converter – Kristian Martinsen Oct 31 '17 at 9:39
• Your phone charger works at a much higher frequency than the mains supply, which means a much smaller transformer can be used. – Finbarr Oct 31 '17 at 9:46
• @KristianMartinsen because that 20 gram ac/dc uses active switching circuitry. There will still be a transformer in there. The trick is that that device will use a much higher frequency (it will go from the mains directly to DC without and voltage conversion, and then turn that into a very high frequency AC signal). At higher frequencies, we can use a smaller transformer to achieve the same results. – Joren Vaes Oct 31 '17 at 9:46
• For your phone charger to provide 1A at 5V from a 240V supply using just resistors (and diodes/capacitors) it would need to dissipate over 300W in heat! – Finbarr Oct 31 '17 at 9:52
• Ok, I understand, sort of anyway, thanks for explaining that, will look into something like that in the future, after reading and practicing a lot more. Hehe, that sounds like one huge resistor I don't want, and expensive, my 35watt ones are already at 5\$ a piece. My solutions requires 0.2a at 31v. I've built the converter and was about to put on transformer, but the transformer factory had 1-36v in their model list and turns out they only ment they could create constant voltage from 1 - 36. so my transformer will need 230in and can only deliver 31. – Kristian Martinsen Oct 31 '17 at 9:59

Big power resistors are expensive unless you use disposable light bulbs, then you can get advantage of the constant current characteristics when operating the bulbs in the 25% V rated range. This is due to the PTC non-linearity. Using fixed R's is dumb for cost and bulk reasons. Consider several series light bulbs (halogen) such that you have 25% of rated V on each lamp in series (3) dissipating 28W of waste heat to produce 6W of output at 48V. Low MTBF but it can work, but then 3 Halogen bulbs may cost more than buying a SMPS.

I use this method on occasions when I need a cheap and dirty power drop in the garden for DC power drop resistor for driving high power LEDs from a higher voltage on a long cable for convenience.

When cold, R tungsten is about 10% of R_hot=V^2/W . Warm will be a higher %, you can figure this out.

For 6 Watts at 48V you could drop 222V in a X-rated cap with 28VAR dissipation.

The impedance Zc(f)/Load ratio at line (Hz) may be estimated as a voltage divider but then for low ripple (10%) the load cap RC=T needs to be >8x line period thus limiting a C2/C1 (output/input ratio. THis "offline non-isolated" method requires a large primary plastic cap around 0.05 ~0.1uF which tends to be expensive rated for 600V and have MOV or Transil protection.

SMPS line regulators are superior in performance ( efficient, regulated andisolated) and lower cost when purchased, but DIY may cost more with the PCB design so buying is the best option for small qty.