# DIY Peak Volt circuit alternative

Discussion in 'General Electronics Discussion' started by caribet, Dec 20, 2014.

1. ### caribet

6
0
Dec 20, 2014
I want to make up this little circuit as shown in the image. However I don't have on me a 22mF 400v capacitor. I only have a 47mF 450v capacitor. Can I use this capacitor in its place or would I have to change the resistor too. In addition I am only expecting voltages up to 300v. Your help is much appreciated.

(don't know a lot about electronics so could someone explain/confirm if the resistor is in the circuit to slowly bleed off the charge of the capacitor. If that is the case, would using a 1/2 Watt 1M ohm resistor just discharge the capacitor faster? )

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2. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Hi there and welcome to Electronics Point

I guess when you say "mF" you actually mean microfarads, µF? Because mF means millifarads and a 22 mF capacitor is 22,000 µF! You need to be careful about these things.

A 47 µF capacitor will work instead of 22 µF. If you want to keep the same discharge behaviour, you need to reduce the resistor in proportion to the increase in capacitance, so it would reduce from 1 MΩ to (22/47)= 470 kΩ.

The power dissipation at 300V will be less than 0.2W with a 470 kΩ resistor, and less than 0.1W with a 1 MΩ resistor, so a 0.5W resistor will be suitable. 300V is quite a lot of voltage to have across a resistor so you could split the resistance into two resistors in series, with each resistor being half the total resistance.

I would use a 1N4007 for that diode. It has a 1000V maximum reverse voltage. A 400V diode's reverse voltage specification will be exceeded if you're feeding AC into it.

Be extremely careful with that circuit. The voltages and currents there can easily kill you. Make sure it is held firmly in place so it can't move and touch you. Don't connect free flying leads to it or use alligator clips - always use secure terminations. Keep one hand in your back pocket if you work on it live. If you didn't already know those precautions, research electrical safety on the web first!

3. ### caribet

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Dec 20, 2014
I need measure some peak voltages in the ignition system for my boat Outboard motor. Do the same safety issues apply?

4. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Yes. A 47 µF capacitor charged to 300V is dangerous. If you connect yourself across it from one hand to another, so current flows through your heart, it can kill you. This is one reason why electricians work with their left hand in their back pocket (assuming their heart is on their left - some people apparently have it reversed!)

There's some good general safety advice at http://www.repairfaq.org/sam/safety.htm but he doesn't cover any special safety considerations related to boats or motors.

Can you do your work on dry land? A constantly moving floor would just compound the existing dangers!

5. ### caribet

6
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Dec 20, 2014
Is a 22 microF much less dangerous? Testing is on land

8,393
1,271
Nov 28, 2011
7. ### caribet

6
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Dec 20, 2014
Does the capacitor have to be fully charged to 47micro farads before the peak voltage will be shown on multi meter or is peak voltage shown before capacitor is fully charged?

8. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
The capacitor will charge up to the peak voltage fairly quickly, depending on the characteristics of the signal that is driving the circuit. The capacitance doesn't make a great deal of difference; it determines how quickly the capacitor will discharge between peaks, and when the input voltage is removed.

If you're using this to monitor voltages in an engine ignition system, you will need to use a faster diode - for example a UF4007. The pulses are too steep for a 1N4007. It's designed for mains applications.

47 µF is unnecessarily high and the engine may be hard to start because it could take a while for 47 µF to charge up. Something like 1 µF would be better. You can stick to the original 1 MΩ resistance; it's not important.

9. ### caribet

6
0
Dec 20, 2014
I put together a new circuit with the UF4007 diode, the 1 micro Faraday capacitor and the1/4 Watt 1M ohm resistor. I am getting unstable voltage readings though. I am not sure if it is the motor providing unstable voltage or if the capacitor is just discharging too quickly with this resistor to enable a steady reading. Same thing with a 2.2 micro Farad cap. If this is the case, any suggestions as to what resistor I should use?

10. ### duke37

5,364
769
Jan 9, 2011
If you have a mechanical switch (points) then I would expect that the pulse height will vary. If you can see the points, then the spark moves across the points.

A transistor switch should be much more consistent.

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11. ### caribet

6
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Dec 20, 2014
I am not sure what I have. I just connected the circuit to the pulser coils on my outboard just under the flywheel.

12. ### Rory Starkweather

77
7
Nov 13, 2014
Can you get another 47 uF capacitor, preferably of the same type?

If so, what about connecting two in series? For capacitors that should be equivalent to putting them in 'parallel', which would give you approximately 27uF of capacitance. And it might have some additional benefits.

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