WickedBlade Posted January 30, 2010 Report Share Posted January 30, 2010 (edited) Hello all, apologies if this was asked before and/or if it's posted in the wrong place. I did a search but didn't find an answer to my question. So here it is. I use a 12V non-switching PSU for my project because I need more voltage than the CORE module needs to feed relays and stuff. So currently, my 12V PSU goes into the CORE 7805 vreg. The problem is that 12V to 5V is apparently quite a drop and the vreg gets really hot. There's no room on my CORE board to fit a heatsink so I'm stuck. I was wondering if there was a replacement available that would maybe use another technique to avoid so much heat loss... (I've seen the LM2575T for instance but it has too many legs...) Would it work? If not, I'd like to hear about other options. The obvious one would be to offboard the vreg and use a heatsink, but I'm thinking of using an intermediary vreg (say, a 7809 that I have handy here) to feed the 7805 (bypassing the rectifier bridge, or rather, placing it before the first vreg, of course), maybe both would produce less heat together than the 7805 alone... Thoughts? Edited January 30, 2010 by WickedBlade Quote Link to comment Share on other sites More sharing options...
philetaylor Posted January 30, 2010 Report Share Posted January 30, 2010 Hi. 7805's do tend to get very hot at 12v especially without a heatsink, I wouldn't be surprised if you get a thermal cutout... There is a switch mode voltage regulator available from http://www.dimensionengineering.com/DE-SW050.htm which is pin compatible with the 7805 "but" it is pretty expensive, $15. The spec says it will work up to 30V !!! Cheers Phil Quote Link to comment Share on other sites More sharing options...
lylehaze Posted January 30, 2010 Report Share Posted January 30, 2010 Hello all, apologies if this was asked before and/or if it's posted in the wrong place. I did a search but didn't find an answer to my question. So here it is. I use a 12V non-switching PSU for my project because I need more voltage than the CORE module needs to feed relays and stuff. So currently, my 12V PSU goes into the CORE 7805 vreg. The problem is that 12V to 5V is apparently quite a drop and the vreg gets really hot. There's no room on my CORE board to fit a heatsink so I'm stuck. I was wondering if there was a replacement available that would maybe use another technique to avoid so much heat loss... (I've seen the LM2575T for instance but it has too many legs...) Would it work? If not, I'd like to hear about other options. The obvious one would be to offboard the vreg and use a heatsink, but I'm thinking of using an intermediary vreg (say, a 7809 that I have handy here) to feed the 7805 (bypassing the rectifier bridge, or rather, placing it before the first vreg, of course), maybe both would produce less heat together than the 7805 alone... Thoughts? The amount of heat produced by a linear regulator is a product of voltage drop (12 - 5 = 7) times current used.. So going with multiple linear regulators, or a resistor before the 7805, will all produce exactly the same amount of heat, though it will be spread out among more components. A switching alternative uses "different math", and can be a LOT cooler. But switching regulators tend to add HF switching noise to the power supply (that's how they work). If your project is all digital/MIDI, no problem. But if it handles audio signals, you may get unwanted noise from the switching power supply. Another option, though it's a bit "over the top", would be to use a switching regulator to drop the 12 volts down around 8, then use a 7805 to regulate your final 5 volts. Check the datasheet on your 7805, some require up to 7.5 volts to operate properly. This method will REDUCE the switching noise in your 5 volt supply. How well it reduces it depends a lot on the capacitor choices you make between the two regulation stages. Have Fun, LyleHaze Quote Link to comment Share on other sites More sharing options...
WickedBlade Posted January 30, 2010 Author Report Share Posted January 30, 2010 Thanks for the help guys. philetaylor, I'm afraid that $15 is a bit too much for me, but thanks for confirming that the heat comes from a big drop. lylehaze, thanks for the formula... My project currently uses 3 voltages, and hence 3 vregs. One for opamps in the audio path (~10V), one for the relays (5V), and one for the MIDIBOX modules (5V). The two 5V vregs are currently separate because I wanted to avoid noise whiplash in the relay coils that could reach the audio path (call me paranoïd). So the 5V from the CORE Vreg is only digital. I guess that a switching vreg would work, then... Do you have a part reference I could use? The LM2575 is not a direct replacement (it has 5 legs) and I would prefer that, if there is such a thing... Also, the vreg I use for the relays is a L78S05 so it can handle a bit more current and maybe it could handle the digital path too, but I've already added a heatsink because it's very hot as it is... Quote Link to comment Share on other sites More sharing options...
lylehaze Posted January 31, 2010 Report Share Posted January 31, 2010 (edited) Thanks for the help guys. philetaylor, I'm afraid that $15 is a bit too much for me, but thanks for confirming that the heat comes from a big drop. lylehaze, thanks for the formula... My project currently uses 3 voltages, and hence 3 vregs. One for opamps in the audio path (~10V), one for the relays (5V), and one for the MIDIBOX modules (5V). The two 5V vregs are currently separate because I wanted to avoid noise whiplash in the relay coils that could reach the audio path (call me paranoïd). So the 5V from the CORE Vreg is only digital. I guess that a switching vreg would work, then... Do you have a part reference I could use? The LM2575 is not a direct replacement (it has 5 legs) and I would prefer that, if there is such a thing... Also, the vreg I use for the relays is a L78S05 so it can handle a bit more current and maybe it could handle the digital path too, but I've already added a heatsink because it's very hot as it is... I do not have a part number.. But there may be another choice.. How about multiple transformers? Each one can be chosen to supply the best voltage for that circuit. Putting the op amps on a private transformer can't hurt the isolation, either. :) Good Luck, LyleHaze Oh,. just for the record, I have run PIC projects by feeding 24VDC to a 7805 quite a few times. Yes, a small homemade heatsink was required.. Yes, the total current was quite low (really just the PIC and a few LEDs) but it can be done reliably. Edited January 31, 2010 by lylehaze Quote Link to comment Share on other sites More sharing options...
WickedBlade Posted February 1, 2010 Author Report Share Posted February 1, 2010 I do not have a part number.. But there may be another choice.. How about multiple transformers? Each one can be chosen to supply the best voltage for that circuit. Putting the op amps on a private transformer can't hurt the isolation, either. :) Good Luck, LyleHaze Oh,. just for the record, I have run PIC projects by feeding 24VDC to a 7805 quite a few times. Yes, a small homemade heatsink was required.. Yes, the total current was quite low (really just the PIC and a few LEDs) but it can be done reliably. Thanks. I'll try to move the CORE 7805 somewhere where I'll be able to use a heatsink... Or I'll try to use a common 7805 for the digital path and relays... Quote Link to comment Share on other sites More sharing options...
WickedBlade Posted February 2, 2010 Author Report Share Posted February 2, 2010 I don't know if I'll try that yet but you might be interested to know that Recom makes a switching regulator that is pin compatible with the good ol' 7805. It is the R-785.0-0.5 and R-785.0-1.0. The first one is rated for 500mA output, second one for 1A. They're a bit expensive, but they might be just the thing. Rapid online offers them, but I'm aware of no other shop having them europe-side. If somebody does, do share! Quote Link to comment Share on other sites More sharing options...
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