C-Bus Voltage Calculation

Transcription

1 D E S I G N E R N O T E S C-Bus Voltage Calculation Designer note number: Designer: Darren Snodgrass Contact Person: Darren Snodgrass Aroved: Date:

2 Synosis: The guidelines used by installers to determine whether a C-Bus Network design is within secifications are sometimes not adequate to cover more comlex cases. This Design Note rovides formulas that can be alied to a network design to determine the voltage dros across the network. Commercial In Confidence The following document is issued commercial in confidence and cannot be reroduced or transmitted to unauthorised ersonnel without the exressed written ermission of Clisal Integrated Systems Pty Ltd.

3 Introduction Most C-Bus installers are familiar with the general guidelines of installing C-Bus to ensure that the C-Bus voltage is sufficient: No more than 100 units er network No more than 1000m of cable Make sure there are enough ower sulies for the units Unfortunately, these simle guidelines are not sufficient to comletely redict whether a C-Bus network will have adequate voltage at all oints on the network. In the following examles it is assumed that there are sufficient C-Bus ower sulies for the C-Bus units used (the C-Bus Calculator will rovide this information). For a long network with lots of C-Bus units, like the one in Figure 1, there may be considerable voltage dro along the C-Bus cable. Power Sulies C-Bus Figure 1 For the case of 100 C-Bus inut units saced at 3m intervals along a C-Bus cable, the voltage along the network is shown in Figure 2. C-Bus Voltage 100 units, saced 3m aart, each drawing 18mA Voltage Distance Figure 2 The voltage at the far end of the network is around 5V, which will revent the network from oerating correctly, even though the simle guidelines have been followed. It should be noted that adding more ower sulies to the network at the same lace as the others will not make any difference to the network voltages and will not solve the roblem. There are generally two solutions to the roblem : 1. Sread the ower sulies around the network 2. Sread the network around the ower sulies

4 Sreading the Power Sulies Around the Network By lacing ½ of the ower sulies at each end of the network (as shown in Figure 3), the voltage dros are reduced considerably. For the above examle of 100 units at 3m intervals, the voltage along the network is shown in Figure 4. Voltages are well within the C-Bus oerational limits. Power Sulies C-Bus Power Sulies Figure 3 C-Bus Voltage 100 units, saced 3m aart, each drawing 18mA Voltage Distance Figure 4 Placing ower sulies at additional oints along the network will imrove the voltage again. It is not always convenient or ossible to sread the ower sulies around the network. In these cases, it may be ossible to sread the network around the ower sulies. Sreading the Network Around the Power Sulies By lacing ½ of the network on each side of the Power Sulies (as shown in Figure 5), the voltage dros are also reduced considerably. For the above examle of 100 units at 3m intervals, slit into two searate runs, the voltage along the network is shown in Figure 6. C-Bus Power Sulies Figure 5

5 C-Bus Voltage 100 units, saced 3m aart, each drawing 18mA Voltage Distance Figure 6 Dividing the network into additional runs will imrove the voltage again. A comarison of the network voltages for the above examle with a single network run, and with the network divided into 2, 3 and 4 runs is shown in Figure 7. C-Bus Voltage Voltage run 2 runs 3 runs 4 runs Distance from Power Suly Figure 7

6 Calculating Voltage Dros The voltage at the end of a C-Bus Network with ower sulies at one end and where the units are evenly saced and have the same current consumtion is found from: Vend I N R = L I ( n + 1) R Vend 36 ( L) I n (for n > 10 with a single run) Where : V end = voltage at end of network L = length of C-Bus network I = current er unit (18mA for key inut units) n = number of C-Bus units on the run N= total number of C-Bus Units on all runs R = outut imedance of Power Sulies (see Table 1) = number of Power Sulies Notes on alying the formula: Where the network is sread around the ower sulies, the above formula should be alied to each run to determine whether the voltages are within range. Where the ower sulies are sread evenly around the network, the formula can still be used. In this case, the half way oint between two ower sulies is like the end of a run. The voltage will be given by using the formula with half of the number of units (on the run between the two ower sulies) and half the length of the cable between the ower sulies. Where the units have different currents, use an average value. The calculated result in these cases will be less accurate. In an installation where most or all of the units are at the far end of a network (rather than being evenly distributed) the end voltage will be lower than the formula redicts. If the units are mostly closer to the ower sulies, the end voltage will be higher. C-Bus Power Suly 5500PS (350mA) Din Rail Outut Units with 200mA ower suly Outut Imedance 20Ω 35Ω Table 1

