Circuit behaves completely differently on Veroboard from Breadboard? - Page 1
Forum section: electronics / beginners

author: thejoggingmat
author_rank: Regular Contributor
timestamp: August 15, 2015, 11:52:47 am
content: Hi, so I transferred a circuit from breadboard to veroboard.Schematic:It works on breadboard as expected. However, the moment I transferred to veroboard, everything changes.The circuit acts as if the PWM stuff didn't exist and the LEDs drive at full brightness. Measuring the PWM frequency read 0Hz (not PWMing). The 555 timer heats up and smokes after ~1min. I replaced the 555 with another few and it just heats up quite quickly).I have:-Redone the same circuit on 3 veroboards: Same problem-Used DMM's continuity to 'reverse engineer' to check the circuit is indeed correct: No problems-Checked if there are any shorts where there shouldn't be: None whatsoever-Check all passive (R/Cs) components are in spec: No problems-Removed Q1 & IC1 (TIP41C/555 respectively) & planted on the breadboard (obviously I used the non-smoked 555 & threw away the spoilt ones): Working, no problems-Planted a new & working (in the breadboard) set of TIPs & 555s into the veroboard: Not working.What is the matter with this circuit?Veroboard circuit:

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2015, 12:09:12 pm
content: Firstly, get rid of C3. All it will do is make Q1 dissipate more power than necessary.Check the connections, especially that there are no unintended connections resulting from bits of copper strip which haven't been properly cut.

author: c4757p
author_rank: Super Contributor
timestamp: August 15, 2015, 12:11:03 pm
content: Way more power, actually, and I'm somewhat suspicious of the pulse power dissipation in that transistor (which is now connected solidly via solder rather than having the relatively poor connection of a breadboard between them).

author: Andy Watson
author_rank: Super Contributor
timestamp: August 15, 2015, 12:15:22 pm
content: First thing that leaps out is:- there is no decoupling of the power supply - 555s are sensitive to glitches on their power supply.C3 ? What is it doing? May be this was supposed to be the power supply decoupling ? If so, I would add a parallel 0.1 across the supply too. At present, C3 appears to add extra stress to Q1 whilst maintaining full brightness on the LEDs.I don't see a series resistor to limit the current in the LEDs - could be on the reverse of the LED panel? If not - you need to limit the current.R2 - check that the resistor that you have used is actually 470 and not 47.

author: thejoggingmat
author_rank: Regular Contributor
timestamp: August 15, 2015, 12:23:57 pm
content: Quote from: Andy Watson on August 15, 2015, 12:15:22 pmQuote from: c4757p on August 15, 2015, 12:11:03 pmQuote from: Hero999 on August 15, 2015, 12:09:12 pmBodging C3 was added as a last resort as it was the only solution that quietened down the LEDs (seehttps://www.eevblog.com/forum/beginners/irritating-high-pitched-noise-with-555-pwm-driving-leds/msg717459).The supply (3x18650s) were already decoupled at the lipo protection circuit module side (not pictured); I used a combination of 10n and 1µ for each 18650 (with protection module) to be extra safe.Checking for unwanted connections was one of the first things I did (see first post).

author: Chris C
author_rank: Frequent Contributor
timestamp: August 15, 2015, 04:14:41 pm
content: Quote from: thejoggingmat on August 15, 2015, 12:23:57 pmYou're intentionally switching 800mA. That part you know about.This part you might not. Each time the 555 changes output state, it also briefly consumes 200mA. Basically, it shorts the pull-up and pull-down transistors on its output together ("shoot-through"), so this consumption occurs whether you're using the full 200mA rated output or not.Either of these transients can cause the voltage differential between Vcc and ground to momentarily collapse, if not properly decoupled. The decoupling capacitors should be as close to the point where large currents are used, because all the wires and traces leading up to that point have a tiny bit of resistance and inductance. This becomes significant when working with large currents, even just the 555's own shoot-through; preventing their stored charge from quickly flowing to where it is needed. The decoupling caps on the batteries are too far away to be very effective, connected by relatively long wires. And too small as well.So what happens when you don't decouple properly, and the Vcc-to-ground voltage collapses? The 555 generates reference voltages for 2/3rds and 1/3rds of Vcc via a simple voltage divider. And then uses a comparator to compare the timing cap voltage against its reference voltages. With Vcc collapsed, the reference voltages change, and it can false trigger, then retrigger almost immediately after Vcc rebounds. Which means it's possible it's consuming 200mA almost continuously. This can be up to 12V*200mA=6W, and you can see why the 555 is heating up.The wiring on your breadboard version was by luck, just good enough that this didn't happen. Or perhaps you were using a DC power supply with larger internal caps, rather than the batteries. Whatever the case, you need to add that decoupling! Try temporarily connecting the + lead of the 1,000uF capacitor to the + lead of the LED. Also put the largest ceramic cap you have (preferably at least 0.1uF) parallel to that electrolytic cap. Nevermind if it whines, just see if it solves your immediate problem. If so, then you know the only issue is decoupling, and you can work towards a more permanent and elegant solution.

author: Gyro
author_rank: Super Contributor
timestamp: August 15, 2015, 04:19:04 pm
content: Nooo, It's come back to haunt mehttps://www.eevblog.com/forum/beginners/irritating-high-pitched-noise-with-555-pwm-driving-leds/Please see and review advice on 1000uF cap and Supply decoupling in your original thread.