#Power transistor tester free#

The heatsink should be designed so that transistors can be mounted and unmounted easily and quickly, or the task will become a chore very quickly. D5 is used to suppress the relay coil's back-EMF when the button is released. When SW2 ('Test') is pressed, the relay activates and applies power to the test circuit and the DUT. RL1 is a relay, typically with a coil resistance of around 270 ohms and with a contact rating of at least 10A at 12V. The internals of the tester should be electrically floating.

Thermal compound (aka 'thermal grease') is essential, and the heatsink should ideally be insulated from the case because there is a polarity switch. The regulators need a good heatsink, but since the ground pin (pin 2) is connected to the case, the two regulators can be bolted directly to the heatsink. This ensures that the test voltage is stable, and not influenced by mains voltage variations. Ideally, you'd use the regulated version of the supply. It won't change anything other than reduced supply ripple, which is of no consequence for this kind of test anyway.

#Power transistor tester free#

Feel free to increase the capacitance if it makes you feel better. A 100VA transformer will be more than sufficient. I suggest that the power transformer should be fairly substantial, or excessive voltage sag will be experienced during testing.

The 1N5404 types shown are suitable for an output current of no more than around 6A. The bridge rectifier needs to be a high current type, and given that 35A bridges are quite cheap, this is a good choice. Normal variations will not cause large errors, and the final result of 'matched' devices will still have some variations - finding two identical transistors is not normally expected or obtained. Mains voltage variations will affect the accuracy of measurements. The power supply should ideally be regulated, but if you don't have one, an unregulated supply will still work. The meters will most commonly be digital, and a current range up to at least 2A may be useful (but not essential). Ideally, you will have two multimeters, but one can be used if you don't have two. WARNING - The rated maximum source to gate voltage of MOSFETs must never be exceeded. However, it is more complex than the circuit described here. It uses a selectable emitter resistor to determine the current, and can provide very accurate results. This is a constant collector current h FE tester for transistors, and because it uses constant collector current, it allows for more accurate matching. The supply voltage is deliberately quite low (about 12C DC) so that SOA problems should not occur.Īn alternative to the method show here is described in Project 177. MOSFETs are a little more forgiving, but will still fail if pushed too far. You must also be aware of the safe operating area of the device, since exceeding this will destroy BJTs very quickly. You will also need a fairly substantial power supply if you want to test at high current. For these tests, using mica washers is out of the question (too time consuming), so the heatsink will be at the potential of the collector (BJT) or drain (MOSFET). Simpler actually makes it more complicated for high current tests, since you will have to mount the transistor before you start. Manufacturers get around the heatsink problem by pulse testing (so the device does not get a chance to get hot), but this requires expensive equipment to do, so a simpler test is needed. Ideally, a quick clamping method will be utilised so the devices don't have to be screwed down each time - this can get seriously time consuming. Bear in mind that some of the tests are potentially destructive if the DUT (device under test) does not have an adequate heatsink, so the provision of a fairly substantial heatsink is essential. This article is not intended to cover all the possibilities, since the equipment needed is out of the range of the average hobbyist. The situation is made a little harder when you have PNP and NPN (or N-Channel and P-Channel) devices, since the simpler comparison (or bridge) techniques don't work due to the opposite polarities. Matching power transistors is theoretically easy, but in reality there are quite a few parameters that should be matched to get true matched pairs. Some retailers sell matched devices, but they are rather expensive, and not commonly available. Whether you use Bipolar Junction Transistors (BJTs) or Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), there are many circuits that suggest (or require) matched transistor pairs.