7 Examle Calculations Examle 1 If you have 100 inut units, 3 meters aart, each drawing 18mA (as for the original examle): L = 300m n = 100 N = 100 I = 0.018A (18mA) R = 20 (assuming DIN rail ower sulies) = 6 (6 x 350mA = 2100mA) V end = 36 (20 / x 300) x x 100 (using aroximation formula) = 5.7V (this network is not OK) Examle 2 If you have 8 touch screens sread over a 1000 meter long network, each drawing 40mA: L = 1000m n = 8 N = 8 I = 0.040A (40mA) R = 20 (assuming DIN rail ower sulies) = 2 (2 x 350mA = 700mA) V end = / 2 x 8 x x 1000 x x 9 = 16.6V (this network is just OK, but should be imroved) Examle 3 If you have 3 touch screens and 20 key inut units on a network which is 200m long: L = 200m n = 23 (3 + 20) N = 23 I = 0.029A (average value = (40mA x mA x 20) / 23 ) R = 20 (assuming DIN rail ower sulies) = 2 (2 x 350mA = 700mA) V end = / 2 x 23 x x 200 x x 24 = 23V (this network is OK)

8 Examle 4 If you have 100 inut units, 3 meters aart, each drawing 18mA, but with a ower suly at each end: L = 150m (use half the network length where ower sulies are sread around) n = 50 (use half the number of units where ower sulies are sread around) N = 100 (there are 100 units in total) I = 0.018A (18mA) R = 20 (assuming DIN rail ower sulies) = 6 (6 x 350mA = 2100mA) V end = / 6 x 100 x x 150 x x 51 = 23.8V (this network is OK) Examle 5 By rearranging the formula in the Aendix, the relationshi between the number of inut units and the length of cable on a single run is: Vs Vend n = L I 1 and Vs Vend L = I ( n + 1) This is shown grahically in Figure 8 assuming: V s = 32V (tyical case where the number of ower sulies is just adequate) V end = 18V (allowing a few volts of margin) I = 18mA Maximum C-Bus Units (at 18mA each) Units Cable Length Figure 8

9 Conclusions It is ossible to determine the aroximate voltage at the end of a C-Bus network. Provided the end voltage is within the C-Bus oerational voltage range (15V 36V), the network should oerate satisfactorily. It is always best to allow a few volts of margin to allow for errors, so if the formula redicts a voltage of less than 20V, it is advisable to consider restructuring the network. Where the aroximations do not hold (for examle, there are units of different currents, their sacing is uneven or there are different tyes of ower sulies), then averages can be used. In this case the result will be less accurate, and a wider safety factor should be alied (for examle, if the result is less than 24V, then the network could have a roblem).

10 Aendix Network Voltage Formula Derivation Caution : Maths Ahead The voltage dros along a C-Bus network can be calculated using ohms law: V = I R Where V = voltage dro I = current R = resistance However, this gets a bit comlicated when there are multile C-Bus devices laced along a network as in the examles above. In this case, the voltages are calculated as given below. Power Sulies V s ki C-Bus 2I d I V end V k V d Assuming (see Figure 9): evenly saced units (distance d aart) Figure 9 all units drawing the same current (I) The voltage at the end of the network is equal to the ower suly voltage minus the voltage dro: V end = V s V d Voltage Dro The voltage dro is found from : n V d = V k k = 1 where n is the number of units on the run. The voltage dro between each unit is the current (k I) x the resistance (R), giving: V d = n k = 1 k I R

11 V = I R d n k = 1 k 1 V d = I R n( n + 1) 2 The resistance of C-Bus cable (with airs wired together) is around 0.09Ω/m, so V d = d I n( n + 1) The total length of the cable L = n d Hence V d = L I ( n + 1) For values of n greater than 10, the aroximation can be used: Vd L I n Power Suly Voltage The nominal Power Suly voltage for a single ower suly is: V = 36 I s o R o Where I o = outut current R o = outut imedance The outut imedance for various C-Bus Power Sulies are shown in Table 1. Where there are several ower sulies in arallel, the effective imedance will be: 1 = Ro k = 1 1 R k Where R k is the outut imedance of the kth ower suly is the number of ower sulies If all of the ower sulies are the same, this simlifies to: R R o = Where R is the outut imedance of the ower sulies. is the number of ower sulies The outut current I o is found from: I o = I N Where Hence V s = 36 I = current consumtion er C-Bus unit (tyically 18mA) N = total number of C-Bus units (on all runs owered from the Power Sulies) I N R

12 Outut Voltage Hence the voltage at the end of the network is: I N R Vend = L I ( n + 1) Where there is a single run, n = N, giving: R Vend 36 ( L) I n (for n > 10 and with a single run)