Electrolytic Cap or not?

[angie1199]

Hi Guys

Spent all day working this out. Finally got it working then found I didn't have a normal capacitor for C5. I've tested this in TINA and it 'appears' to work but is there a reason why I shouldn't use an electrolytic cap instead of a normal cap?

**broken link removed**

Angie

 

[dknguyen]

Your question is like asking "Should I get a red car? Or should I get a normal car?"

An electrolytic capacitor is just one of the many types of capacitors. Namely- other capacitors have better frequency characteristics and stability. but are only available in small sizes (sometimes very small sizes). For the types where larger sizes are available, they are EXTREMELEY expensive they don't even come close to the mid range of sizes possible with electrolytics. The chief advantage of the electrolytic is large capacitances and the low cost you can get these capacitances for.

 

[bychon]

Electrolytic capacitors can be a little leaky and inaccurate in a critical circuit, but your application is not a critical circuit. Go for it.

 

[angie1199]

Your question is like asking "Should I get a red car? Or should I get a normal car?"

An electrolytic capacitor is just one of the many types of capacitors. Namely- other capacitors have better frequency characteristics and stability. but are only available in small sizes (sometimes very small sizes). For the types where larger sizes are available, they are EXTREMELEY expensive they don't even come close to the mid range of sizes possible with electrolytics. The chief advantage of the electrolytic is large capacitances and the low cost you can get these capacitances for.

As I know very little about electronics I didn't know the difference between the two.

The original schematic showed the non-polarized symbol, thus my question.

Angie

 

[angie1199]

Electrolytic capacitors can be a little leaky and inaccurate in a critical circuit, but your application is not a critical circuit. Go for it.

Thank you, that's all I needed to know, a short explanation and an answer.

I'm easy to please really

Angie

 

[bychon]

No need to explain yourself. We get so much worse here! You included a drawing that we could read, used whole sentences, and knew what you wanted. No problem**broken link removed**

 

[mneary]

Just a comment on prefixes. Tina might read mF as millifarads. I know this is true with LTspice. 50 years ago mF used to be a microfarad, but computers don't remember this.

Better to call it a uF and remove the uncertainty.

 

[MikeMl]

Independent of the capacitor question, I have some questions about your circuit.

What is the circled thing between the 555 pin 3 and the base of the transistor?

Do you care that your switch shorts the trigger input to ground?

 

[angie1199]

Just a comment on prefixes. Tina might read mF as millifarads. I know this is true with LTspice. 50 years ago mF used to be a microfarad, but computers don't remember this.

Better to call it a uF and remove the uncertainty.

I typed in 1000u and TINA converts it to mF. I know what you mean tho'

 

[angie1199]

Independent of the capacitor question, I have some questions about your circuit.

What is the circled thing between the 555 pin 3 and the base of the transistor?

Do you care that your switch shorts the trigger input to ground?

The circled thing is an ammeter to let me know the current whizzing along that wire. Seems I should have paid moe attention to it as it had over 600mA and it blew the 555

I've added a 5k resistor and the current is now 1.94mA, enough to activate the NPN but not enough to fry the 555. Now to check every output/input for same.

The circuit is from a schematic **broken link removed**. I just intertwined it with my two original circuits to add a delay due to the method of switching being intermittent.

Angie

 

[audioguru]

Since you did not limit the current from the output of the 555 to the base-emitter junction of the transistor then the low current transistor is probably also fried.

 

[angie1199]

Since you did not limit the current from the output of the 555 to the base-emitter junction of the transistor then the low current transistor is probably also fried.

Just tested the npn and it's fine, tho' I did get 100 of them last week due to the number of these circuits I have to build.

I'm just guessing I need a resistor in between the output and the npn. I know very little about this 'lectronics stuff so it's just trial and error here ya know, well with a little logic thrown in

Angie

 

[audioguru]

The max allowed output current of a 2N3904 transistor is listed on its datasheet as 200mA but it performs poorly above 100mA.
It is shown to turn on well when its base current is 1/10th its collector current.
So if its collector current is 100mA then its base current should be 10mA.

Your schematic does not show a supply voltage. If it is 9V then the output high from the 555 as shown on its datasheet is 7.6V and the base voltage of the transistor is 0.9V. then the base resistor has a voltage of 7.6V - 0.9V= 6.7V and its value should be 6.7V/10mA= 670 ohms which is not a standard value. Use 680 ohms.

You don't use trial and error when you see and calculate with the spec's of the parts on their datasheets.

 

[angie1199]

The max allowed output current of a 2N3904 transistor is listed on its datasheet as 200mA but it performs poorly above 100mA.
It is shown to turn on well when its base current is 1/10th its collector current.
So if its collector current is 100mA then its base current should be 10mA.

Your schematic does not show a supply voltage. If it is 9V then the output high from the 555 as shown on its datasheet is 7.6V and the base voltage of the transistor is 0.9V. then the base resistor has a voltage of 7.6V - 0.9V= 6.7V and its value should be 6.7V/10mA= 670 ohms which is not a standard value. Use 680 ohms.

You don't use trial and error when you see and calculate with the spec's of the parts on their datasheets.

Got the 1/10th of 'best performance' and that I should ignore the datasheet when it says max current 200mA.

Supply is 12V. Not having that degree in this stuff, datasheets are somewhat of an enigma with all those numbers. However, since your comment regarding output compared to Vcc I see that both 15V and 5V input lose 1.7V at the output, thus 12 - 1.7 = 10.3V output.

When you say the transistor base voltage, do you mean the base-emiiter voltage? If so I see on the datasheet that the saturation voltage (max) is 0.85V, saturation voltage (min) of 0.65V. So, going for the median, 10.3V - 0.75V = 9.55V. So following on your quote, 9.55V/10mA = 955R. 955 Ohms is not a standard value, use the next upwards, which is 1k. This I'm assuming is the V/I = R formula. As I've never seen it in action I had no idea how it fitted in with circuit design. Now I think I know a little more

I'm no spring chicken, but I do learn fast. However I can't learn from books as those books I have looked at tend to speak in a language that is directed towards those who already understand. I learnt ASP in 3 weeks building my first online shopping site with full checkout system etc. I learn quickly. But I had a friend I was able to ask what many would consider stupid questions, as I needed to know. I also had the internet which had various examples and explanations to draw on. It was then explained in words I could understand.

From your reply, although each part wasn't explained as to how you got each value, I was able to fill in the blanks. I feel I have learned a little more thanks to those who have taken the time to respond to this thread. Seeing the figures (current) in TINA has also helped in linking everything together. I didn't even know how a resistor effected a circuit, or why it was used before.

I wish books for learning electronics were written this basically. Maybe more kids would get back to building things, rather than buying ready made.

Again, thank you to all.

Angie