# Arduino PCR (thermal cycler) for under \$85

## Step 5: The circuit- resistors

Split and strip the the power adapter as shown in the first image. Hook up the resistors as follows:

Black wire -> pin 1 (AC pin) on the relay
White wire-> one of the resistors. Connect the second resistor in series. Connect the second resistors to pin 2 (AC pin) on the relay.

Connect arduino to the relay
Arduino pin 7 -> pin 3 (+DC pin) on the relay
Arduino GND -> pin 4 (-DC pin) on the relay

Cover all connections w electrical tape!!!

---- some math ---
With 2 150ohm resistors in series, the total resistance should be 300 ohms. So with the U.S. outlet voltage being 120V, the current should be 0.4Amps, which makes the wattage 48Watts. The resistors are rated for 50 Watts, so this should be OK, but please double check the math, especially if your outlet runs on 230V (you'll need different resistors with higher Wattage).
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makerbuilderbaker says: 1 year ago
Although those wattage calculations are correct for 120 volts DC, they might not work so well with 120 volts AC, which is what you are giving them. As you probably know, Ac voltage in the states oscillates from negative to positive at about 60 hertz. While we think of our AC power as 120 volts, it is actually higher and lower. 120 is the RMS voltage (the root mean square http://en.wikipedia.org/wiki/Root_mean_square). This means that the "average voltage" of the entire sine wave is 120 volts. This is calculated by taking the peak voltage of the sine wave and dividing by sqrt(2). See this for details: http://en.wikipedia.org/wiki/Alternating_current#Example

If the RMs is 120, the peak voltage is 120*sqrt(2)=169.706

so for a large fraction of the operating time, the resistors will be getting above 120 volts, all the way up to 170 volts.
I=V/R I=170/300 I=.56
P=VI P=170*.56 P=68 watts

Although you are not operating at 170 volts all of the time, you are exceeding the wattage rating for a significant amount of time every second. I sincerely suggest you use 100 watt resistors. These will protect you from peak voltage as well as any power surge/ripple.

Besides that, I love the product! I am currently working in a bio lab with fancy thermo cyclers and they are essential to our work.

I am sure you can large wattage wirewound resistors on the internet. Good luck.
steveastrouk in reply to makerbuilderbaker1 year ago
This is wrong.

The RMS value is THE SAME HEATING value as the equivalent DC. FORGET the peak.

Your numbers are completely wrong. We cannot talk about "peak power", there is no such thing.

Steve
staceyk (author) in reply to makerbuilderbaker1 year ago
Hey, thank you so much for this, really appreciate it!! If I stick with the size that fits the block, I might use 2 of these in series, 300ohms @ 50 watts.

I = 170/600 = 0.28A
P=170*.28 = 48Watts (at peaks)

Does that seem right?
makerbuilderbaker in reply to staceyk1 year ago
Your math is correct, and those new resistors will be much better suited to the task.
However, to guarantee proper operation and long life, you might want to go even higher. Resistor wattage ratings are usually absolute maximum ratings which can be tolerated for only a short amount of time. With 300 ohm resistors, you go to 96% of the absolute maximum it would likely receive. In the name of caution, I only use resistors at up to 1/2 to 2/3 their rated wattage capacity. Depending on price and availability, I would recommend a total resistance of more than 750 ohms, this will place you below 40 watts at peak.
A few reasons for this caution are outlined below.

The tolerance on your wattage may be several percent, so your capacity could be from 47.5 to 52.5 watts with a 5% tolerance. Unfortunately tolerance on wattage is less prevalent than tolerance on resistance.
Also, grid surges and ripples do occur and you want to build your device to handle the worst case scenario.

It is good practice to err on the side of caution and never trust your components.
XTL in reply to makerbuilderbaker1 year ago
For 230V countries: Peak voltage = 325 V

For power of 40W - implies 120mA current
Resistors would be 2.6 kOhm.

Using preferred values:
- two 1500 Ohms resistors (choose devices with 25W or greater rating. Preferably 40W)
- gives us 110mA and 35 Watts.

or: - two 1200 Ohm resistors (choose devices with 30W minimum ratings. Preferably 40-50W)
- gives us 135mA at 44 Watts.

Use an osPID to control the SSR driving the load.
Use a cheap (low temp 400C) k-type thermocouple to connect to the osPID controller.
steveastrouk in reply to XTL1 year ago
FORGET IT. RMS values are all you need to consider.