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Electronics Forum / Basics / April 2008"Goosing" with a higher voltage
Hi guys: I have an application where I'm driving a device with 5 volts DC. From time to time it is necessary to give it a short pulse of 24 Vdc then revert back to 5V. I have a power supply that has both 5V and 24V outputs. Can I just pulse the 24V right onto the 5V line (with a power transistor or relay) as long as I include a diode on the 5V output to avoid back current? Or is this foolish? Don > Hi guys: > [quoted text clipped - 6 lines] > relay) as long as I include a diode on the 5V output to avoid back > current? Or is this foolish? It sounds like it could work. I would use a Schottky diode to minimize the voltage loss when the 5 volt supply was providing the load with current. I even that loss is too much, you might use a MOSFET as a synchronous rectifier (with the channel conducting in parallel with the body diode when the 5 volt supply was driving). But then, you have to produce a gate drive signal that is synchronized (non overlapping) with the disabling of the 24 volt drive control.  SignatureRegards, John Popelish >> Hi guys: >> [quoted text clipped - 14 lines] > produce a gate drive signal that is synchronized (non overlapping) with > the disabling of the 24 volt drive control. Be careful. If there are any bypass caps downstream the voltage on them might still be higher than 5V when the 5V sync-rectifier comes back on -> phssst ... *POOF*. SignatureRegards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. >>> Hi guys: >>> [quoted text clipped - 18 lines] > might still be higher than 5V when the 5V sync-rectifier comes back on > -> phssst ... *POOF*. Yep, that was one of the reasons for the non overlapping warning. I would probably turn the MOSFET on only after the body diode was carrying the load during the 24 volt switch off, and turn the MOSFET off before turning the 24 volt switch back on. That assumes the system could handle those brief sags below 5 volts. SignatureRegards, John Popelish Sounds like I can use a Schottky diode. I found one with a voltage drop of about a half a volt. My chips downstream appear to be able to handle that drop. I suppose I'll have some adjustment of the 5V at the power supply too. If I can crank it as high as 5.5V I might be able to compensate. Thanks for the help guys. Don > >>> Hi guys: > [quoted text clipped - 32 lines] > > - Show quoted text - I'd say Don should use OR-control, that way nothing can go wrong. He already has 24 volts to drive a high-side mosfet. 24v | +-----+----, | | | | | | |\ | 6k2 | ,------|+\| | | | | \ | | | LM393| >----+ | | | / | o| | ,--|-/| | |--power | | |/ | | o| switch 10k 10k gnd | | | | | | | '-----------|----+ | | | | ,------+---+ | | | | | | | 18v | |_ | | zener 10k __ | | | | || | | 5v-+------+---''---+--load | | s d | | n-channel mosfet | \ SPP80N03 | / '--->\1k / \ | | 50k | | gnd I didn't draw a .1 uF bypass cap for the comparator, but it should probably have one. The mosfet needs oriented source to the 5 volt supply and the drain to the load. The 1k pot nulls out any negative offset in the comparator, to make sure it trips when the source voltage exceeds the drain voltage, preventing the mosfet from ever conducting back into the 5 volt supply. If the comparator has positive offset, it will oscillate at light loads under a few tenths of an amp or so, which probably won't hurt anything. But if one prefers, using an op amp instead of a comparator would get around that. I like the comparator 'cause it's fast, cheap and robust. > > >>> Hi guys: > [quoted text clipped - 86 lines] > > - Show quoted text - D'oh, can't have that gate-source zener and resistor that way. Do away with the zener. Run the 10k resistor from gate to ground instead of to the 5 volt supply, and change its value to 15k. Hope that does it. > > >>> Hi guys: > [quoted text clipped - 6 lines] > > >>> relay) as long as I include a diode on the 5V output to avoid back > > >>> current? Or is this foolish? OR control, latest version: > 24v > | [quoted text clipped - 31 lines] > | > gnd |
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