Electronics Tips and Tricks Thread

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Thread Starter


Joined Nov 9, 2007
It has occurred to me that a thread where posters could pass on useful tips, tricks, rules-of-thumb, and conventions that may be of use for when you need a 'how to answer' not an in depth understanding. Please don't use this thread for asking questions.

I will kick off with a very simple but useful formula for making a non standard resistor by adding a parallel resistor to a standard one.

\(R_{1} = \frac{R_{2}R_{d}}{R_{2} - R_{d}}\)

\(R_{1}\) is the required shunt resistor

\(R_{d}\) is the desired non preferred value

\(R_{2}\) is the (next) higher preferred value
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Joined Jul 17, 2007
In retaliation for StudioT's post... ;)

Attached files:
1) Series_ParallelResistanceCalculator.zip - contains an Excel spreadsheet that has StudioT's formula in it, and also does series of lookups on standard E6 thru E192 series resistance tables to ensure use of only the standard values in the results. Just put in the resistance you want where indicated, and resistor combinations for all E-series are displayed.
2) E24PLL.ZIP - contains E24PLL.TXT, a rather large (224kb) text file that you can scroll through to find pairs of resistors to use in parallel to obtain a particular resistance.

Here's a handy web page that does both series and parallel calculations for you, for E12 and E24 series resistors:
Note that you must select E12 or E24 each time you enter a new value, or you will receive an error message.

3) CapSMTMarkings is an excerpt from a KEMET Packaging and Marking document, for identification of SMT capacitor values.
If there are two letters and a number, the manufacturer is the first letter. If just a letter and a number, only the value and multiplier is specified.



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Joined May 16, 2005
Rule of thumb for calculating power supply filter capacitors:

C = 0.7(I)/ΔE(f)

Where I = load current, ΔE = acceptable ripple voltage, and f = pulses per second from the rectifier.

For full wave rectified 60Hz, this works out to:
C = 0.00583 * I / ΔE


Joined Nov 4, 2008
If any of you can get a copy of the November 2008 QST, I have an article in Technical Correspondence called "Making the Glass Half Full." I discuss in detail the reciprocal impedance parameters: conductance, susceptance, and admittance. These GREATLY simplify otherwise daunting parallel impedance problems.




Joined Nov 28, 2008
If your 9V battery connection is too short for the power supply, why not try the following and kill your project.

Mod Note:E
Caution if you extend a power lead in this way,
ie: by using two identical button type connectors, the polarity of the supply to your project will be reversed, which will no doubt damage your project...

You won't need to power it up anymore.


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Joined Jan 9, 2009
When designing power supplies be sure to size the input protection fuse so that the equipment being power blows up first so as to protect the power supply fuse from blowing open first.

Er... aren't fuses meant to protect circuitry? I think you meant so that the 'fuse' blows up first to protect the circuitry and not the other way around.

On the topic of fuses, make sure your fuse is placed as the first part of the system, before any surge protection. I recently had someone put 415VAC across a design intended to only allow 300VAC, so the VARistors popped, but the fuse downstream from the VARistor didn't, so then 600+VDC creeped into the circuit unimpeded. In reality, when the VARistor started to suck large amounts of current, the fuse should have been upstream so that it got a chance to blow from the VARistor current.

Thread Starter


Joined Nov 9, 2007
Er... aren't fuses meant to protect circuitry?
No, the primary function of fuses and other disconnection devices in mains equipment is to avoid electric shock to users from faulty equipment.

The secondary function is to avoid electrically caused fires to property.

Protection of the equipment itself is only a tertiary consideration, and a bonus if that can also be acheived.


Joined Feb 27, 2009
this one helped me when I first started learning about transistors just passing it on
when looking at a schematic of a transistor the arrow always points to the n device
this is true in fets also

Thread Starter


Joined Nov 9, 2007
Please this thread is meant to be a list of Tips, not a discussion thread.

Here is another one.

π does not appear in plane formulae.
1/2π appears in cylindrical formulae
1/4π appears in spherical formulae.

For example

The field E above an infinite plane charge density Q,

E = Q / ε

Field near a long line, linear density Q

E = Q / 2πεr

Field surrounding a point charge

E = Q / 4πεr\(^{2}\)


Joined Jan 9, 2009
Thermal time constants of resistors de-rate them under duty cycle. A 5W resistor at continuous might be thought to allow 50W at 10% duty cycle (i.e. 50W pulses). This is incorrect! Generally the de-rating at 10% would allow more like 10W pulses and not 50! The internals of them heat up much faster than they can dissipate it into ambient!

I recently got 'burned' on this. (Pun intended.)
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