converting 12v to 120 - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: July 30, 2011, 12:11:30 pm
content: Right, having given up on GTI's as my last one blew up I have decided in the short term to use my 90" of panels to charge a car battery by day and once charged use it to power my PC.The easiest way of doing this is to boost the 12V to 120ish VDC as the power supply will run from 110-250V. How should i do this ? I want a cheap solution. Can I used a voltage multiplier fed by a high frequency H bridge ? or should I use a proper SMPS inverter circuit ?

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: July 30, 2011, 12:53:17 pm
content: The PC power supply is switched so it accepts DC too. I had luck before with a dirty simple halfbridge step-up. In a dead power supply from a PC you allready have a transformer and TL494

author: Simon
author_rank: Unknown
timestamp: July 30, 2011, 12:55:34 pm
content: what do you mean by half bridge step-up ?

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: July 30, 2011, 01:46:04 pm
content: Quote from: Simon on July 30, 2011, 12:55:34 pmTake a TL494, drive a half-brige with it into a transformer from dead pc power supply (transformer running backward), rectify it and add a cap.http://www.electronicproducts.com/images2/F129ONSE0507a.gifIf you want 50 Hz AC just feed what you get in a halfbridge and chop it with 50 Hz

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2011, 04:26:34 pm
content: You might need more than 120VDC because the peak voltage of 120VAC is 170V, although it will be designed to run at lower voltages than 120VAC and there will be some ripple on the DC bus so you'll certainly be able to get away with lower voltages than 170V. An easy way to test is to connect the PC PSU to a variac and isolation transformer and measure the voltage on the DC bus capacitor with a 'scope.How long do you plan to run your PC?Car batteries aren't designed to be discharged much. You're better off with a proper SLA.

author: Simon
author_rank: Unknown
timestamp: July 30, 2011, 05:09:43 pm
content: well i could aim for 150V I just don't want to push higher than i need to as that way I think it is more efficient. Of course I'm talking flat DC output not pulsed the specs on the PSu are for RMS not peak

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2011, 05:19:45 pm
content: Don't forget, at lower voltages, the PSU will almost certainly draw higher currents and may not be working at its peak efficiency at the lowest possible working voltage.

author: Simon
author_rank: Unknown
timestamp: July 30, 2011, 05:28:27 pm
content: I thought small difference in Vi and Vo made it more efficient too or does that not apply because this is transformer based ?

author: Unknown
author_rank: Unknown
timestamp: July 30, 2011, 05:40:08 pm
content: Quote from: Simon on July 30, 2011, 05:28:27 pmThis only applies to linear regulators. Unless your PC is something like thirty years old, it's very unlikely to use a linear regulator for more than a few percent of its power. Switching power supplies are often more efficient at higher voltages. Efficiency at 230V is usually better than 110V, for example.

author: Simon
author_rank: Unknown
timestamp: July 30, 2011, 05:49:30 pm
content: err what about there being less voltage across the inductor which also has a resistive component ?

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2011, 06:46:42 pm
content: Quote from: alm on July 30, 2011, 05:40:08 pmI think Simon is talking about a single inductor boost converter which of course is totally impractical for much more than doubling the voltage at high power levels.Quote from: Simon on July 30, 2011, 05:49:30 pmThe inductor should have negligible resistive component. The problem with the single inductor boost converter is the duty cycle becomes very high at high input-output differentials which is both bad efficiency wise and will cause lots of ripple on the output as the power will be delivered in very short pulses when the current in the inductor is interrupted.You need to use a transformer based topology for such high power and input-output differentials.

author: Simon
author_rank: Unknown
timestamp: July 31, 2011, 07:49:56 am
content: I was actually refering to efficienciesin the pc's PSU.not id not really think a flyback smps will meed my needs, I was thinking a voltage multiplier

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2011, 04:04:25 pm
content: Quote from: Simon on July 31, 2011, 07:49:56 am

author: Simon
author_rank: Unknown
timestamp: July 31, 2011, 04:09:26 pm
content: well i was thinking about itI am actually thinking of just getting another GTI, but I'll not plug a lawnmower in next to this one and possibly put a resistor in series with it to limit any fault current

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2011, 06:30:43 pm
content: I think adding a resistor in series with a GTI might make matters worse because the output voltage will be higher, being a constant current source PSU.

author: Simon
author_rank: Unknown
timestamp: July 31, 2011, 06:59:49 pm
content: well providing it feeds back into the grid I don't really care, for a 200W unit your looking at an average current of under an amp so even if just one ohm it will not loose much and hopefully take up some slack in the event of a fault