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Help sourcing components

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Thanks to MikeMI I have this pwm circuit diagram to work to..

Problem is, having made what I considder an acceptable mosfet arrangement I'm left with very little room to place the pwm..


That ringed area is the best space I have, around 20mm square and 3.5mm deep.
I've been looking into making a micro timer circuit and I'm happy I can fit one in there but I've no idea what to buy :(
So far I've found lm, ts, se, & ne 555's, which one will I need?? The polar capacitor, I found what I thought would be the one but after reading about it its a "tantalum" that should be "derrated for saftey"o_O..
All the searching has led me to the conclusion that sourcing the micro components is well above my grasp of electronics!
Can anyone help me make a components list to realise my circuit diagram in micro please.

(Sorry for the huge pics:facepalm:)
 

KeepItSimpleStupid

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Tantalum is just unforgiving with polarity reversal, but it's OK in the role it's being used for. Using voltage ratings of at least 2x the expected voltage is good design practice.

The SE, TE and NE is just the manufacturer. The prefix doesn't matter. The suffix determines the package and other stuff.

To minimize size, you can use SMT components.

The datasheet http://www.ti.com/lit/ds/symlink/lm555.pdf#page=15 suggests something different that the 10 uF capacitor. Two different type of capacitors in paralel.

For the timing capacitor, you would typically use a metalized polyester capacitor which is usually availabe with +-5% tolerences.

This https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 is a weird adapter that would mount a surface mount 555 timer in the SOIC-8 package vertical.

It may or may not buy you some space. Surface mount pitches (space between pins) are all over the place.
 
Thanks KISS.

After climbing out of the Google hole with a list of smd components (and a headache) I thought I should make absolutely sure of the expected voltage to the two capacitors that go to ground, and the 10u that's suggested I should be replacing with two unequal ones in parallel.. do I assume that together they should equal the 10u original value? I want to be sure the '.ic V(tau)=2.3' written on the diagram is the timers working voltage and all capacitors should suit that?
 

alec_t

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I want to be sure the '.ic V(tau)=2.3' written on the diagram is the timers working voltage and all capacitors should suit that?
That is just a starting voltage for Mike's simulation. The cap working voltage must be greater than the supply voltage (7.6V in the sim) by some margin, so I'd suggest at least 12V.
 

KeepItSimpleStupid

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The 10 uF is not a value that sets timing. Mike likely picked something for the simulator.

Usually you follow the manufacturer's suggestion for bypass capacitors in terms of type and value. In MANY cases is something like a 0.1 uF ceramic capacitor. Universally, the requirement is they need to be close to the pin with as short of a lead as possible. Since each type of capacitor has different characterisics, the parallel combination exploits each of them. Sometimes 3 different types are used in parallel.

What the capacitor does is compensate for the minute amount of inductance that exists in the lead to the IC. Without it, intermittent things may happen.

==

Some capacitors have a WV or Working Voltage and a Surge Voltage. 2x the expected voltage in a system is a good design number. The lower the voltage rating, the smaller the size. Really bad things happen when you use a capacitor rated for 10 kV in a circuit that expects 1 V,but for 5V system, you might find, 6, 12, 25 or 50 V rating parts.
 

MikeMl

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The 10 uF is not a value that sets timing. Mike likely picked something for the simulator...
Actually, Mike put it there as a power-supply bypass right across the Vdd and Gnd pins on the 555. I have seen 555's mis-trigger if that bypass isn't there. The simulator doesn't care if it is there or not; the real chip does.
 
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Lucky I just bought a job lot of everything then. Chinese ebay shops are great :D
Managed to get every 0805 resistor and capacitor I'll ever need, a bunch of 555's, smd pots, a diy circuit etching kit, etc ect.. life is good :)
Now all I need is a clue how to use them haha.

While I've got an open thread.. I've another ask that I'm not entirely sure how to phrase correctly, so a layman description ensues.

I've got a small readout chip, sort of a volt/ammeter that reads resistance, battery life, v under load and does the math to calculate wattage too.

Issue is, if I use pwm the readout will jump around. Is there any sort of buffer I can use to spur off my mosfet load side to even out the voltage to the readout?

I'd guess I need something that holds the load voltage constant for a second so the readouts input voltage doesnt drop through the pulsing?
 

MikeMl

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A low-pass filter made from a series resistor and a shunt capacitor would likely do it. We would need to know the PWM rate, and the input impedance of the voltmeter, though. Filtering the current input is going to be a bit tougher.
 
The pwm rate will be used through its full range the pot allows. The impedance.. I've taken a few readings, not sure they're going to help but here they are.
Pic 1 is the battery side, vcc I guess?
Pic 2 is the load side
Pic 3 is from the battery across to the load side
There was no resistance across the earths.
 

MikeMl

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The measurements above were not helpful.

One idea is to use an analog meter movement. It will ballistically average the pwm and present a steady needle.

To read the average voltage across the load, try the addition to the circuit:

22a.png

I'm guessing that the input impedance to the voltmeter is > 1meg?
 
The measurements above were not helpful.
Haha, why does that not surprise me :sorry:.
If impedance isn't resistance I've no idea how to measure it. The volt meter doesn't come with a data sheet, that'd be too easy.

I like the idea of averaging out the voltage to the display, a lot. I'd actually much prefer to see the average voltage! If it works through the full duty cycle it'll be dam near perfect!! I assume it goes past the 3v on the graph, to represent the 8.4v battery through the 1-96% duty cycle?
 

tomizett

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I've nothing useful to contribute regarding the design, but just wanted to congratulate you on a really nice bit of metalwork there... that's the kind of electronics I like!
 
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