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About bahstrike

  • Birthday 07/03/1986

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  1. This particular one is solved. Turns out the cable for another connected device was tying my -12v to chassis. On it's own, this was "fine" and didn't really cause a problem. The problem arose when connecting to the external device, which ties its (and my) 0v to chassis, thereby completing a short to -12v supply through mains GND prong. Crazy to have a silly DC short going through wiring in the wall. Unfortunately, I don't know whether this is the same scenario in a couple other reports of similar-sounding issue.
  2. I'm continuing some investigations and found that the external device ties its DC GND to the actual case Ground. Should I be doing the same?
  3. Hi all, I have a device that outputs a regulated +12, -12, +5v supply for it's own use. The current design ties GND to the center-tap of the transformer. This device detects a low voltage DC signal from an external source. To provide a common reference, I tie my GND to external source's GND with essentially no impedence. Usually this works fine. However in a couple instances, it causes what appears to be a dead short in my device- the transformer/regulators get hot and the ICs are not powered up. When I disconnect the external source, the device works fine. The external "active" signal is detected through 50k or higher ohms of resistance, so I cannot imagine that is the problem. So the fact that the GND are connected directly together indicates to me that either my device, or the external device, has a rapidly changing GND potential and is sinking/sourcing substantial current- appearing as a short. Attached is the schematic of the power supply, with a representative model of the external signal detector. Does anything here look obviously misguided? Should I reduce the 100k input resistor down to 50k, and add the other 50k to the external GND? I'm not sure if this would destabilize up my A/D converter.
  4. Thank you Hero, I really like V2 for the benefit of no switching waste. I'll try that design with my next attempt at a DC->AC coil driver, where higher currents are necessary. From simulating, my existing receiver (RS232 -> TTL) should work OK with ~8mA maximum switch waste (on output). For the transmitter (TTL -> RS232) it looks like I can use my existing PCB layout by hacking on one new resistor, changing a couple resistor values, and removing the pull-ups and base resistors at each transistor-to-transistor. For now, I have to stick to the original layout as much as possible since I foolishly already ordered 12 PCBs!! This solution should work properly with only a ~40uA maximum switch waste (on output).
  5. I was under the impression that if base voltage is equal to emitter voltage, the transistor is totally off and the collector is essentially high-Z. That's why I have pull-ups on base-emitter of most transistors. So you're saying that I basically have current flowing from pull-up of transistor A, through its base/collector, and then through the pull-up of transistor B.. and that's enough to establish an "on" on transistor B, because its base is also connected to this pathway? I actually shot myself in the foot by adding all these "safety" pull-ups? But luckily, maybe the whole design can still work by just leaving out the pull-ups? Back to LTSPICE to test this theory.. I'll be so relieved if that's all it is!
  6. I have a circuit that already provides +12 and -12 volt supplies. The microcontroller Vdd is 3.3v. I tried designing a transistor / resistor system to handle the invert and voltage shifts, without using a MAX232, but unfortunately it does not work.. It overheats and I could only salvage the rest of the system by snipping off these transistors. Before I ordered the PCBs I tested the design in LTSPICE and sure enough the TTL <-> RS232 voltage shift and invert worked great, but admittedly I didn't check the current running through the components... simulating again I see that the lower-right-most transistor has 1.1A running through collector-emitter.. Attached is the schematic.. what did I fail to anticipate? I'm now thinking I only need some strategically placed diodes to prevent mis-bias the transistor bases.
  7. For this project, I only want to detect the presence of a voltage, regardless what that voltage may be.
  8. In this project, it's OK to draw current from the source. But in my next project, it's NOT OK to draw any noticable current, so a true DVM would be applicable. Luckily for that scenario, I can pretty much gaurantee the voltage will be between 5v and 24v DC, therefore electrical isolation is not as important. So perhaps I can use a high-impedance input fed through opamp and then into an ADC; to get a limited range, but true, DVM. Yet, on this project, I already know 120VAC will show up often. And I know that 12VDC is also commonplace :-\ And I know I can't trust a technician to actually check the 3rd party device schematics (if even available) and use a knob to select the correct range :'( The signal doesn't appear unless you're actually operating the device from another room, so it's not trivial either to just check beforehand with a voltmeter.
