Hi Mauricio hola: I've seen those NTC in PC switching power supplys, but at the primary side instead... He visto esos NTC en fuentes de PC, pero del lado del primario... Regarding your relay solution: it's certainly not much elegant, but if it works for you it's ok. Con respecto a tu solucion a reli: no parece muy elegante, pero si te funciona eso es lo importante. Nevertheless I've got a suggestion: why don't you connect N.O. contacts across your NTC, so as when your cap reaches voltage enough, the contacts close and you got two paralell paths for your current. As I see there is no need to open the NTC path, and it will help if your relay contacts are no perfect. Sin embargo tengo una sugerencia: porqui no conectas los contactos N.A. en paralelo con el NTC, de manera que cuando el capacitor alcanza la tensisn suficiente, los contactos se cierran y tenis dos vmas en paralelo para la corriente. Me parece que no hay razsn para desconectar el NTC, y puede ayudar a los contactos del reli. Regards to all. Besos y abrazos para los gauchos. Marcelo Fornaso www.sysameri.com/marcelo/ ----- Original Message ----- From: Mauricio Jancic To: PICLIST@MITVMA.MIT.EDU Sent: Monday, August 23, 1999 10:06 PM Subject: Re: [OT] Designing power supplys Hi, >> Hi Mauricio, do you connected the load directly to the rectifier output >> or directly to the cap? Directly to the cap. >> I understand you have the NTC between the >> rectifier and the cap... I want to know if the NTC was involved in the >> current flow. Well... I solve it in a diferent way... I conect the coil of a 28v (or 24v) relay to the cap. The NC connector of the relay makes all the circuit current to pass trough the NTC untill the cap voltage is the required to open the NC contact and close the NA contact wich conects the capacitor directly to the rectifier. I know this needs a "hard" relay that supports all the circuit currents, but I think this is, some how, a easy solution that complies with all the requirements, anyway yours is a good Idea but I know that the "relay idea" will work at once and thats what I need right now. Mauricio -----Mensaje original----- De: Wagner Lipnharski [SMTP:wagnerl@EARTHLINK.NET] Enviado el: Miircoles, 23 de Febrero de 2000 09:24 p.m. Para: PICLIST@MITVMA.MIT.EDU Asunto: Re: [OT] Designing power supplys Hi Mauricio, do you connected the load directly to the rectifier output or directly to the cap? I understand you have the NTC between the rectifier and the cap... I want to know if the NTC was involved in the current flow. This is somehow a crazy idea. Imagine currents, at the secondary 28Vac, primary 117Vac, ratio is about 4.17, take the 0.17 as losses, it means that 10A at the secondary would be 2.5A at the 117V primary. Now, it would be much easier to control the 117Vac @ 2.5A than the 28Vac @ 10A. Suppose you connect a 400V x 8A triac (Q4008L4-ND Digikey $2.00) in series with the primary to the 117Vac. Suppose you connect this triac to keep conducting all the time, using a resistor between Anode and Gate, then you connect the output side of an opto-coupler to switch off (when active) the triac. The LED side of the opto-coupler would be fed by a voltage comparator, like an LM358, the "dirty one". Use an easy 5V zener to feed a trimpot, center point fo to the LM358's (-) input, while the (+) goes to a voltage resistive divider (3V) connected to the Cap + side. Whenever the cap Voltage is higher than the trimpot preset, the LM358 will drive the opto-coupler LED, that will cut off the triac at the transformer's primary. Without supply, the cap will loose Voltage until the resistive goes below the 3V (trimpot preset) and the triac will be free to conduct again, recharging the cap and so on. VCAP 110Vac-------. .-------. .-------o--------o---------o----o Output Neutral | | | | | | | 3 C ~ | |+ | R2 22k R4 2k2 3 C .----------. | large | | 3 C | | +++ CAP o--. .---o 3 C | BRIDGE | --- | | | | 3 C '----------' | 3k3 R3 | | .---' 3 C ~ | | - | | | | A 5VZener | | | | | Gnd | | | | '-------' '-------o | | Gnd o-----. | | | | | GND +VCAP | '------. | | | | | --- R1 20k ? .---. | R5 10k Triac V/A | / +|---' V* | |\ | .------< | | | '---o----. Opto | \ -|----------->TRIMPOT | | | '---' 22k Ohms | V <-_ V LM358 | | --- --- | R6 10k Phase | | | | | 117Vac-------o----------' | | | Gnd Gnd Gnd Optocoupler should be a Zero Crossing Triac Output, p/n MOC3040IS-ND Digikey $1.60. The output of the optocoupler should conduct in both sides of the sinewave. Another opto could be the PS3601LNEC-ND a dual inverted photo diodes ($1.15 at Digikey). Trimpot Voltage swing from 1.13V to 3.63Vdc, with the circuit above it will regulate VCAP from 8.66 to 27.84 Vdc. Change R5=22k, TRIMPOT=10k, R6=33k, Trimpot will regulate from 19.6 to 25.52 Vdc. The zero-crossing opto would give you smooth noise and triac will work cool (I guess), so you probably would need a small heatsink (if any)... pretty different from the linear regulator with transistors, etc... Problem is; your Voltage regulation would be as fast as 16ms. Changes can be made to change phase at the triac, still regulation at 16ms. It also requires a fixed resistor at output as a small load to help the regulation. I don't know how this circuit will behave at power on... perhaps needs some fix to avoid 358 latching up, because voltages at the LM358 would be crazy, so its output. Paul? anything you see? probably a 9V zener in series to the LM358 Vcc to make it works only after Vcap goes above 18V? Wagner. http://www.ustr.net Mauricio Jancic wrote: > > Hi Wagner, > Thanks for your reply. I have been looking and I ended in connecting the load directly to the rectifier (with the cap of course!). Now I can draw about 8 Amp and I have 28.5v. I didn't try with 10A coz I don't have resistors that big (power) right now. So now, with out the regulator it works ok, I think that if sometime I'd like to use the regulator (wich I think is the right thing to do) I must buy a higher output voltage trafo. > Again, thanks Mauricio