What is a Bleed Resistor

A bleed resistor (or bleeder resistor/start capacitor resistor) is a resistor in series with the start capacitor in order to bleed off the charge on the start capacitor. It is important to bleed off the capacitor in order to reset it when the motorcycle engine starts running.

A bleed resistor is also called a bleed wire or bleeder wire. The bleed resistor helps retain the voltage across the terminals of the capacitor while keeping current low enough so that there is no danger of overcharging and damaging the capacitor.

Functions of Bleeder Resistors

In motorcycles, there are bleed resistors in the ignition coil, CDI, and in a few other places. Bleed resistors allow capacitors to be stored for a longer time without self-discharging.

The bleeder resistor in a capacitor is not actually “needed” for the capacitor to retain energy – it just makes everything more reliable and safe. However, the bleed resistor does have other functions. Let’s look at what these might be:

1) Preventing Electrical Shock When Turning off Appliances

Have you ever had an appliance that has a removable power cord? You unplug it, and the whole thing shuts down – you know, like when you pull out a lamp. But sometimes… there’s still current running through them! It can be a shock even though there’s no apparent reason why this should be happening.

2) Preventing Damage to the Capacitor

Some capacitors (not all) bleed a small amount of current even after their power source has been disconnected. This bleed-off may be necessary for proper capacitor functioning, but bleed resistors ensure that these currents are very low and not capable of doing damage to the capacitor. The bleeder resistor provides insurance against this sort of problem.

3) Preventing the Capacitor from Discharging too Quickly

If there is no bleeder resistor, then at switch-off the bleed current will be drawn through the load (rather than through the bleed resistor). This bleed current can flow very quickly and may exceed the safe threshold of any loads. For instance, if you have a circuit with an electrolytic capacitor with a bleed resistor, it’s less likely to trip breakers or blow fuses.

4) Providing an Extra load to Discharge the Capacitor Through

A bleeder resistor provides you with electrons which can help bleed off excess energy from the capacitor. This also serves as an auxiliary load which ensures that most of the current is drawn through the bleed resistor and not through your expensive electronic gadgets!

Test a Capacitor using Bleeder Resistors

1) Locate the bleeder resistor near the positive terminal of the battery or power supply. A bleeder resistor is designed to bleed off an excess charge from a capacitor in order to avoid arcing between the terminals.

2) Connect one side of the bleeder resistor to any terminal on the capacitor and then connect the other side of the bleeder resistor to the ground with lead wires.

3) Check if there is any sparking between the bleeder lead wires and the case where they are grounded. Sparking may indicate that there is an issue with the bleeder resistance, which would need to be replaced before proceeding with testing.

4) Discharge the capacitor by touching one bleeder lead wire to the ground while allowing the other bleeder lead wire to come in contact with the negative bleeder resistor terminal for a brief moment. If there is no spark or arcing, then the capacitor is good and may be used in the circuit.

5) If the capacitor bleeds off charge without sparking or arcing between bleeder lead wires, then it may be used in the circuit.

How to Choose a Bleeder Resistor

If you pick a tiny, low-value resistor, it will aid in fast bleeding. It uses more power, however, if you choose a high-value resistor.

So, the designer has to pick out a high-value resistor that is not too high to interfere with the power supply and yet low enough to discharge the capacitor quickly.

A bleed resistor should be able to bleed current at the same rate that the power supply delivers current. It should not bleed the power supply faster than it can deliver current, in order to avoid damaging the capacitor.

It is also good practice to choose a bleed resistor of high wattage, in order to avoid overheating. The bleed resistor can dissipate more power when it is made of wire rather than a piece of copper foil.

Conclusion

The bleeder resistor is important, but most people never see it. It’s an uninteresting part of a circuit with no flashy LEDs, buzzers, or LCDs. But at its heart, it serves a valuable purpose that deserves some attention from all of us… safety! Easybom hopes this does help you with your bleed circuits design.