  9. ??? I don't know what you mean by constant current source. Is it a special component, like a Current Sense resistor? The 5v - 240v is generated by a 3rd party device that I don't control, and the voltage level may vary depending on the manufacturer so I cannot predict what it will be. However, the voltage level on any one particular device will not change. That's why I say it's acceptable to have a 'learn' procedure to find the appropriate range, then store in EEPROM which relay to always activate from that point forth. Unless you have a magic trick Hero999, I think the relays & resistors might still be the easy&cheap way out of this.
  10. DVM circuit would be wonderful. I seem to have found DVM schematics with either a lot more components, expensive ICs, or ones that are based on ICL710x which has an integrated LED segment driver.. far beyond my needs of a simple on/off. I figured that by using reed relays with varying resistance, I can achieve the goal with a relatively simple prerequisite 'learn' procedure that involves testing at highest resistance- if that does not detect the signal then test again at the next lower resistance- until the signal is detected.. Store that range identifier in EEPROM and the system is now compatible with the source. Am I on the right path or is your fully-featured DVM cheaper to build? EDIT: I'm sorry- I did fail to specify that although the circuit should have the initial capability to deal with 5v - 240v, it does not change once the source is identified.
  11. I'm trying to simply detect the presence of a voltage, either AC or DC, across a wide range (5v through 240v). I attached a picture of my current schematic. LTV817 opto-isolator gain should be around x10. However I think I can simplify this design to fewer resistors and relays. Range A: 1M Resistor 240v / 1M = 0.00024 * 10 = ~2mA 120v / 1M = 0.00012 * 10 = ~1mA Range B: 100k Resistor 48v / 100k = 0.00048 * 10 = ~5mA 24v / 100k = 0.00024 * 10 = ~2mA 12v / 100k = 0.00012 * 10 = ~1mA 5v / 100k = 0.00005 * 10 = ~500uA I suppose since the 5v case might only produce half a milliamp, I should increase the pull-down resistor to 100k and perhaps eliminate the 10uF capacitor altogether. In the case of AC, I can have microcontroller detect and ignore zero-crossings so the capacitor might not be needed anyway. Does this seem reasonable, or my original schematic is best, or is there an even simpler approach I am not thinking about?
  12. KevinIV: its alright, the diodes are actually the DB104 chip (bridge rectifier 1A 400V). the current consumption predictions I made were only to show that the Top-Center output (~2.4 watts) is a lot higher than Top-Bottom (~0.84 watts) ..which leaves the bottom half of the coil used at a lighter load, and used in conjunction with the already loaded top half.. I didn't know if that might cause problematic funkiness with reactances/inductances/what-have-you.. Hero999: that's an interesting circuit.. i started to type "doesn't the GND need some sort of rectification, especially with additional smoothing and regulation"... until i saw the 1n4004's.. :) i will give the alternative design some thought for a future use, but i am comfortable with using two rectifiers and my particular circuit does not require common GND between the supplies. since my 24v usage is so limited (only needed to switch on an optoisolator in a remote device), i have given thought about just using a voltage boost IC. I am only building a prototype; I may look again for one in the end-user design of this device. If you know of a good boost IC feel free to recommend :D
  13. I have a 12VA CT transformer, from which I would like to get both 24vdc and 12vdc. As shown, I use Top-Center to get 12v, and Top-Bottom to get 24v. The transformer is rated for 500 mA @ 24v series output, and 1 A @ 12v parallel output. I expect to simultaneously draw maximum 35mA @ 24vdc, and maximum 200mA @ 12vdc. This is my first circuit utilizing mains AC. Do you think this configuration with these loads could cause unexpected behavior? I am concerned that the load on first half of the secondary might affect the 24v series output. (?)
  14. although i consider you guys to be the masters here, seeing as it's an electrical engineering website.. i still want to toot my programming horn! please check out the video for a 1-minute demonstration of instant digital x-ray. i have reasonable understanding of the internals of the devices involved, but my only claim to fame here is what you see working on the computer. the acquisition software is a C# application interfacing with two company-internal APIs for medical PACS integration, and (at most) 8 different 3rd party processing libraries and SDKs for controlling radiography-related devices http://www.youtube.com/watch?v=O5sQMZ_6LM0
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