A request from a non member

[richard296]

lawrence the reading on resistor was 19.1 volts. I took the readings every 15 mins no fluctuation. Thank you all. Richie

[Skywave]

Another example of what some of us - over the years - have learnt the hard way. Four things tend to reduce the life of electronic assemblies:

1. Heat, excess of; lack of ventilation.
2. Dampness and moisture, ingress of.
3. Excess voltage - which usually leads to #1 anyway.
4. Poor mechanical integrity: vibration; thermal expansion & contraction.

Over the years I have seen a lot of commercial kit which is especially weak on item 1 above. It is often possible to relocate hot-running items to elsewhere on the chassis where the heat can escape - even if that means drilling ventilation holes, using higher-rated wattage components and, occasionally, a minor design change.

Al.

[richard296]

skywave thank you for your comments and taken on board I will remedy the hot capacitor as you have stated. I'm new to all this. Your last comment using higher wattage components. Do you not mean use better quality components. As there are 4 values in the equation. The charts say give any 2 and the other 2 can be calculated. I'm puzzled because i think by changing the wattage value you change the other factors. By changing these factors does this not make the other equations wrong. Or am i totally lost. Thank you richie

[richard296]

sorry hot resistor to busy thinking how to word my reply. richie

[Refugee]

The wattage is just a maximum rating and will make no difference to the currant or resistance.
Increasing the wattage will make it run cooler and give the part an easy life.

[richard296]

Thankyou for that refugee. I hope i understand what your saying. So therefore if the higher wattage number doesn't count. You would then only need 1 value then you can work the other 2 values of the equation needed. Or should i stick to to being a mp and robbing you all blind. thank you richie

[pwdrive]

Thankyou for that refugee. I hope i understand what your saying. So therefore if the higher wattage number doesn't count. You would then only need 1 value then you can work the other 2 values of the equation needed. Or should i stick to to being a mp and robbing you all blind. thank you richie

Richie, you might be getting confused between the wattage (power) rating of a resistor and the wattage (power) that the resistor is dissipating.

The wattage rating is the maximum power in watts that the resistor is designed to dissipate while maintaining it's specified resistance value.

The power dissipated by the resistor is dissipated as heat as a result of the current flowing through it.

The wattage rating of the resistor should always be higher than the power the resistor is dissipating for safety and long term reliability.

The power (in watts) that the resistor is dissipating is a product of the current squared (in amps) that is flowing through it and the resistors value (in ohms)

The current flowing through it can either be determined by connecting a test meter in series with the resistor and set to measure current or the current flow can be derived by using ohms law by measuring the voltage across the resistor in volts and dividing that voltage by the value of the resistor in ohms.

Lawrence.

[AlanBeckett]

In days of old, when such things were allowed, it was the done thing to put a low value Wirewound Resistor across a High Current PSU, wait until it got red hot, and use it light your Fag.
Over-dissipation in action

Alan

[richard296]

I think i understand now. It's the readings taken on the components or the load it's taking or the circuit that feeds it. Not what someone has added later. Am i getting there. Reason for confusion the smallest resistor i could get in alloy case is 25 watts. I thought this would throw all calcs out. What i did was place a sheet of alloy on top of the ceramic resistor. 50 x 50 x 8mm no paste. I may be imagining things but the plate wasn't even warm. I could even keep my hand on the resistor. It had been on for 2 hours or so. If this is ok. I will solder resistor other side of board it will then protude through the top of alloy case after cutting a hole. I will then fasten plate to resistor using contact paste. What do you's reckon.

[pwdrive]

I think i understand now. It's the readings taken on the components or the load it's taking or the circuit that feeds it. Not what someone has added later. Am i getting there. Reason for confusion the smallest resistor i could get in alloy case is 25 watts. I thought this would throw all calcs out. What i did was place a sheet of alloy on top of the ceramic resistor. 50 x 50 x 8mm no paste. I may be imagining things but the plate wasn't even warm. I could even keep my hand on the resistor. It had been on for 2 hours or so. If this is ok. I will solder resistor other side of board it will then protude through the top of alloy case after cutting a hole. I will then fasten plate to resistor using contact paste. What do you's reckon.

Yes, it's the voltage/current measurements that count, 15 watts written or stamped on a resistor doesn't mean it's going to dissipate 15 watts, it's the actual current flowing through it that determines the actual power it will dissipate, that's why earlier on we asked for the voltage measurement on the input and the output side of the resistor, from that you can work out what power the resistor is dissipating.

Once you know the power it's dissipating you can choose a wattage rating to suit, the higher the wattage rating is above the dissipated power the cooler the resistor will run.

Lawrence.