# How can I build a battery load tester that switches between 0.8 and 2.0 ohms? [closed]

I would like to build a load tester that switches: 2 seconds at 2.0 ohms and 1 second at 0.8 ohms and back to 2.0 ohms to repeat.

I gather that I can have a circuit with two resistors and a switch between them or between one and both.

What kind of device can switch in this way?

## closed as too broad by Brian Carlton, Andy aka, laptop2d, Dmitry Grigoryev, brhansJun 27 '18 at 20:54

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• The switch is called "MOSFET". – Ale..chenski Jun 22 '18 at 2:51
• What voltage are you expecting to apply to this load? – WhatRoughBeast Jun 22 '18 at 4:16
• @WhatRoughBeast - Approx 4 volts. – B Seven Jun 22 '18 at 14:18
• A relay can also be used as a robust switching element. – KalleMP Jun 25 '18 at 20:04

Sure thing! Assuming DC voltage input. This schematic did not turn out beautiful, but should do the trick.

simulate this circuit – Schematic created using CircuitLab

• To be clear for the OP, there are many other MOSFETs you could use here but you need to check the datasheet for (a) what voltage you need to apply to the gate to get it to turn fully on (for the IRF530 it's about 10 volts) (b) what the drain-source resistance (Rds) will be (for IRF530 about 0.16 ohms). – nekomatic Jun 25 '18 at 12:51

Use a 2 Ohm load with <0.1 Ohm switch and a keep motor on while switching another R of 1.33 ohms including FET on/off to obtain your test result. Use “very” short probe and ground leads to measure scope results to avoid >10Mhz noise.

Use a CMOS relaxation Schmitt oscillator with RC=1/2f approx. and bias input to skew duty cycle as desired which also raises f slightly using a larger R to V+ or gnd with Vdd =>3x Vgs(th) for low RdsOn to drive low side of R load with Nch FET.

One can also use NPN power Darlingtons but R values must be computed for correct currents and proper power ratings with heatsinks as required.

• I don't have enough knowledge to use this. Can you provide some links, references or tutorials? – B Seven Jun 22 '18 at 14:20