Calculator Inputs
Example Data Table
| Case | V0 | Vsafe | Capacitance | Target Time | Maximum Resistance |
|---|---|---|---|---|---|
| Low voltage converter | 48 V | 5 V | 2200 µF | 10 s | 2.0097 kΩ |
| Intermediate bus | 120 V | 12 V | 1000 µF | 30 s | 13.0288 kΩ |
| High voltage link | 400 V | 50 V | 470 µF | 60 s | 61.3913 kΩ |
Formula Used
The capacitor voltage during bleeder discharge is:
V(t) = V0e-t/(RC)
The maximum resistance that reaches the safe voltage in the target time is:
R = -t ÷ [C × ln(Vsafe ÷ V0)]
Discharge time for a selected resistor is:
t = -RC × ln(Vsafe ÷ V0)
Initial current, initial power, and capacitor energy are:
I0 = V0 ÷ R
P0 = V02 ÷ R
E = 0.5 × C × V2
The suggested wattage rating uses the tolerance worst case and the selected safety factor.
How to Use This Calculator
Enter the converter output voltage before shutdown. Enter the voltage that you consider safe for service or testing. Add the total output capacitance connected to that node. Enter the desired discharge time in seconds.
Leave the installed resistor field blank to get a recommended value. Enter a real resistor value to test an existing design. Add tolerance, safety factor, resistor count, and series or parallel arrangement. Then press Calculate.
Check the result above the form. Confirm the time target, current limit, power budget, suggested wattage, energy removed, and standby loss. Use the CSV or PDF button to save the result.
Understanding DC-DC Bleeder Resistors
A DC-DC converter can keep charge after input power is removed. Output capacitors, snubbers, and test fixtures may hold voltage long enough to surprise a technician. A bleeder resistor solves this problem by adding a predictable discharge path. The resistor is usually placed across the output capacitor or across a selected storage bank.
Design Balance
The main design balance is simple. Lower resistance discharges faster, but it wastes more power during normal operation. Higher resistance wastes less energy, yet it may leave the bus charged for too long. This calculator shows that tradeoff with time, current, energy, and resistor wattage in one result.
Discharge Time
Discharge time follows an exponential curve. One time constant equals R multiplied by C. After one time constant, capacitor voltage falls to about 36.8 percent of its starting value. Safety targets are often lower than that, so several time constants may be needed. The tool rearranges the exponential equation and finds the largest resistance that still meets the requested shutdown time.
Power Rating
Power rating is just as important as resistance. At the first instant, the resistor sees the full starting voltage. This creates the highest current and highest power. The rating should cover that value with margin. A safety factor allows heat, tolerance, enclosure temperature, and part aging to be considered.
Tolerance and Assembly
Tolerance can change the real discharge behavior. A high actual resistance makes discharge slower. A low actual resistance increases current and power. The calculator reports both worst cases, so the selected part can be checked before it is placed in a prototype.
For multi-resistor assemblies, equal parts can be used in series or parallel. Series parts divide voltage and power. Parallel parts divide current and power. The total equivalent resistance must still match the design goal.
Engineering Review
Use the result as an engineering estimate. Confirm component voltage rating, pulse rating, thermal rise, creepage, clearance, and converter standby loss. For hazardous voltages, follow qualified electrical safety rules and verify the output with a meter before handling.
In regulated products, discharge time may be set by an internal company rule or a safety standard. When no rule is available, pick a conservative safe voltage and document the assumptions. Review measurements under maximum capacitance, maximum voltage, and ambient heat.
FAQs
What is a bleeder resistor?
A bleeder resistor is a resistor placed across a charged capacitor or output rail. It drains stored energy after power is removed and helps the voltage fall to a safer value.
Why does a lower resistor discharge faster?
A lower resistance gives the capacitor a stronger discharge path. That reduces the RC time constant. The tradeoff is higher standby current and higher heat during normal converter operation.
Can I use the exact calculated resistance?
You can use the calculated value as a design target. In practice, choose a standard resistor value and verify time, power, tolerance, voltage rating, and thermal performance.
Why is the highest power at the start?
At the start, the capacitor voltage is highest. Since resistor power equals voltage squared divided by resistance, initial power is the maximum value during discharge.
What safety factor should I use?
A factor of two is a common early estimate. Use a larger factor for hot enclosures, poor airflow, high voltage, uncertain capacitance, or long continuous operation.
Should resistors be in series or parallel?
Use series parts when voltage rating is the main issue. Use parallel parts when current sharing or power sharing is needed. Always verify equal part ratings.
Does the calculator include capacitor leakage?
No. It assumes the bleeder resistor dominates the discharge path. Leakage may help discharge, but it is usually uncertain and should not replace a defined bleeder design.
Is the output safe after the calculated time?
The result estimates voltage decay for ideal parts. Always measure the output before touching a circuit, especially with high voltage or high energy converter links.