# Resistance Ratios for the high-voltage resistive divider

The output voltage is read by the SHVPS by way of a resistive divider. The SHVPS micro-controller (Arduino micro) has a 10 bit analog-to-digital converter (ADC) which maps an input voltage from 0 V to 5 V into a value between 0 and 1024. The resistive divider should therefore divide the maximal output voltage by a given ratio, so that an output value of 5 V is obtained when the SHVPS is at its maximum voltage rating (for example, the ratio would be 1:1000 for a 5kV unit). In practice however, we have chosen to use slightly higher ratios, so that the voltage on the ADC input is roughly at 4.8 V when the SHVPS is delivering its full output voltage. This prevents the ADC from saturating, should the output raise higher than the maximal rating of the HVPS, and therefore allows detecting (and dealing with) this eventuality.

Furthermore, once the ratio is defined, it is necessary to choose the values of resistances to use. We have noticed that the current “wasted” in the voltage measuring circuit, has a strong influence on the ability of the EMCO DC/DC converter to rapidly change its output voltage. The more current going through the circuit, the faster the response to a voltage set point change. However, this means less current available for the load. We have tested different resistance combinations and the optimal values for a few EMCO Series A models are given below:

EMCOR6 (MΩ)R9 (kΩ)RatioFb Volt. @ Vmax (V)C1HV Resistor #LV Resistor #
5kV10095.310504.761.05031550763 (Farnell)
2327394 (Farnell)
3kV5080.66214.831.0356SM102035005FE-ND (Digi-Key)P80.6KFCT-ND (Digi-Key)
2kV501204184.791.0442SM102035005FE-ND (Digi-Key)P120KFCT-ND (Digi-Key)
1.2kV2288.72494.821.037622MGBCT-ND (Digi-Key)2327378 (Farnell)
500V222151034.841.033322MGBCT-ND (Digi-Key)2327103 (Farnell)

Columns description:

1. EMCO: The voltage rating of the EMCO to be soldered on the board
2. R6: Value of resistance R6 (HV resistor)
3. R9: Value of resistance R9 (LV resistor)
4. Ratio: relation between the total (HV) voltage applied to the divider and the low voltage output (i.e. (R9+R6)/R9)
5. Fb Volt. @ Vmax (V): Feedback voltage (i.e. voltage applied on the ADC pin) when the EMCO is delivering its full output voltage. This the EMCO rating (col 1) divided by the ratio (col 4).
6. C1: This is the theoretical correction factor assuming linear relationship between the true HV output and the value read by the ADC. This value can be used as calibration for the HVPS, using C0=0, C2=0, and setting C1 to the value given in this table. However, this is not recommended, and a full calibration should be done in order to have a good voltage accuracy over the complete voltage range of the HVPS. The value in this column is obtained by dividing 5 (i.e. the full scale voltage of the ADC) by the result of col 5.
7. HV Resistor #: This is the order number (and retailer) for the HV resistor
8. LV Resistor #: This is the order number (and retailer) for the LV resistor