Capacitor Discharge Pen 5,000 µF 1000 V - Recommended "Bit of Kit"
- Paul Taylor
- Apr 30
- 3 min read
Updated: Apr 30
A quick review of a handy and convenient safety device from AliExpress. I paid about US$4.
The item is available in two different capacities. I purchased the slightly more expensive version that is capable of discharging larger capacitors, both in voltage rating (1kV) and in farad rating (5 mF).
Not costing much more money than a flat-blade screwdriver - it's much safer for you, the screwdriver, and the capacitor. Did you know capacitors can be damaged by a sudden short?
The device is meant to reduce the voltage in the capacitor by dissipating the stored electrical energy in a safe manner. The device is to be held by the handle region, while the two spring-loaded gold-plated pogo pins applied to each terminal of the capacitor. A thick transparent shrink-wrap helps protect you in the case your fingers aren't at the optimum position of the stick.
NOTE: The device is only for discharging electrolytic capacitors. Batteries are specifically prohibited.
The unit's terminals are not polarized. A bridge rectifier redirects the voltage internally regardless of way the probs touch the capacitor. Guided by the control circuitry, the energy is "dumped" into a IGBT (part number H25R1203).
During discharge, a red LED lights to indicate discharging is in process. After the LED goes away, the capacitor has been brought down to about 7 volts. Afterwards, the voltage of the capacitor is safe for handling. Unfortunately, you'll need to make sure you have the unit flipped to the proper side to see the indicator - there isn't one on the back side.
The remaining voltage may still be dangerous to sensitive test devices, such as a component tester, capacitance meter, or LCR bridge. A different final discharge method will be necessary to bring down the voltage even further. I recommend a 100 ohm resistor, which will continue the discharge from 7 volts using a similar 70 mA current which the device uses.
How do I know the current the capacitor discharge pen consumes? Well I tested it of course! Here you can see that the device is drawing 75 mA at higher voltage (I assume this constant current goes beyond my test which stopped at 40V). One reason the device can't take the voltage the whole way down to zero is the multiple semiconductor junctions of the discharge circuitry, each of which requires sufficient voltage to initiate current flow.
The probe pins can be repositioned in and out, as long as the screw isn't tightened too much. This enables the touch-points to match the capacitor leads. However, as supplied, moving the joints can loosen the screws. I added a low-profile split-ring washers underneath each of the screw heads, and now repositioning can be repeated without loosening.
I really like this item. The constant current discharge through a high performance IGBT is safe for capacitors - and for me. Far superior to using resistors, which are constant load not constant current.
I have this new unit proudly hanging on a peg board above my workbench. By the way, I also have other form factor capacitor discharge units - one of which I may feature soon which has a digital display. This one is self-powered, probes are always attached, and covers a wide range of voltages. So I'm going to grab this one most often, I predict.
Safety devices should always be handy so you are more likely to use them.
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