OHM’s Law Help Sheet
Definition
A potential difference of 1 Volt will force a current of 1 Ampere through a resistance of 1 Ohm, or
Voltage (V—Volts) = Current (I—Amps) x Resistance (R—Ohms) or more simply:
V = I x R
Power (W-Watts) = Voltage (V—Volts) x Current (I—Amps) or more simply:
W = V x I
The diagram below shows the variations on this formulae.
Examples:
A heater in a tool measures 20 Ohms. Assuming a voltage of 240V, what is the expected wattage of the heater?
W = V x V / R = 240 x 240 / 20 = 2880W.
A controller has a fused output of 15A on a voltage of 240V. What would be the expected maximum output wattage of the controller?
W = V x I = 240 x 15 = 3600W.
A 24V tip is rated at 250W. What would the expected current demand be?
W = V x I, or I = W / V = 250 / 24 = 10.42A
If the connecting cable to the 24V tip is rated at 0.1R per meter and is 3m long, how much heat will be dissipated by the cable?
Total R of cable = 3 x 0.1R = 0.3R. W = I 2 x R = 10.42 x 10.42 x 0.3 = 32.57W per cable.
If the system is changed to a 240V tip, all other parameters remaining the same, what will be the current demand be and how much heat will be dissipated by the cable?
I = W / V = 250 / 240 = 1.042A
Total R of cable = 3 x 0.1R = 0.3R. W = I 2 x R = 1.042 x 1.042 x 0.3 = 0.323W per cable.
Page:1
Tel: 01323 647375 Fax: 01323 410355 e-mail: tech-support@fastheatuk.com

Resistor Networks
OHM’s Law Help Sheet
Resistor Colour Code
Page:2
Tel: 01323 647375 Fax: 01323 410355 e-mail: tech-support@fastheatuk.com

Resistance / Wattage / Amperage Chart
For any given resistance measurement, the table below shows the expected wattage and current load based on a 240V ac system. Please note that this table is provided as a ‘rule of thumb’ and the values shown are rounded for simplicity. It will also be necessary to take into consideration the resistance of the cables when measuring loads with a high wattage.
150
160
170
180
110
120
130
140
190
200
38
39
40
34
35
36
37
31
32
33
Ohms
1
2
3
4
5
8
9
6
7
10
1.6
1.5
1.4
1.3
2.2
2.0
1.8
1.7
1.3
1.2
6.3
6.1
6.0
7.0
6.9
6.7
6.5
7.7
7.5
7.3
Amps Ohms
240 11
120 12
80 13
60 14
48 15
40 16
34 17
30 18
27 19
24 20
300
320
340
360
220
240
260
280
380
400
48
49
50
44
45
46
47
41
42
43
384
360
339
320
524
480
443
411
303
288
1,858
1,800
1,745
1,694
1,646
1,600
1,557
1,516
1,477
1,440
Watts
57,600
28,800
19,200
14,400
11,520
9,600
8,230
7,200
6,400
5,760
192
180
169
160
262
240
221
206
152
144
1,405
1,371
1,340
1,309
1,280
1,252
1,225
1,200
1,175
1,152
Watts
5,236
4,800
4,430
4,114
3,840
3,600
3,388
3,200
3,030
2,880
Page:3
0.8
0.75
0.7
0.67
1.1
1.0
0.9
0.85
0.63
0.6
5.00
4.90
4.80
5.45
5.33
5.22
5.11
5.85
5.71
5.58
Amps
22
20
19
17
16
15
14
13.3
12.6
12
38
29
23
19
231
144
115
58
14
11.5
90
95
100
70
75
80
85
55
60
65
Ohms
21
22
23
24
25
26
27
28
29
30
Tel: 01323 647375 Fax: 01323 410355 e-mail: tech-support@fastheatuk.com
250
400
500
1000
1500
2000
2500
3000
4000
5000
1,047
960
886
640
606
576
823
768
720
678
Watts
2,743
2,618
2,504
2,400
2,303
2,215
2,133
2,057
1,986
1,920
6.25
8.33
10.4
12.5
1.04
1.67
2.08
4.17
16.7
20.8
2.7
2.5
2.4
3.4
3.2
3.0
2.8
4.4
4.0
3.7
Amps
11.4
10.9
10.4
10.0
9.6
9.2
8.9
8.6
8.3
8.0