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  • Writer's pictureGustav (PCB Grinder)

POWER-UP (work in progress)

Updated: Nov 19, 2020

The bulk of problems Ive seen over the years with DIY projects are

  1. Bad/rushed soldering

  2. Supply voltage discrepancies

In this post, Ill try to address them, so you have the best odds for a smooth power-up of your freshly built unit. (This is a work in progress, and I will be adding some pictures for illustrative purposes).

1. Bad/rushed soldering

To avoid bad/rushed soldering, its best to start by being careful from the beginning of your project, rather than rush it. If you are patient, even as an absolute beginner, you will quickly get the hang of it (there is a "how to solder" post in the blog here).

If you already rushed it, and have blobs, cold joints, and shorts all over the place, reflow all the joints again before moving on to power-up, and use a solder sucker to get rid of excess solder/shorts.

Tip tip! A lot of inexperienced builders get frustrated when trying to reflow joints, but the trick is to add a little tin to your tip, even if you are trying to reflow to remove solder. The flux, which helps the solder flow, is gone from the settled joint, so youll need a fresh dap of solder to get it moving.

(I will insert examples of bad/good solder joints) 2. Supply voltage discrepancies The best way to make sure your supply lines are right, is powering up in stages, starting with the power transformer. There is a previous blog post on how to wire up the transformer here. After the Power Transformer The power transformer puts out alternating current, so set your meter to AC, and measure the output on the secondaries before connecting the transformer to the circuit. Most DIY projects call for a 2x15V center tapped transformer configuration, so in most cases, you are looking to see 15VAC from center to either side, and 30VAC from side to side, before you connect the transformer to the circuit. (I will insert a picture doing the measurement).

When the transformer is connected, do not install active components! (ICs/VCAs/transistors, if socketed) until the end of this guide.

If you are not seeing the expected voltages.

  • Check wiring of the transformer

  • Check rating of the transformer

  • Check measurement device setting (AC/DC).

After Rectification

The first thing we do is convert AC to DC (rectification), and this is done with 4 diodes, usually packaged in a round diode bridge for this common purpose.

Putting it simply, AC is measured in RMS, while DC is measured in peak, so as we convert our supply from AC to DC, the voltage measured will be different by a factor of about x1.4 (square root of 2).

For the purpose of powering up your circuit, you dont need to understand the maths or physics of it, but you need to know, that 15VAC corresponds to about 15VAC x 1,4 = 21VDC, which will actually be slighty lower, because there is a forward voltage drop/loss over the diodes in the rectifier. None of this is expected to/needs to be 100% exact. Your 15V trafo may put out 16V, and you may end up with a different DC reading relatively. What you need to do here is to make sure, you are in the general territory of what you expect to see, and relative to the next stage, we need about 3VDC of excess to start regulating properly.

If you are not within a few volts of the expected readings

  • Check transformer hook-up

  • Check for shorts in circuit supply

  • Check for components with wrong orientation

  • Check measurement device setting (AC/DC).

After Regulation

The most commonly used regulators in DIY, are the LM78xx and LM79xx type. At this point, the current is direct (DC), and can be either positive or negative. the LM7(8)xx types regulate positive voltage supplies, the LM7(9)xx types regulate negative lines. The 3rd and 4th digit indicate the voltage the regulator aims to spit out, so an LM7815 regulator aims to spit out +15 Volts of direct current (15VDC). To check if the regulators supply voltage output is correct, set your meter to measure DC, put one probe on the output pin of the regulator, and the other probe on the ground reference (center tap of the trafo secondary will work as reference, if you are unable to locate a ground plane on the PCB)

Tip! You may be thinking "but how do I know which one is the output pin"? You can always check the data sheet for a given component, but that may be a little confusing, so here is an easier way. Go to google image search, type in the name of the component you need to know the pin-out of, followed by "pinout", and you should see something you can use as a reference, i.e. "LM7815 pinout"

If you are not relatively close to the expected output reading

  • Check for shorts in circuit supply

  • Check for components with wrong orientation

  • Check for orientation of regulators

  • Check for type of regulator

Active Components

Now, we can start placing the active components, ICs, VCAs et al.

If you can muster the patience, I recommend placing one, powering up, checking the supply lines, powering down, placing another, powering up, and checking the supply lines, one by one. This will guarentee that you instantly know which one, if a single IC malfunctions.

If adding the active components mess up the readings on your supply lines.

  • Check IC socket for correct supply voltages with IC removed*

  • Check for the type of IC

  • Check orientation of IC

*Tip! You may, again, be thinking "how do I know what to measure", but you can use the same "trick" as before. Go to Google image search, type in the name of the component, followed by "pinout", and you should see something you can use as a reference, i.e. "5532 pinout"

The pinout only shows which pin we expect to receive the positive and negative supply voltages (on the 5532, thats + on pin 8, - on pin 4), so the actual values depend on the supply voltages coming from your regulators.

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