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Forum Index : Microcontroller and PC projects : Flash 3 common catode LED lamps.
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CaptainBoing Guru  Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
for bipolar stuff you have to limit the current running through their various junctions or they get very stressy. your last circuit showed DRIVE IN connecting straight to the base of Q, if that in turn connects directly to the Arduino output, when it goes high, you'll get effectively unlimited current through the BE junction of Q... It will definitely turn on with that amount of juice... for a few moments... destroying Q, possibly the internal Arduino output driver or even the entire thing. Pretty much any small signal trannie would be a good fit for Q (and new-Q if you go that way); they are fast and only require a small Ibb so the base resistor can be quite big. In my modification I went for a 1K (5mA) to give a good kick and make the little nerk switch hard, but you could probably go up to 47K with something like BC547. If you have tons of old trannies you think you'll never use - this is the place for them. They are just being used as switches so their characteristics are unlikely to come into play and it uses up old stock. Edited 2021-08-06 16:44 by CaptainBoing |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
I'm afraid the lights went out in London long ago. Very intimidating, expensive crap hole these days   |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7830 |
The golden rating is "260mW" absolute max. Yes, you can get 2A through the TPL350, but every mA produces heat. At 2A (the recommended running max) the heat is so great that the silicon will melt - but not if the pulses are very short. That rating is used to charge and discharge the gate capacitance on some pretty big mosfets. The TLP350 almost certainly *does* have a continuous current rating (although not specified because it's not designed for those applications). You need to find the volt drop across it at your desired operating point then divide that into 260mW to find it. One of the graphs shows about 2.5V drop (but not operating near it's maximum limit). If that holds true at 28v then it should be able to source or sink about 100mA max. It may not do. It could well be much worse. It's a lovely chip, but not for this job. :) @Tinine I *did* say SSR... ;) @Bob.Steel Sorry I got the quantity wrong. I knew it had a 2 in it and at least one 0, but I couldn't scroll back to check. :) The method of switching by driving the emitter of a transistor is already well known. Of course, the base of the transistor needs a resistor to 5v - never a direct connection - because the base current also has to flow out through the emitter. As we are driving a mosfet the collector current only needs to be a couple of mA - say 5mA. If the transistor has a gain of 20 (it will be higher) then it's base current will be 250uA so the arduino would have to sink 5.25mA (it will be lower). That's well within it's capabilities. The base current would have to be limited to that by using a base resistor of about 17.5k to 5v. 18k should be fine. The mosfet pull-up resistor would have to be about 5k6. That may lead to a bit of a slow turn-on with consequent power dissipation, but so be it. Edited 2021-08-06 17:03 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
You did! and early on.  I use a variant of that very SSR (flotek) to switch a 1Kw fan heater on my "frost guard" in the conservatory over the winter. It looks like an identical package so three of them would be quite bulky. That SSR has given sterling service with not a moment's problem. That's jinxed it. I'd still homebrew, despite the thrashing this thread has got, it isn't that big of a problem (I wouldn't event use a microcontroller for the light flashy/turny-on logic - 555, 4017 and a BT149) . Edited 2021-08-06 17:25 by CaptainBoing |
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| bob.steel Senior Member  Joined: 27/02/2020 Location: AustraliaPosts: 188 |
Its up and running well without a resistor on the input .However I will add one now to be sure. Arduino's can only source/sink 40 mA if I remember rightly. BTW altering the code to invert that signal means leaving each pin on for long periods and then flipping it off to get the effect then back on so I would assume its power demand would change drastically n the arduino, that's why I was reluctant to alter the code. Edited 2021-08-06 18:29 by bob.steel |
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| bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188 |
Don't let it concern you. I'm looking at all options for a permanent solution and 20 off them so I find it interesting to look for what I consider to be the best approach . Don't let me keep you here . It might help reduce the size of the thread considerably if you would delete your huge pictures . I have one of the units depicted in your last and it is too expensive . Simple as if you read the rest of the thread. Thanks . |
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| bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188 |
No I was refering to the guy that sleeps beside her . She's waking up beat! Here a tinnie is an alluminium dingy used to access the reef for huge coral trout and they are often found lost and forelorn floating out at sea . Nobody knows where they are trying to go. |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Bob. I'm a bit concerned about you putting a resistor in the input. The base needs a resistor but the emitter is connected directly to the Arduino pin. I did try to send a circuit but my scanner refused to scan then it did but now I have forgotten how to add drawing. Why me? Peter |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7830 |
