sitelights
07-05-2001, 07:14 PM
How can you determine the quality of a low voltage outdoor lighting transformer?
First, visually inspect the unit in question. Is it in a heavy gauge stainless steel R-3 case, are there any sharp edges, does it have louvers for cooling...forget it, a louvered case (bug heaven) is merely an historical artifact (or an under-sized housing)..., is it fully potted (usually an epoxy/sand mix), does it have thermal overload protection with auto-reset, does it have resettable breakers for dead shorts (not those damn fusestats that have to be replaced and provide some goofy "diagnostic" signal...yes the fusestat blew, now put another one in and try again), does it have a barrier between the primary and secondary wiring compartments (the "Faraday Shield")?
You could completely destroy the transformer by dismantling and counting the windings and weighing the core (finer windings, heavier coil, purity of metals = higher quality)or you could make a simple tests.
Test method (A): select the transformer you wish to test, energize the transformer and check the input primary voltage (usually 115v to 125v) and the output secondary voltage (usually 11.5v to 12.5v) with a digital volt meter that reads to .00 volts and record the results; then connect the secondaries to a 500' reel of low voltage cable and connect the meter to the other end of the cable; energize the transformer and record the voltage at the end of the cable; then check the voltage at the transformer secondaries. Note that in this test the 500' of low voltage cable acts as the load. If transformers of the same output rating are compared the transformer that provides the highest secondary voltage at the cable end has better "regulation" which is the term for maintaining output under load.
Test method (B):quantify an existing low voltage installation by testing and recording the parameters listed above (primary input voltage, secondary output voltage at the transformer and secondary voltage at the last lamp or cable end) and record the results. Then replace the existing transformer sequentially with the transformer(s) you wish to test. It is the ratio of the secondary output voltage at the transformer to the output at the last lamp or end of cable that is important. As an example, if unit "weak" has a ratio of 12.0v at the transformer and 8.0v at the end and unit "strong" has a ratio of 12.0v/10.0v, under identical conditions, the known superiority is now self-evident.
Amps, however, is another matter to be addressed in a later post (done: search "ampacity", "amps" and "capacity"). Note that if you are not using a good quality digital volt meter routinely in your work, you are merely a novice who mistakenly thinks that if all the lamps glow you have a succesful installation.
Effective low voltage lighting provides a maximum of 12v and a minimum of 10.5v to every lamp on that transformer. A difference of more than 10% voltage between first and last lamps can be detected visually without recourse to a volt meter or a footcandle meter (more on this item later...done search "meter").
Remember that energy costs are a factor in any lighting project; unintentionally dim lights are using the same energy as bright lights but the energy in the dim system is lost through: voltage drop caused by resistance in the main cable, poor transformer regulation, bad connections and as heat in the supply cable due to undersizing the cable.
I use RadioShack digital multimeters ($30.00 or so, buy one on sale for $20.00 see Product Reviews) that test accurate to within +/- .01v and there is no need to calibrate if you use the same meter for every test. You don't know what you are doing until you can quantify your results. Energy is precious.
First, visually inspect the unit in question. Is it in a heavy gauge stainless steel R-3 case, are there any sharp edges, does it have louvers for cooling...forget it, a louvered case (bug heaven) is merely an historical artifact (or an under-sized housing)..., is it fully potted (usually an epoxy/sand mix), does it have thermal overload protection with auto-reset, does it have resettable breakers for dead shorts (not those damn fusestats that have to be replaced and provide some goofy "diagnostic" signal...yes the fusestat blew, now put another one in and try again), does it have a barrier between the primary and secondary wiring compartments (the "Faraday Shield")?
You could completely destroy the transformer by dismantling and counting the windings and weighing the core (finer windings, heavier coil, purity of metals = higher quality)or you could make a simple tests.
Test method (A): select the transformer you wish to test, energize the transformer and check the input primary voltage (usually 115v to 125v) and the output secondary voltage (usually 11.5v to 12.5v) with a digital volt meter that reads to .00 volts and record the results; then connect the secondaries to a 500' reel of low voltage cable and connect the meter to the other end of the cable; energize the transformer and record the voltage at the end of the cable; then check the voltage at the transformer secondaries. Note that in this test the 500' of low voltage cable acts as the load. If transformers of the same output rating are compared the transformer that provides the highest secondary voltage at the cable end has better "regulation" which is the term for maintaining output under load.
Test method (B):quantify an existing low voltage installation by testing and recording the parameters listed above (primary input voltage, secondary output voltage at the transformer and secondary voltage at the last lamp or cable end) and record the results. Then replace the existing transformer sequentially with the transformer(s) you wish to test. It is the ratio of the secondary output voltage at the transformer to the output at the last lamp or end of cable that is important. As an example, if unit "weak" has a ratio of 12.0v at the transformer and 8.0v at the end and unit "strong" has a ratio of 12.0v/10.0v, under identical conditions, the known superiority is now self-evident.
Amps, however, is another matter to be addressed in a later post (done: search "ampacity", "amps" and "capacity"). Note that if you are not using a good quality digital volt meter routinely in your work, you are merely a novice who mistakenly thinks that if all the lamps glow you have a succesful installation.
Effective low voltage lighting provides a maximum of 12v and a minimum of 10.5v to every lamp on that transformer. A difference of more than 10% voltage between first and last lamps can be detected visually without recourse to a volt meter or a footcandle meter (more on this item later...done search "meter").
Remember that energy costs are a factor in any lighting project; unintentionally dim lights are using the same energy as bright lights but the energy in the dim system is lost through: voltage drop caused by resistance in the main cable, poor transformer regulation, bad connections and as heat in the supply cable due to undersizing the cable.
I use RadioShack digital multimeters ($30.00 or so, buy one on sale for $20.00 see Product Reviews) that test accurate to within +/- .01v and there is no need to calibrate if you use the same meter for every test. You don't know what you are doing until you can quantify your results. Energy is precious.