First power supply build woes - Page 1
Forum section: electronics / beginners

author: Dr_Krieger
author_rank: Contributor
timestamp: August 21, 2013, 08:16:11 pm
content: I'm currently troubleshooting an LM723 power supply circuit I put together from a schematic on instructables.comMost of it's put on stripboard, and as far as I can tell everything is connected proper; however when I turn it on I don't get variable voltage coming off pin 3, but pin 4 and if pretty much any load is connected to pin 4, the pot starts to arc and smoke.As for parts used, almost everything was replaced with a similar value, except for the 10w resistors (now anyways), 2n3055s and lm723.The "LT10A04" was left out too, I doubt it has anything to do with this problem though.Now I will say when I was first putting it all together and on the first or second power up, I got smoke coming off the lm723 because of a short I believe (it's been fixed since then); could my issue be cause I damaged the lm723? I kind of expect no voltage or erratic voltage or something if that were the case though.Anybody got any ideas on what I should check?Need more info?Danke fürs Lesen.

author: w2aew
author_rank: Super Contributor
timestamp: August 21, 2013, 08:32:06 pm
content: It's likely that you damaged the '723. Also, you should not connect a load to pin 4. Anything that emitted some smoke is probably damaged (the pot, the 723, etc.). Note that you won't be able to sustain any kind of heavy load on a stripboard, so don't try to pull more than several hundred mA until it is properly built on a PCB or hardwired.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 21, 2013, 08:51:44 pm
content: Thanks for the confirmation; guess I won't be finishing this project any time soon. :/

author: Jebnor
author_rank: Regular Contributor
timestamp: August 22, 2013, 01:59:19 am
content: Quote from: Dr_Krieger on August 21, 2013, 08:16:11 pmThe first thing I notices looking at the schematic is the crossing connection from the 40VDC rail, through the 1k8 and the LED. The connection dots on this schematic are on the small side for my liking, but, one does not exisit on that connection. Check to ensure that you don't have the 40VDC rail connected to Vout?Maybe?That might account for the smoking pots. 40V at the bottom end of 20K and through a 1k2 could be more wattage than it can handle.Just a thought.

author: Jebnor
author_rank: Regular Contributor
timestamp: August 22, 2013, 03:08:37 am
content: I too was looking at building a power supply with a 723 ( I have 6 of them hanging around for some reason)Awesome datasheet.http://www.ti.com/lit/ds/symlink/lm723.pdf

author: Paul Price
author_rank: Super Contributor
timestamp: August 22, 2013, 01:00:35 pm
content: Your circuit is prone to zapping the internal driver transistor in the 723..it just needed a approx. 220-ohm /1 w resistor feeding the collector of the internal transistor. If you want to repair your circuit add this resistor to the collector of the driving transistor and replace the 723 and it will work fine only so long as you don't short circuit the output with the current setting pot set to max. output current, because then it will blow out the 3055 pass transistors. Bad design, but you could add a trimmer pot to limit the max current setting.Try this attached circuit, it really works. Just use the power transistors you already have, they are all you need and already wired correctly, just add the resistor.Some modifications would improve it:Use .1-ohm emitter swamping resistors for the '3055's, not the .5ohm shown in the schematic and they are already the ones you have in your existing circuit.Replace the 1n4001 D8 diode with a 25-amp diode.The fan controller is turned on/off by a bi-metallic snap switch..not so good idea.Instead:Use a fan from a discarded PC power supply that only requires about .125 amp at 12V to cool the heatsink.Add a fan controller to control the fan that sets fan speed based on the temperature of the heat sink, just requires a 12V regulator (on small heatsink a several watt capableNPN (such as TIP42) with resistor-zener diode), a .5amp PNP fan driver transistor and a 10k to 47k neg. coeff. thermistor thermally attached to the heat sink and a 5V zener across the PNP transistor to set the minimum speed or a min. speed control (instead of zerer can use a resistor collector to base across the thermistor of the PNP fan control transistor to set min speed). Use a TIP42 instead of the TiP31 in fan control ckt. The fan only needs to idle at the lowest quiet steady speed to prevent fan motor startup chatter.VR2 sets output current, VR3 sets output voltage.You get the idea?Note: There are two secondary windings to this power transformer. You would need to add a tiny 24V (<250mA AC is all that is needed) transformer to achieve the same purpose to supply the bias control voltages needed for driving the main high power section using the other winding that is the transformer you already have.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 22, 2013, 07:54:55 pm
content: Thanks for the comments guysPaul, thank you for your detailed post and suggestionsI'll be saving the schematic you posted and note your alterations, maybe some day I'll build itNow, when you say "add this resistor to the collector of the driving transistor", do you mean the "BD 243C" listed on the schematic I posted?

author: Paul Price
author_rank: Super Contributor
timestamp: August 22, 2013, 08:13:24 pm
content: No, to the internal drive transistor inside the LM723 that connects directly to the BD243C. Both collectors would benefit to be connected to the same resistor. that would connect to the ++V 40V input.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 22, 2013, 08:45:23 pm
content: So if I understand you correctly, I should add a 50-ohm resistor on the 40v line before pins 11 and 12?Does it have to be 50-ohm 5w? would a 3w work?I don't have a lot of high watt resistors and I know none of the 5+w are 50-ohm.

author: Paul Price
author_rank: Super Contributor
timestamp: August 22, 2013, 09:46:10 pm
content: I am now more sure a 20W resistor would be better than the 5W value I suggested, add a 220-ohm/1W but connect only to the Vc pin 11 connection (collector of the output transistor) of the LM723 to +40V and use the 20W resistor in series with the collector of the TI31C/BD243 to the +40V. The VDD connection of the LM723 goes directly to the +40V.In this way using a 5 ohm 20-watt resistor will work. The real serious problem is only with a complete short circuit of the output at max voltage and current settings and these resistors prevent the internal 723 output driver transistor from zapping (shorting then microseconds later opening up) after the BD243 has been destroyed which goes on to destroy the 2N3055's .. a real meltdown situation. Thus the two resistors, the 220ohm protects the LM723 and the 5-ohm protects the BD243 from killing the 2N3055's and itself as well in a short circuit of the output event.According to my calculations I would also bet that just adding the 220-ohm 1W transistor to the pin 11 Vc connection might be all that is needed, but only short-circuiting the output would prove the point. Variations in current gain of the individual transistors under these conditions creates some degree of uncertainty, so also adding the 5-ohm resistor might be the safe way to protect against catastrophic failure. In this way, the next repair might be just replacing a zapped 220 ohm or 5-ohm resistor.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 22, 2013, 11:33:32 pm
content: I don't have any 20w resistors (didn't even know they made them that big), so I hope you bet correctly.I've got 3 lm723s coming in the mail, so I'll be able to test within the next week or two.Thank you for all the help and insight Paul.

author: Paul Price
author_rank: Super Contributor
timestamp: August 22, 2013, 11:54:24 pm
content: Let me know how it works. Good Luck!