I see.Thanks again SgtWookie. I know I have a bunch of similar threads, but this is because I have am on revision 19 of my idea.
This one is missing a digit; "6090" points to an old, completely unrelated thread.I have produced 6 different similar families of devices. This one is skk-18.104.22.168 I went back through my posts on all different devices and found the ones that have gone into my current design, the others were dead ends. I think this is the useful list of parts that went into skk-22.214.171.124:
Perhaps pure luck?The caps you suggested (.33uf and .1uf) for the voltage regulator are new to me. I see they are what the manufacturer recommends, but I have been using this 100uF 7805 regulator circuit for like 5 years now with no (apparent ) noise problems. Why does this work for me at all.
I don't get why you have not experienced problems either - unless your circuits are VERY low frequency.I have used many different micro controllers and different logic components all without issue. I have used smt, and thru hole with no problem. My designs power multiple devices, sometimes, off 1 regulator also with no problems. I run from battery or from transformer. I don't get it.
I really don't have enough information about your circuits to tell you WHY they are working anyway; I have yet to see a board that you have created. However, I CAN tell you that we very frequently have new members that are having difficult-to-explain problems with their circuits, and it turns out that they did not include bypass capacitors; most of the time adding the caps takes care of the problem.I will defiantly take your advice and add these caps, but why does it work for me at all? I suspect probably because all the IO signals come from bouncy switches anyway, and everything else just drives LEDs, or is inside the uController.
They greatly improve the transient response. You're going to be switching several hundred mA current on and off quickly; that'll create rather large transients.What are they doing?
You'll have to price them out. MLCC's (multilayer ceramic caps) would be fine. You can get 0.1uF's in as small as an 0402 package, but that's too small for me to deal with. 0603's the smallest I deal with anymore. 0805's are much easier. The larger they are, the more expensive they get.2nd issue, decoupling caps. I have none because I have no noise problems, at least so far. However this latest design has doubled the number of latches and added shift registers, so sounds like a good precaution. I think I want to use smt parts, what type would be the best / most cost effective?
OK, good deal.I will upload my board and updated schematic when I get the new shift register drawn and the caps in.
I've never seen SMT/SMD IC's stacked. You usually do that with just DIPs. I've stacked SMT/SMD caps before where physical strength wasn't an issue, as the part was going to be filled with Eccofoam anyway.I have a question about this stacking two parts on top of one another: Has anyone actually make this work for a TSSOP-16? I think I am pretty good at hand soldering smt, but I wouldn't even try that...
If you go by the standard Ib=Ic/10 rule, then you're sinking 35mA using the MM74HC595, which is their absolute maximum source/sink current. I don't recommend operating them at their absolute maximum. ±20mA would be more like it; which would mean 200mA Ic for the 2N2907's; unless you want to start qualifying them individually to make certain the Vce(sat) won't be excessive with the lower Ib.I went back and breadboarded this design. My current source is a single pin of the mm74hc595 ( per 7 segment LED ) driving a 2N2907 giving me 350mA max.
OK. Your maximum current limiter is still the 2N2907. It's going to have more Vce due to Ib being lower than it might.The current sinc is a tpic6c595 giving me 100mA drain per pin or 500mA for max for the device.
OK, you're good so far.My only voltage source ( VCC ) is +5V. The 7-segment display being multiplexed 1/3rd of the time is a HDSP-C1A1 common anode display:
Since the usual forward voltage for the 2 leds is 4V, I have a bunch of 51 Ohm resistors for current limiting resistors on each segment of the 7-segment display. Assuming 20mA per segment, and rounding the resistance up.
If you look at the datasheet again, the absolute maximum current for 100% duty cycle is 25mA/segment, and for 10% duty cycle, 80mA/segment and 0.1mS duration. If you tried running it with 60mA current for 1/3 of the time, you'd probably shorten the life of the displays.Except whoops, the LEDs are only on 1/3 of the time so I need more current. I assume I need 60mA per segment on 1/3rd of the time to achieve full brightness. However, this is how I hooked it up, with 51 Ohm resistors, and the brightness is fine. How can this be?
Did you measure the actual current with the resistors, and without the resistors?I put another 7 segment next to the others and just hooked it up directly to vcc and gnd with no current limiting resistors at all, and it did not appear much brighter than the multiplexed ones, certainly not 2/3rds brighter.
What am I missing here, is this just a trick of the eye?
It's less current total, so it's less power expended. Some of the power will be dissipated in the resistors as heat. Since diode Vf is ~4, and you're dropping ~1v across the resistor, that'll be ~4/5 of the power in the LEDs and ~1/5 in the resistor and Vce(sat).If I increase the current limiting resistors do I save power or am I just converting it to heat in the resistors?
Absolutely. In that case, you will need ~15mA base current.And as for the base resistor on the transistor. If I have a 51 Ohm resistor on each of the led segments, that would give me a max current of 20mA * 7 + 10mA or 150mA. Do I even need a base resistor?
Use the typical Vf.Also with the LEDs, do I use the max forward voltage or the usual forward voltage to compute the current limiting resistors?
Because the base-emitter junction "looks" like a forward-biased diode. The Vf of a forward-biased diode starts out fairly low at low current (around 500mV @ maybe 250uA), but that increases as the current increases.One more thing I don't understand. If I have the current through the LED limited to 150mA by the current limiting resistors on the segments, why do I need the base resistor?
Yes, it should be ~180 Ohms. 200 or 220 would be fine, too.Also the value of the base resistor should be > 6.6 ohms correct?
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by Jake Hertz
by Jake Hertz