converting a radio shack PSU to a 0 to 10v adjustable supply - Page 1
Forum section: electronics / projects

author: mkube396
author_rank: Regular Contributor
timestamp: August 23, 2014, 11:35:31 pm
content: I need to convert a radio shack model 22-504 PSU to a 0 to 10v adjustable supply it has a dead transformer.I want to wire it up to 1 of my mega Watt s-400-12 running at 13.8~14VI all so have a 10 turn pot at 2K.so I am wondering if you guys can help take the guess work out and help me with what to replace to expand the adjustment on this if it can be done.. I am looking at resistor 7 and 10

author: Simon
author_rank: Unknown
timestamp: August 24, 2014, 08:00:16 am
content: it has a dead transformer ? so you have just lost the most valuable component. What exactly are you converting then ? your best bet would be to start from scratch. New transformer and then put a nice SMPS after it.

author: mikerj
author_rank: Super Contributor
timestamp: August 24, 2014, 11:04:37 am
content: Sorry to be the bearer of bad news but that is a terrible starting point for a bench supply. You won't be able to reduce the voltage output by changing R10 and R7 since the voltage reference is a 12v Zener (ZD1) so you can only increase the output voltage by changing these resistors, not decrease it.Reducing the reference voltage will lower the minimum output, but you won't get anywhere near zero volts without some significant circuit changes (e.g. adding a negative voltage rail).Most importantly this circuit has no proper current limiting, something which is essential on a bench supply. In practice the current will probably be limited to some (high and poorly defined) value by the limited drive capabilities of Q3 that will eventually starve the 2N3055 of sufficient base current. You could hack some form of current limiting into the circuit, but quite honestly it would be polishing the proverbial excrement.

author: mkube396
author_rank: Regular Contributor
timestamp: August 24, 2014, 11:17:30 am
content: Quote from: mikerj on August 24, 2014, 11:04:37 amI did see the 12v Zener after I posted I would be happy to even see 1.5 to 10 volts. as for current limiting I don't care about that just yet till I can afford a a proper bench supply..I would just like to get this to work well enough for now to get me moving forward..thanks

author: Simon
author_rank: Unknown
timestamp: August 24, 2014, 11:20:38 am
content: To be honest just start from scratch and reuse the case. if you don't mind a bit of noise you can use a switcher which can be current limited without generating lots of heat.

author: mkube396
author_rank: Regular Contributor
timestamp: August 24, 2014, 11:22:30 am
content: Quote from: Simon on August 24, 2014, 11:20:38 amI don't have the skills to do that just yet, I am just a puppy amongst the big dogs...

author: Simon
author_rank: Unknown
timestamp: August 24, 2014, 11:41:19 am
content: They are simpler than you think and no more complicated than the linear regulator. I made a simple one with a comparator directly driving the mosfet with a random inductor and capacitors from my junk box using a 78L05 as a reference. I was thinking of putting it through it's paces later and doing a video on it.

author: mikerj
author_rank: Super Contributor
timestamp: August 24, 2014, 12:33:19 pm
content: Quote from: mkube396 on August 24, 2014, 11:17:30 amNote that the voltage reference drives the base of transistor Q2, so the Vbe (base-emitter voltage) of this transistor increases the effective voltage reference by about 0.7v. To get down to 1.5v, you'd need a reference of 1.5v-0.7 = 0.8v.However, Vbe is a temperature sensitive voltage, it reduces by about 2mv per Celsius. When coupled with a high voltage reference, this has relatively little effect e.g. in the original circuit the total reference voltage is about 12.7v (12v zener+Vbe) and 2mv in 12.7v is only 0.015%/Celsius). If you replaced ZD1 with a 0.8v reference, then this would increase to 0.13% Celsius. That means with your output set to 10v, the voltage would change by 13mv for every Celsius change that Q2 sees. It wouldn't be unreasonable to see a temperature rise of 20 or 30 Celsius within a loaded linear PSU, so you nominal 10v could drift down to become 9.6v.If you really don't care about voltage regulation (i.e. you are happy to keep adjusting the voltage control as the PSU warms up), then you could actually use the Vbe of Q2 as the reference and get rid of ZD1 entirely. This would allow you to adjust down to around 0.7v, but temperature stability would be pretty terrible. If you want to try then1) Bypass ZD1 (e.g. replace with a wire link)2) Change R10 to around 100 Ohms3) Replace VR1 with your 10 turn pot4) Replace R7 with 150 Ohms.I really wouldn't recommend this though, some kind of current limiting is really mandatory for a bench supply unless you enjoy blowing stuff up. You'd be far better off getting one of the cheap Chinese adjustable voltage and current modules from eBay for a few dollars.EDIT: Actualy scrap the above. Looking at this further the start-up circuit around Q1 is going to cause problems since your divider chain will now have a much higher impedance. You'd need to make quite a few more modifications to get this working safely and reliably. Seriously, don't bother.

author: mkube396
author_rank: Regular Contributor
timestamp: August 25, 2014, 01:34:18 am
content: ok thanks guys I just put in an LM317 just to get me going ill save up for the proper bench supply..right now the wife is laid off so no crazy stuff right now....