hehe... These things happen, Peter. Correct - the emitter has to go directly to the arduino output. With this arrangement you want it to swing the full voltage range really. The current into the arduino pin is the base current + the collector current. Both are small because they are limited by the base and collector resistors so it doesn't matter. The arduino takes the same current whether an output is low or high. They are "totem pole" mosfet outputs so one of them is conducting all the time anyway. It's the load that decides if any current will be drawn from the supply. Keeping the LED mosfet gate pulled down to keep it off will draw a few mA from the 28v supply. Using the figures I gave previously, 5mA will come from there and 250uA will come from the arduino 5v rail. This circuit needs to be tested away from an arduino! I don't want 28v to get to the arduino pin under any circumstances. That will depend on the c-b diode of the transistor junction. If that breaks down the arduino output will release magic smoke. The transistor spec must be checked and tested before an arduino is risked. A BC547, for example, should stand 50V on the c-b junction so that should be ok. It depends on what's available. :) Edited 2021-08-06 19:29 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Bob. I've been reading and rereading your reason for not changing your code. I'm not sure what you are saying but an Arduino will draw the same current no matter what the output pins are at. Can I also say that modern micro's are very nearly indestructible. Excess voltage is probably the only way to kill one. I have had them almost glowing red...well almost. Mick's concern is fair enough but I think all modern transistors will support 30V c-b. But that does relate to my argument for isolation. Good luck. |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day All I have spent the day working out how to attach a drawing to a TBS post. Then finding my copy of TURBOCAD. Then working out how to use it. I don't know if this is what Bob has got and I'm not even sure it will work but for what it is worth. And now I know how to do it until the next time.  |
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| bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188 |
Thanks Peter I'll give it a go soon. |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
The bootstrap circuit will exceed the 20V rating for Vgs of the IRF540. It might be a good idea to connect a 20V zener from gate to source. Peter Edited 2021-08-07 16:29 by PeterB |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2589 |
For a 1Hz flash rate the capacitor will need to be much bigger, 10uF might be enough. And don't forget that zener. You have tried similar bootstrap circuits before, so we are going around in circles. |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
Definitely. VGSth is max 4V (so this is worst case requirement to bottom-out the trans-conductance curve i.e. for a good low RDS). Zener ratings are nominal and if you get an unlucky combination of a high zener and low fet, it could be enough to destroy the gate insulator. It isn't likely but 20V is too close for comfort in my books. Also I would like to protect the Arduino from any 27V spikes when Q opens up. I would chuck something around 5V in there. We all have loads of 4V7 and 5V1 zeners kicking about right? An opportunity to use up more of that old stock. jus' sayin' Edited 2021-08-07 17:34 by CaptainBoing |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6268 |
Wouldn't that result in a leakage current through the load? Gate to transistor collector is where I would put it. Jim VK7JH MMedit |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
I have started something haven't I. Phil, the only discharge path for the 0.1 is leakage once the zener has got the voltage down to 20V. From then on it's anybody's guess. I have no idea what the required flash rate is so it's all a bit of by guess and by God. CB, going for 5V would mean wasting all that stored charge. I suspect Bob has been using this circuit without a zener and with no problems but it is all a bit iffy. Jim, Once the voltage has dropped to 20V there is no more leakage but I agree this is not an elegant solution. I was hoping one of you would compliment me on my very beautiful schematic. You blokes have no style. Peter Edited 2021-08-07 18:00 by PeterB |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7830 |
I will certainly compliment you, PeterB, being a CAD fan myself. :) Got real style me.... And nanoCAD. :) I've never used a bootstrap circuit with a mosfet so I'm not qualified to figure this one out. Personally I'd do anything to avoid it. I don't like circuits that could be marginal in operation. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| bob.steel Senior Member Joined: 27/02/2020 Location: AustraliaPosts: 188 |
Top is 20v so Ill go with 16 or less. Maybe try 5 v too. Left the 1 hertz some time back . Its White on for about 3 secs then 4 flashes of red in 1 sec then 4 flashes of blue then white back on . When I get something that works and I can see the effect I'll play with that and may leave the white on full time or drop it to half current maybe . Dunno until I get there. Well I will . Any schematic I can read is good for me and that one is very clear . Just wondering in the light of whats been said what I should actually use for a circuit.? Edited 2021-08-08 00:32 by bob.steel |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
This is good to know. I have a bunch of both the AC and DC models and was unsure if they were trustworthy. I have a small low-budget project and so I think I'll take the risk  |
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